Microalgae–material hybrid for enhanced photosynthetic energy conversion: a promising path towards carbon neutrality

ABSTRACT Photosynthetic energy conversion for high-energy chemicals generation is one of the most viable solutions in the quest for sustainable energy towards carbon neutrality. Microalgae are fascinating photosynthetic organisms, which can directly convert solar energy into chemical energy and electrical energy. However, microalgal photosynthetic energy has not yet been applied on a large scale due to the limitation of their own characteristics. Researchers have been inspired to couple microalgae with synthetic materials via biomimetic assembly and the resulting microalgae–material hybrids have become more robust and even perform new functions. In the past decade, great progress has been made in microalgae–material hybrids, such as photosynthetic carbon dioxide fixation, photosynthetic hydrogen production, photoelectrochemical energy conversion and even biochemical energy conversion for biomedical therapy. The microalgae–material hybrid offers opportunities to promote artificially enhanced photosynthesis research and synchronously inspires investigation of biotic–abiotic interface manipulation. This review summarizes current construction methods of microalgae–material hybrids and highlights their implication in energy and health. Moreover, we discuss the current problems and future challenges for microalgae–material hybrids and the outlook for their development and applications. This review will provide inspiration for the rational design of the microalgae-based semi-natural biohybrid and further promote the disciplinary fusion of material science and biological science.


INTRODUCTION
Microalgae, including cyanobacteria and green algae, represent the most important biological systems for producing biomass and high-value products [ 1 ].As the most important primary producer, microalgae are found almost every w here and have a profound impact on Earth's atmosphere and biosphere [ 2 , 3 ].Currently, excessive exploitation and utilization of fossil fuels have produced a large amount of carbon dioxide emissions, contributing to increasing global warming.Climate change is a crisis facing mankind, while carbon neutrality is a common interest and responsibility of mankind.The Chinese government has announced to the world a timetable for carbon peaking and carbon neutrality-that is, China aims to peak its carbon dioxide emissions before 2030 and strives to achieve carbon neutrality before 2060.Energy transition is the fundamental guarantee for the realization of carbon-peaking and carbon-neutrality targets.Photosynthesis can convert carbon dioxide and water into biomass and valuable products, which is one of the most important pathways for carbon neutrality in nature.Microalgal photosynthesis could not only absorb the carbon being put out by fossil fuel combustion, but also produce hydrogen that is fully carbon-neutral [ 4 , 5 ].Further, microalgae are suitable raw materials for photosynthesis-derived fuels like biodiesel, biohydrogen and bioethanol [ 6 ] because their cultivation does not need to occupy arable land and can use saltwater and wastewater [ 7 ].Therefore, exploiting microalgae photosynthetic energy is a promising approach to energy transition for carbon peak and neutrality.
Microalgae production technologies, including microalgae selection, breeding and engineering, are playing a more and more important role in biofuel production [ 8 ].However, the photosynthetic conversion efficiency is a major limitation for microalgae biofuels production.General ly, wi ld-type strains of microalgae can capture 43% of the sunlight energy in the photosynthesis, but only ∼4-8% of the light energy can be converted into chemical energy in the form of biomass (theoretical maximum is 9%) [ 9 ].Therefore, the microalgae production of biofuels cannot meet human energy needs.Genetic engineering and metabolic engineering can be used to improve the microalgal production of biomass and lipids [ 10 ] but these methods require complicated operation and high cost with a narrow scope for application.
In nature, organisms can form organic-inorganic composite materials with complex structures and excellent biological properties through biomineralization, such as bones, teeth and shells [ 11 , 12 ].These biomaterials have a highly ordered hierarchical structure from nanoscale to macroscale, which can prov ide organisms w ith functions such as mechanical support, protection, movement and signal sensing [ 12 -15 ].For example, diatoms can use silicate in the environment to form a finely structured silica shell on the cell surface, which can provide them with mechanical protection, photonic crystals and pH buffers [ 16 , 17 ].In addition, there are some bacteria that can use internal magnetite crystals (Fe 3 O 4 or Fe 3 S 4 ) as magnetic sensors [ 13 , 18 ].Inspired by the natural biomineralization phenomenon, an artificial cell-material hybrid has received increasing interest for green chemistry and engineered living biomaterials [ 19 , 20 ].In order to exploit the microalgal photosynthetic energy, scientists have been inspired to interface nature's photosynthetic organisms with synthetic materials so as to impart the organisms with new properties.We define the artificially generated biomorph as a microalgae-material hybrid (MMH), which is mainly based upon microscale interactions, such as chemical bonds or non-covalent interactions.In the early research stage, the silica solgel method was used for immobilizing cyanobacteria to enhance photosynthetic CO 2 fixation [ 21 ].In pursuit of new photosynthetic energy conversion, silicification-induced multicellular aggregation was developed for inducing photosynthetic H 2 production under aerobic conditions [ 22 ].Further, cyanobacteria have been coupled with electrodes for bioelectrochemical energy conversion [ 23 ].These MMHs offer a great incentive to develop seminatural photosynthetic systems for solar-to-chemical energy conversion.In addition, the MMHs have recently been used in photo-driven biomedical therapy due to their characteristics of photosynthetic H 2 or O 2 evolution [ 24 , 25 ].It follows that the use of MMHs for biological regulation is becoming an emerging field.In this feature article, we review the recent achievements in the material-based modification of microalgae, as well as their application in solar energy utilization, water environment protection and biomedical therapy (Fig. 1 ).Furthermore, we elucidate the mechanisms of MMH for functional regulation from the perspective of the cellmaterial interfacial interaction.We also discuss the current problems and outlook for future challenges of MMHs for enhanced photosynthetic energy conversion.

CONSTRUCTION OF THE MMH
In nature, many organisms have evolved to utilize inorganic materials to achieve optimal function or protect themselves against harsh environments.Among them, diatoms are the typical cell-material hybrids, which can form sophisticated nanostructured silica shells for mechanical protection and photonic response [ 15 ].However, most microalgae cannot spontaneously form cell-material hybrids due to the lack of biomineralization-relevant proteins.In the recent decade, scientists have explored three material approaches to improve microalgae.

Cell immobilization
The long-term stability of cell activity is crucial for microalgal applications but microalgal cells are usually sensitive to a disturbed environment.Cell immobilization can confer high stability and extend the lifetime for microalgae because the matrix for immobilization provides a stable microenvironment that can protect the cell from harsh external environments, such as temperature fluctuations, pH fluctuations, organic solvents, shear forces and toxins [ 26 -28 ].Furthermore, the immobilization can offer easy regulation of the culture environment for cells [ 26 ].In addition, cell immobilization can afford the system a high cell density.
Microalgal cells could be immobilized by different material strategies.Over the years, three cell immobilization methods such as adsorption, covalent binding and entrapment have been successfully developed [ 26 ] (Fig. 2 ).Adsorption is a mild and reversible process that depends on the physical binding between the cell surface and the material surface, such as van der Waals interactions, ionic forces and hydrogen bonds.Also, immobilization by adsorption permits the reuse of microalgae cells.However, easy leaching limits the development and practical applications of this method.Unlike adsorption, covalent bonding is an irreversible process that can improve the stability of a cell-material hybrid but may have a negative effect on cell activity (Fig. 2 ).Covalent binding immobilization is based on the covalent bond between cells and the support matrix in the presence of a binding agent.The binding agents are usually cytotoxic and can cause loss of cell viability (Fig. 2 ) due to conformational transitions or structural changes of organic molecules on the cell surface [ 26 , 29 ].Therefore, the covalent binding method is not very suitable for whole-cell immobilization of microalgae.Entrapment is a promising way to preserve biosystems [ 27 , 30 ] that is usually based on the soft synthesis of inorganic materials with biocompatible precursors.The advantages of cell entrapment include an easy synthesis process, tunable porosity and good mechanical strength and stability (Fig. 2 ).For example, the generated porous structures produced by the sol-gel method can provide entrapped cells with an aqueous environment and physiological stability.However, the disadvantages of cell entrapment are also obvious, such as the limitation of cell proliferation and scale application.General ly, cel l entrapment is the most appropriate immobilization method for enhancing the stability of microalgae but functional modification of microalgae by materials is limited.
The selection of the material is key to the immobilization process, as it affects the viability and functionality of microalgae.First of all, the biocompatibility of the material is the primary consideration, which means that the material should provide an inert environment for the immobilized cells to keep the biological activity of the enzymes.Second, the cost is also important for material selection, for it directly affects application prospects.Currently, three types of materials are available for efficient immobilization, including inorganic materials, organic materials and composite materials [ 24 ].Inorganic materials include sol-gel-based materials, carbon materials, and oxide and metallic nanomaterials [ 21 , 31 , 32 ], while organic materials involve natural materials (chitin, agar, alginate, etc.) and Achieving the optimal outcome relies on the rational matching of material properties and microalgal functions.Furthermore, different levels of microalgae-material hybridization may be necessary, depending on the desired outcome.
synthetic materials ( poly vinyl alcohol, polypropylene ammonium, polyurethane, etc.) [ 33 ].The composite materials represent a combination of organic and inorganic materials, which provides the integration of the two types of performance, implying the superiority of composite materials [ 26 ].According to different properties, polymers, carbons, oxide and metallic nanomaterials, and composite materials are usually used in adsorption and covalent binding [ 26 , 33 ], while sol-gel-based materials are mainly used in cell entrapment [ 21 , 26 ].

Unicellular shellization
Unicellular shellization is an emerging technique of cell-surface engineering [ 34 , 35 ] that emphasizes the protection of living cells against external stresses in vivo .Compared with cel l immobi lization, unicel lular shellization allows cell density control and cell behavior monitoring [ 34 , 35 ].Inspired by biomineralization, cell-in-shell structures are formed through biomimetic mineralization on the cell surface, which can suppress or retardate cell growth and division, but enhance cell survival under harsh conditions.Such a hybrid method facilitates light penetration and mass transfer from the external environment to the enclosed cells, which is suitable for photosynthetic microorganisms.In nature, most microalgae exist as single cells, so unicellular shellization can offer a new approach to improving microalgae.However, it is a great challenge to confer microalgal cells with artificial shells due to the lack of 'mineralization sites' .
In order to construct artificial shells for microalgae, mineralization-related molecules have to be introduced onto the cell surface.Layer-by-layer (LbL) assembly is an efficient method for cell-surface modification, which can form thin films on the cell surface by depositing alternating layers of oppositely charged or molecular interactive materials [ 36 -38 ].The microalgal cell surface is usually negatively charged, so positive polyelectrolytes can be absorbed onto the cell surface to form an electropositive layer (Fig. 3 ).In the assembled layers, the polyelectrolytes act as 'branch' that not only connects the underlying oppositely charged polyelectrolytes, but also increases the functional groups on the cell surface.Inspired by diatoms, an artificial biosi lica shel l was first introduced onto a cyanobacterial surface with the help of poly(diallyldimethylammonium chloride) (PDADMAC-a cationic polyelectrolyte with positively charged quaternary amines) and poly(styrene sulfonate) (PSS-a biocompatible polyelectrolyte with a negative charge) [ 39 ].PDADMAC was first introduced onto the cyanobacterial surface [ 39 ] then PSS was absorbed onto the PDADMAC layer to form an electronegative surface layer.By alternating the two polyelectrolytes with several cycles, a nanofilm with abundant quaternary amines can be formed on the cyanobacterial cell surface and the exposed PDADMAC acted as a catalytic template to induce the silicification on the cell surface.Besides, other functionalized silica shells can also be conferred on a microalgae cell with the help of a modified LbL technique, such as Chlorella with a SiO 2 /TiO 2 shell [ 40 ] and cyanobacteria with a mesoporous silica shell [ 41 ].
Currently, LbL-assisted si lica shel lization is one of the most frequently used material technologies to improve microalgae at the single-cell level (Fig. 3 ).However, excess polyelectrolytes during each polyelectrolyte deposition treatment need to be removed by centrifugation and separation, which are complicated and time-consuming.Moreover, the treatment processes may induce unexpected cell injuries.Therefore, LbL is commonly suitable for robust cells with rigid cell-wall structures such as cyanobacteria and Chlorella , but may frequently cause damage to the cell membrane of green algae without cell walls, which are sensitive to chemical and environmental changes.Direct adsorption of nanoparticles is another tactic to improve microalgae for shell formations (Fig. 3 ), such as cerium dioxide nanoshell for Chlorella [ 42 ], which is based on electrostatic interaction.The electrostatic interaction mainly depends on the zeta potentials of both the nanoparticle and the cell surface.As the surface of microalgae cells is usually negatively charged, direct adsorption usually needs positively charged nanoparticles.General ly, unicel lular shel lization is an approach to making microalgae more robust but current methods cannot be applied on a large scale.

Multicellular aggregation
Multicellularity was a transformative evolution for life on Earth that greatly increased biological complexity through forming new biological structures [ 43 , 44 ].For example, multicellular organisms have evolved sophisticated functionality via cell cooperation with complementary behaviors [ 44 ].The multicellularity of cyanobacteria evolved nitrogen fixation function [ 45 ], while the multicellularity of green algae eventually led to the emergence of plants [ 46 , 47 ].Therefore, multicellular transition is a promising approach to trigger new functions of microalgae.There are essentially two ways to make a simple multicellular entity out of a single cell: (i) a single cell divides and its offspring stick together; (ii) several solitary cells aggregate to form a colony (Fig. 4 a) [ 44 , 48 ].In nature, some environmental stresses can induce many unicellular microalgal cells to form clusters (Fig. 4 a) [ 49 ].For example, predation by a small-mouthed ciliate can result in the generation of cellular clusters of the previously single-celled Chlorella [ 50 ].Currently, most approaches to experimental transitions of multicellularity focus on the regulation of ecological conditions that would favor the formation of cellular clusters [ 51 ].These approaches to form cellular clusters are induced by environmental stresses and controlled by genes, but they lack universality, are always time-consuming and are usually based on cell lineages with established multicellular developmental programs or lack of separation following a cell divi-sion [ 52 , 53 ].Hence artificially induced multicellular aggregation is desired for microalgae functionalization.
Artificially induced multicellular aggregation can be achieved by cell flocculation and mineralizationinduced cell aggregation.Flocculation is a common method to form larger aggregates from single cells.Approaches for microalgal flocculation range from chemical flocculation to biological flocculation and particle-based flocculation [ 54 ].Chemical flocculation results in toxicity to microalgae, although the use of natural polymeric flocculants may minimize this problem [ 55 ].Biological flocculation using fungi or bacteria holds huge potential in microalgal wastewater treatment because wastewater usually contains a carbon source necessary for the flocculating microorganisms [ 54 ].However, the operation cost needs to be reduced for scalability.Particle-based flocculation using aminoclay-based nanoparticles, magnetic particles or multifunctional particles can overcome some shortcomings of chemical flocculation such as bio-toxicity and difficulty in chemical recycling, but the manufacturing costs of particles could be a major limiting factor for scalable application [ 54 ].All the above approaches of flocculation-induced cell aggregation are based on electrostatic interaction (Fig. 4 b) but cannot regulate the structure of cell aggregates.Flocculation has been widely used as a harvesting method for microalgae biomass but the effect of flocculation on the physiological function of microalgae was rarely concerned about.Fortunately, we found that biomimetic silicification can induce cell aggregation of Chlorella [ 22 ].The silicification-induced cell aggregation also needs PDADMAC to endow the cell surface with silicification sites following the transformation of the cell-surface potential.Silica can spontaneously cohere PDADMAC-coated Chlorella cells together when silicification occurs.Compared with flocculation, silicification-induced cell aggregation can form more compact and stable MMH and may cause more cell damage.Mineralizationinduced multicellular aggregation is becoming an emerging method for cell functional transformation, with broad application prospects to explore.Anyway, material-induced multicellular aggregation offers a new pathway to improve microalgae.

Improving photosynthetic CO 2 fixation
Microalgal photosynthesis has attracted great attention due to its potential role in renewable energy development [ 56 ].The photosynthetic energy  conversion of microalgae could be exploited to reduce CO 2 emissions, synthesize valuable molecules, decompose toxic chemicals and extract heavy metals [ 1 , 57 , 58 ].For example, an estimated 25 Giga tons per year of carbon from CO 2 can be fixed into biomass by cyanobacteria on a global scale [ 59 ].However, the utilization of microalgal photosynthetic CO 2 fixation is limited by cell stability and reusability.In order to develop a photosynthetic biosystem suitable for CO 2 fixation assimilation, Su and his co-workers first entrapped cyanobacterial cells into a porous silica network to enhance the sustainability of the cells (Fig. 5 a) [ 60 ].The immobilization step could be achieved via the acidification of aqueous colloidal silica precursors [ 60 ].The immobilization of cyanobacteria created large voids around the cell (Fig. 5 a) [ 60 ], which could provide space for cell division.Thus, the macrovoids within the mesoporous material would be an ideal microenvironment for cell preservation.The immobilized cyanobacterial cells continued to autofluoresce even after 12 weeks [ 60 ], indicating the extended stabi lity of chlorophyl l a within the immobilized cells and the ability of photosynthesis.Further, the uptake of radio-labeled NaH 14 CO 3 indicated that the cyanobacterial cells fixed CO 2 within the silica gel continuously [ 60 ].Cyanobacterial immobilization with silica gel was a pioneering attempt in using microalgae-material hybrids for photosynthetic energy conversion, which opened a window for improving photosynthetic CO 2 fixation.It should be emphasized that cel l immobi lization with porous silica gel is a feasible method for cell storage of microalgae due to the cytocompatibility and transparency of silica material, which mainly im-proves photosynthetic CO 2 fixation by prolonging cell stability and life.However, the photosynthetic activities and proliferation capacity are inevitably limited compared with the native state.
Currently, unicellular shellization is considered as an efficient way to improve cell activity and function for enhanced ability to adapt to the environment.Inspired by diatoms, we developed a modified LbL method to deposit PDADMAC with the help of PSS [ 39 ] (Fig. 5 b) and the exposed quaternary amines acted as catalytic sites to induce in situ silica shellization (Fig. 5 b).The thickness of the si lica shel l layer was ∼30 nm and the thickness of the shell can be regulated by the number of multilayers [ 39 ].The silica shell can reduce visible light transmission to alleviate high-light stress on the cell [ 39 ].Specifical ly, the effect of al leviation in si licified cyanobacteria (cyanobacteria@SiO 2 ) gradually enhanced with the increase in light intensity, especially under high-light conditions (Fig. 5 c).For example, cyanobacteria@SiO 2 retained ≥50% of the photosynthetic oxygen evolution capacity after exposure to an intense light intensity of 500 μE m −2 s −1 for 2 h (Fig. 5 c).Further, silica-coated cyanobacteria show lower respiratory rates than native cyanobacteria [ 39 ].Therefore, such a silica-based improvement indicates the potential of greatly enhancing the photosynthetic efficienc y of c yanobacteria under highlight conditions, which promotes the photosynthetic production of biomass.Unicel lular si lica shel lization can enhance environmental stability without restricting photosynthesis.However, the LbL-based cell shellization requires multiple steps of centrifugation, which are time-consuming and harmful to cell activity.Further, the interface between the coating materials and cel ls usual ly has an adverse effect on the cell stability.
In order to enhance both the activity and the stability of the entrapped cells, Su and co-workers constructed a mesoporous silica shell for the cyanobacterial cell using the self-assembled Au@L-cysteine biohybrid layer and silica precursors [ 61 ] (Fig. 5 d).The worm-like mesoporous silica shell possesses a high surface area with good mass-diffusion ability due to its nanosized channel structure.Hence, the nanoporous structure can allow the transfer of nutrients and chemicals into cells and the bilayer shell can block the excessive light that may cause cell damage [ 61 ].The L-cysteine molecules in the biointerface can buffer the extracellular microenvironment when the acid/base chemical environment is abruptly changed.Cyanobacteria with the mesoporous bilayer nanoshell performed with higher activity and stability than si lica-immobi lized cyanobacteria.For example, at Day 20, the cells within the mesoporous bi layer shel ls presented 186% ( ±17%) of the initial activity and the viabilities of the shell-encapsulated cells were well preserved for ≥50 days under highlight conditions [ 61 ].This work offers a strategy that can construct a multifunctional nanoshell on a cyanobacterial cell surface with high activity and stability.Based on the strategy, Su and co-workers further develop ordered packing of silica nanocolloids for single-cell encapsulation of cyanobacteria [ 41 ].A space between the capsule shell and the cell was created by the internalization of protamine, resulting in a highly ordered porous yol k-shel l structure (Fig. 5 e and f).The yol k-shel l nanostructure leads to a high cel l viabi lity of > 90% and the rate of photosynthetic oxygen evolution of the yol k-shel l-encapsulated cyanobacteria is 3-to 4-fold higher than the cellcontacting shell-encapsulated cyanobacteria under a large range of light intensities (0-500 μE m −2 s −1 ) [ 41 ].In addition, the ordered yol k-shel l structure can offer the cyanobacterial cell superior protection against harsh external stress, such as resis-tance against high temperatures.Although the yolkshell structure was constructed on a cyanobacterial cell, the strategy could potentially be further used in other aspects of biotechnology, such as biosensors, biocatalysis and controlled delivery therapeutics.The studies of Su and co-workers show that the evolution of material technology can enhance the functionalities of microalgae-material hybrids due to the structure and properties of the materials.Ultraviolet radiation (UV) is also a significant inhibitory condition for microalgae in the sunlight, the biological effects of which mainly involve the formation of reactive oxygen species (ROS) [ 62 ].CeO 2 nanoparticle-loaded silica hydrogels have been used as UV protective matrices for longterm cell entrapment of Chlorella vulgaris [ 63 ].Further, a one-step approach was developed to construct a CeO 2 nanoshell for Chlorella cells by the direct adsorption of CeO 2 nanoparticles [ 42 ], which can efficiently protect the enclosed Chlorella cell from UV-induced oxidative stress [ 42 ].In addition, an SiO 2 -TiO 2 composite shell can also improve photosynthetic activ ity by reliev ing heat stress [ 40 ].
Currently, MMHs improve photosynthetic carbon fixation mainly by enhancing resistance to environmental stresses.Silica is the most commonly used material due to superior biocompatibility and transparency.However, whether the silica-based cell immobilization technique is applicable for all types of microalgae sti l l needs more cases to verify.It should be noted that silica-based cel l immobi lization or shellization can enhance the activity and stability of microalgae but it is hard to endow microalgae with functions that they do not have.A microalgae-silica hybrid can extend the living environment of algae from lakes and oceans to land, and has great potential to be applied to desertification control and outer wilds in the future.Although the existing construction methods are not suitable for large-scale applications in pursuit of carbon neutrality, the microalgaematerial interface technology may play an important role on some special occasions when cost is not the main consideration.

Inducing photosynthetic hydrogen production
In nature, photosynthetic microalgae can photolyse water to produce H 2 and O 2 using photosynthetic machinery coupled with hydrogenase [ 64 ], which is renewable and carbon-neutral.However, this is a transient process because the oxygen generated by the photosystem quickly inactivates hydrogenase [ 64 ].Currently, suppression of O 2 production and timely removal of O 2 are the main strategies to generate an anaerobic environment for photosynthetic H 2 production.Sulfur deprivation is the most commonly used method to suppress O 2 production of green algae [ 65 ] but it would simultaneously suppress photosystem II (PSII) activity and gradually terminate subsequent H 2 production.To decrease intrinsic hydrogenase oxygen sensitivities, scientists have also tried to construct mutants with improved oxygen tolerances [ 64 ] but limited progress has been made.So far, large-scale application of photosynthetic H 2 production is sti l l not realistic.
At present, the research on microalgae photosynthetic hydrogen production mainly focuses on the single-cell level.However, many biological processes and functions in nature involve multicellular aggregation.Aggregation can affect the solute flow and communication between cells, and thus lead to the production of biological characteristics that are different from those of a single cell [ 66 ].It was found that silicification-induced Chlorella ag-gregates could achieve sustainable photosynthetic hydrogen production under natural aerobic conditions for 60 h [ 22 ].The sizes of the Chlorella aggregate could be controlled in the range of 10-500 μm by regulating the cel l density in the si licification medium and the H 2 production rate of the 100-μm aggregates can reach ∼0.35 μmol H 2 h −1 (mg chlorophyll) −1 (Fig. 6 f) [ 22 ].The aggregate can be considered to be an artificially induced multicellular organism with differentiated cell functions (Fig. 6 a) [ 22 ].Here, H 2 and O 2 microsensors were used to examine the interior environment in the aggregates and the profiles indicated that H 2 was evolved by the inner cells rather than the surface cells (Fig. 6 b) [ 22 ].The surface cells retained the native photosy nthetic f unctions and acted as autogenous shells to prevent the penetration of light into the aggregate core, while the inner cells consumed diffused O 2 and photosynthesis-evolved O 2 through respiration, thus creating a hy pox ic domain in the core.The anaerobic condition was mainly a result of the dynamic balance between photosynthetic oxygen evolution and cellular respiration.The PSII activity and hydrogenase activity could be simultaneously maintained in this domain, which guaranteed sustainable photosynthetic H 2 production.Our study provides a novel approach to suppressing O 2 production for sustained photobiological H 2 production.More general ly, a simi lar cel l-material hybrid may be used in other microorganisms for inducing functional transformation.
Blocking O 2 production from PSII wi l l restrict photosynthetic electron sources and ultimately limit photosynthetic H 2 production.Therefore, the timely removal of O 2 without damaging PSII may be a more promising strategy for inducing photosynthetic H 2 production.Recently, enzyme-based oxygen-consuming reactions have been applied in inducing photosynthetic H 2 production through MMH.Huang and co-workers proposed an O 2 timely removal strategy by creating a laccase-modulated oxygen-consuming layer around Chlorella cells for photosynthetic H 2 production (Fig. 6 c) [ 67 ].The laccase-based oxygen-consuming layer was sandwiched between the polydopamine (PDA) layer and the tannic acid (TA) layer, which created an anaerobic balance between photosynthetic O 2 evolution and respiratory O 2 consumption; then the hydrogenase was activated, resulting in the switching from photosynthetic O 2 evolution towards H 2 production.In the presence of TA, the laccase-catalysed oxygen-consuming reaction enabled the formation of an anaerobic microenvironment around the individual Chlorella cell to trigger H 2 production under bulk aerobic conditions.By controlling the amount of TA in the system, the H 2 production could be turned on and off on demand.Significantly, the laccase-PDAcoated Chlorella showed little decrease on the PSII activity and continuously produced H 2 at an average rate of 0.32 μmol H 2 h −1 (mg chlorophyll) −1 for 7 d (Fig. 6 f) [ 67 ].
In addition to laccase, glucose oxidase could also used for timely removal of O 2 but the oxygenconsuming reaction catalysed by glucose oxidase could cause an acidic environment, which leads to hydrogenase inactivation and even cell inactivation.To solve this problem, Fan and co-workers constructed a chemoenzymatic cascade (CEC) system for creating an anaerobic and constant near-neutral environment for sustainable hydrogen production of Chlamydomonas reinhardtii flocculation (Fig. 6 d) [ 68 ].The C. reinhardtii could stably produce hydrogen in the CEC system for 26 days, with an average rate of 0.44 μmol H 2 h −1 (mg chlorophyll) −1 (Fig. 6 f) [ 68 ].In the CEC system, four components were employed in creating the appropriate anaerobic environment: glucose oxidase (GOx), catalase (CAT), glucose and Mg(OH) 2 .The photosynthetic O 2 can react with glucose under the catalysis of GOx/CAT.The intermediate gluconic acid may cause a decrease in pH and suppress the ac-tivity of C. reinhardtii .To maintain the near-neutral environment, the Mg(OH) 2 played an important role in neutralizing the gluconic acid.In addition, Mg(OH) 2 was also used as a flocculant for C. reinhardtii aggregation, which was helpful for creating an anaerobic environment.In this study, CEC played a significant role in quickly removing O 2 , whi le cel l flocculation made a contribution to suppressing O 2 production.Due to the synergistic effects of CEC and flocculation, an anaerobic environment for stable photobiological H 2 production of ≤26 days was achieved (Fig. 6 d and f) [ 68 ].The CEC system helped to maintain the appropriate anaerobic environment without damaging microalgae activity or reducing chlorophyll content, which is key to sustainable photosynthetic H 2 production.This study opens a new avenue to sustainable photosynthetic hydrogen production through rapidly removing O 2 coupled with suppression of O 2 production.
Enzyme-based oxygen-consuming reactions not only require harsh reaction conditions, but also have high economic costs.Therefore, the enzyme-based oxygen-consuming reaction may not be suitable for commercial application in inducing photobiological hydrogen production.Apart from oxidases, some bacteria can also consume oxygen through their respiration.Based on bacteria respiration, dextran-in-PEG emulsion micro-droplets-based algal/bacterial cell communities were developed for photosynthetic hydrogen production in daylight under air (Fig. 6 e) [ 69 ].The localized hypoxia induced hydrogen production in the core of the algal/bacterial microdroplet and demonstrated that photosynthetic hydrogen production can be promoted by enclosing the algal cells within a shell of aerobic bacterial cells.In this way, hy pox ic photosynthesis and aerobic respiration were coupled synergistically to produce hydrogen from the spatially organized algal/bacterial community.The Chlorella / Escherichia coli hybrid spheroids continuously produced H 2 at an average rate of 0.44 μmol H 2 h −1 (mg chlorophyll) −1 for 8 days (Fig. 6 f) [ 69 ].The similarity between the above work is combining microalgae with oxygen-consuming living materials to create an anaerobic environment, which offers a new material strategy for inducing microalgae photosynthetic H 2 production.
MMHs have been an emerging strategy for inducing microalgae photosynthetic H 2 production under natural aerobic conditions, although current methods focus on reducing oxygen content in the microenvironment.With the progress of material technology, hydrogen production rates and the duration of MMHs have been both increasing (Fig. 6 f).We firmly believe that microalgal aggregates have enormous potential in overcoming the bottleneck of sustainable photosynthetic hydrogen production in the future.Although the present approaches are far from practical application, MMH technology offers endless possibilities for improving photosynthetic H 2 production either from the perspective of microalgae or materials, or from the perspective of microalgaematerial interactions.

Augmenting bioelectrochemical energy conversion
From the perspective of energy conversion, the essence of photosynthesis is the conversion of light energy into chemical energy through electrical energy.In photosynthesis, PSII harvests light energy and splits water to generate electrons, then the photosynthetic electrons transfer into high-energy compounds through the photosynthetic electron transport chain (PETC).Therefore, the process of photosynthesis is accompanied by biological currents.However, the bioelectricity only exists inside cyanobacterial cells or chloroplasts.The photosynthetic electrons must be transferred across the cytoplasm to reach the cytoplasmic membrane and exported to external electrodes if the intracellular bioelectricity is to be harnessed.PSII-based electrodes have been used for achieving photocurrents, but PSII is very sensitive to light and can selfrepair every ∼15 min in vivo .In contrast, biophotovoltaic systems (BPVs), which are based on microalgae-electrode hybrids [ 70 ], offer bioelectrochemical energy-conversion technology with enhanced stability and longevity.
The BPV device is composed of an anode and a cathode, like an ordinary battery.Bioelectrochemical energy conversion in BPVs includes three basic steps, which are photosynthetic electron generation, transport of electron to the anode and cathodic reduction of oxygen (Fig. 7 a) [ 70 ].In the anodic side, the oxidation reaction occurs via a photosynthetic process: photosynthetic electrons transfer from the thylakoid membrane to the cytoplasmic membrane, then transfer from the cytoplasmic membrane to the anode electrode [ 70 ].In the cathode side, the reduction reaction is performed by using photosynthetic oxygen as an electron acceptor [ 70 ].The overall activity of BPVs depends on the redox reactions, so the transfer of electrons from microalgae to the anode in BPVs is the key to bioelectrochemical energy conversion.The main principle is based on the electron transfer from anode to cathode through an external circuit, whereas protons diffuse through the cytoplasmic membrane to reduce oxygen at the cathode (Fig. 7 a) [ 70 ].As cyanobacteria are prokaryotes, the photosynthetic electrons cross fewer barriers to reach the cell membrane surface in a cyanobacteria cell than in an eukaryotic algae cell, so cyanobacteria are more commonly used in BPV research.
The electron transportation in BPVs is an extracellular process in which electrons are usually transferred to the anode directly v ia c ytochromes located in the cell membranes (Fig. 7 b) [ 70 ].There are three electron-transfer mechanisms in BPVs: (i) direct electron transfer through pili, (ii) indirect electron transfer through electron shuttling, (iii) direct contact and electron transfer through surface redox proteins (Fig. 7 b).Some cyanobacteria species have pili that can directly transfer electrons to the external anode [ 71 ].In BPVs, pili can act as 'nanowires' to transfer electrons between the cell membrane and the anode surface (Fig. 7 b) [ 71 ].For example, the Synechocystis sp.PCC 6803 cells with pili are better wired to the electrode [ 72 -75 ].Pili mainly control cell communication and phototaxis in cyanobacteria and also can be used as conductive wires [ 71 ].Electron shuttling is another electron-transfer mechanism that involves cyclic diffusion of the soluble mediator into the microalgal cell after oxidizing at the anode (Fig. 7 b) [ 76 ].In addition, direct electron transfer on a cell-material interface can also occur through direct contact between the cell and the anode with the help of redox proteins on the cell surface (typically c-type cytochromes) [ 77 ].
The bottleneck in BPVs is electron-transfer efficiency from the photosystems to the anode across the cytoplasmic membrane and periplasmic membrane [ 70 ].Currently, there are three main approaches to improving the electron-transfer efficiency: (i) modification of the photoanode structure, (ii) regulation of the photoanode-microalgae integration and (iii) modification of the microalgae [ 70 , 78 ].Photoanodes have undergone evolution from flat structures to nanoporous structures and then to microporous structures.It was demonstrated that porous structures offered a large electroactive surface area with aided biocatalyst penetration [ 79 , 80 ], although porous structures were not conducive to mutual contact between microalgae and electrodes.The state-of-the-art photoelectrochemical electrode architecture is an inverse opal (IO) porous structure made from indium tin oxide (ITO) [ 79 , 81 ] because ITO material can exhibit appropriate inertness, excellent conductivity, light scatterability and biocompatibility [ 79 ].Recently, an aerosol jet printing method for constructing hierarchical pi l lar array electrode structures using ITO nanoparticles was developed for semi-artificial photosynthesis in cyanobacteria-based BPVs (Fig. 7 c  and d) [ 75 ].The micropi l lar array electrodes with microbranches were favorable for cyanobacteria loading, light utilization and electron flux output, so they almost doubled the photocurrent of the stateof-the-art porous structures with the same height [ 75 ].When the heights of the micropi l lars increased to 600 μm, the mediated photocurrent outputs could reach 245 μA cm -2 (the closest to theoretical predictions) with an external quantum efficiency of ≤29% [ 75 ].Apart from electrode structures, biofilm structures also affect microalgae-electrode interactions.For example, digitally printed cyanobacteria thin films were also developed for generating a sustained electrical current that was observed to be stable during a 10 h light/14 h dark cycle for 4 days [ 74 ].Further, synthetic biology approaches can not only modify microalgae surface, but also promote photosynthetic electron generation, which enhances bioelectricity from the source [ 78 ].In order to investigate cell-electrode interaction, a host of techniques based on the simpler platforms (electrochemical impedance spectroscopy, confocal fluorescence microscopy, surface-enhanced Raman/infrared spectroscopy, transient absorption spectroscopy and nanoelectrodes) have been established that can prov ide a wealth of underly ing information for BPV systems [ 82 , 83 ].
Although BPV is a promising semi-artificial system for bioelectricity and fuel generation, it is not practical to generate electricity on a large scale in the short term due to its high cost and low efficiency.However, it may be easier for microalgaeelectrode hybrids to be applied by introducing external electrons into the PETC for selective synthesis of high-energy compounds.Recently, electrochemically driven photosynthetic electron transport in cyanobacteria lacking PSII has been demonstrated [ 84 ] and an integrated electron-transfer strategy that converts external electrical energy into biochemical energy has also been developed [ 85 ].The integrated photoelectrochemical architecture shuttles electrons directly to the PETC in liv ing c yanobacteria and drives CO 2 fixation with high energy efficiency (Fig. 7 e) [ 85 ].The cathode interfaces with PETC-modified cyanobacterial cells that lack photosystem II (PSII) activity and cannot perform photosynthesis alone [ 85 ].Illumination of the cathode transfers electrons from an external circuit to the intracellular PETC through PS I, ultimately driving the conversion of CO 2 into acetate in cyanobacteria (Fig. 7 f) [ 85 ].The energy-conversion efficiency for acetate production reached ∼9% under exogenous electron supply and programmed intermittent LED i l lumination (40 0-70 0 nm) when the numbers of photons and electrons received by the cyanobacteria were both taken into account [ 85 ].This approach is applicable for CO 2 reduction by using engineered cyanobacteria and has the potential to produce hydrocarbon fuels from the Sun and CO 2 .
The microalgae-electrode hybrid offers the prospect of financially feasible light energy captureand-conversion technology, along with the added benefits of carbon neutrality.In the future, BPV technology can be applied in environmental bioenergy extraction, landscape architecture power generation, micro-battery chips and so on.The development of MMH technology wi l l improve the efficiency of biophotovoltaic power generation and photoelectrochemical biosynthesis, and provide a carbon-neutral approach for primary energy utilization and secondary energy production.

Boosting biochemical energy conversion
From the perspective of energy metabolism, pathological cells are caused by energy deficiency, energy transmission obstacles and energy metabolic disorders [ 86 , 87 ].Take tumor cells, for instance-the tumor microenvironment is not only anaerobic, but also rich in free radicals.Microalgae photosynthesis can not only evolve a large amount of oxygen to relieve tissue hy pox ia, but also produce hydrogen to scavenge free radicals.Therefore, we propose that microalgae can transfer light energy to tumor cells in the form of chemical energy, which wi l l have a therapeutic effect in tumor therapy (Fig. 8 a).Due to the existence of an immune mechanism in the human body, the direct delivery of microalgae cells into tissues wi l l cause rejection and cellular damage, so it is necessary to camouflage their cell surfaces.Inspired by the basics of the Trojan Horse, researchers consider that MMH may offer a promising strategy of engineering microalgae for alleviating immunoreactions in cross-species energy conversion.
Local hy pox ia in solid tumors is considered a major cause of intolerance to many conventional cancer therapies, particularly for photodynamic therapy (PDT) and radiotherapy (RT), in which oxygen is involved in the process of cel l ki l ling [ 88 , 89 ].Relieving and overcoming tumor hy pox ia could substantially enhance PDT/RT efficacy.Thus, reoxygenation of hy pox ic tumors has been an effective approach to overcoming hy pox ia-based resistance to some conventional cancer therapies [ 90 , 91 ].Although various nanocarriers have been investigated for in situ oxygenic production in tumors, these approaches have been limited due to the serve toxicity of the chemical compositions and immunological reactions in biological systems [ 92 , 93 ].Due to effective photo-controlled oxygen evolution and good biological safety, microalgae have attracted great interest in the in vivo treatment of multiple diseases [ 94 -99 ].
Currently, cyanobacteria and Chlorella are most commonly used in enhancing PDT/RT efficacy.Cyanobacteria coated with upconversion nanoparticles (UCNPs) were constructed for improving the efficiency of PDT for hy pox ic tumors (Fig. 8 b) [ 95 ].In PDT, chlorin e6 (Ce6) is a widely used photosensitizer [ 95 ] but it requires an excitation wavelength of 660 nm, which is ineffective for tumor penetration.When irradiated with excitation light at 980 nm, the UCNPs on the cyanobacterial cell surface convert near-infrared (NIR) light into visible light, which can be absorbed by cyanobacteria (40 0-70 0 nm) and Ce6 (660 nm) [ 95 ].Thereby, the Ce6 can use photosynthetic oxygen evolved by cyanobacteria to produce singlet oxygen for PDT [ 95 ].The survival rate for the Reproduced with permission from [ 97 ].Copyright 2020 American Chemical Society.(d) Schematic illustration of microalgae-hydrogel patch for chronic wound healing by light response dissolved oxygen release [ 25 ].(e) Schematic illustration of the algae −bacteria gel patch for producing H 2 to modulate oxidative stress in diabetic wound healing.Reproduced with permission from [ 24 ].Copyright 2022 American Chemical Society.
tumor-bearing mice with cyanobacteria-UCNP-Ce6 hybrid under NIR light reached 100%, which demonstrated that the c yanobc yeria-UCNP-Ce6 had an excellent antitumor effect under NIR light irradiation [ 95 ].Due to the excellent photosynthetic oxygen-release capacity and abundant chlorophyll content, the living Chlorella cells were also used to produce oxygen in the tumor site to promote the radiation responses of the tumor [ 96 -99 ].The Chlorella@ SiO 2 hybrid was synthesized by using a one-step biomimetic silicification method that could significantly enhance the cytotoxicity and tolerance of the Chlorella cells, improving their living activ-ity in the tumor area (Fig. 8 c) [ 97 ].The obtained Chlorella @SiO 2 was coated by a silica shell with a thickness of 10 0 −20 0 nm [ 97 ].In situ oxygen generation in tumor sites by Chlorella @SiO 2 photosynthesis could relieve tumor hy pox ia, resulting in the enhanced efficiency of radiation therapy [ 97 ].As a natural photosensitizer, the released chlorophyll from Chlorella @SiO 2 could produce ROS to ki l l the tumor cells for the cascaded PDT (Fig. 8 c).For Chlorella @SiO 2 -mediated RT combined with PDT, the tumor volume was reduced by > 90% in the mice, which demonstrated the promising antitumor effect of Chlorella [ 97 ].In addition, the Chlorella cell also can be engineered with a calcium phosphate (CaP) shell to greatly regulate the hy pox ia milieu in tumor tissues [ 98 ].The CaP shell on the surface of Chlorella can provide an immune shield in a hostile microenvironment, leading to oxygen generated in situ and ROS for alleviating the hy pox ia condition of tumor tissues and ki l ling cancer cells [ 98 ].Although synthetic materials can provide camouflage encapsulation for microalgae, they can also cause body rejection.In order to reduce the body rejection, the red blood cell membrane (RBCM) was used to modify the surface of the Chlorella [ 99 ].The RBCMcoated Chlorella can efficiently generate O 2 in situ with their natural photosynthetic system, which can alleviate the tumor hy pox ia condition of tumor tissues [ 99 ].The production of ROS from RBCMcoated Chlorella through laser irradiation can not only enhance the therapeutic effect of RT, but also improve the ki l ling of cancer cells in PDT [ 99 ].In the above studies, the MMH only provided auxiliary enhancement for tumor therapy, but could not directly treat the tumor.Besides, how the MMH gets out of the body is also a problem for clinical safety.Apart from tumors, chronic wounds in diabetes also encounter a hy pox ia microenvironment, which may result in damaged neovascularization [ 25 ].How to relieve hy pox ia in chronic wounds is a key clinical problem in diabetic disease treatment.However, traditional methods such as hyperbaric oxygen inhalation and topical gaseous oxygen therapy are limited in clinics owing to insufficient oxygen in the wound site.Therefore, microalgae-based in situ oxygen therapy was proposed as a new effective approach [ 24 , 25 ].For instance, a patch-like wound dressing made of gel beads containing active cyanobacteria could be utilized for tissue repair in diabetic chronic wounds [ 25 ] (Fig. 8 d).A skin flap of a diabetic mouse with the treatment of an alga-gel patch was completely healed in 6 days [ 25 ].The cyanobacteria in a wound dressing can provide topical dissolved oxygen to human skin and growth factors, which can enhance wound oxygenation, fibroblast proliferation and angiogenesis to improve the healing effect [ 25 ].In addition, excessive ROS produced by hyperglycemia or chronic inflammation can also limit diabetic wound healing [ 100 ].H 2 is a widely used antioxidant for scavenging ROS [ 100 , 101 ], so microalgae-based in situ hydrogen therapy may be a feasible approach to promoting diabetic wound healing.Recently, Wu and co-workers developed a H 2 -producing hydrogel made of symbiotic Chlorella -Bacillus , which can selectively reduce toxic r OH and ONOO − species and relieve inflammation, thereby promoting the healing of chronic wounds by almost 50% at Day 3 [ 24 ] (Fig. 8 e).Moreover, a bioactive cyanobacteria hydrogel made by loading berberine (BBR, a quorum sensing inhibitor and antibacterial agent) can also promote infectedwound healing in diabetic mice through interrupting the quorum sensing of bacteria, relieving hy pox ia and destroying biofilms [ 102 ].Considering that the living microalgal gel could be used in other treatments, such as cytokine therapy and non-invasive stem cell therapy, the microalgal wound dressing may also be used in translational and regenerative medicine, such as tissue and cell regeneration.
Generally, both tumor and inflammatory cells are energy-intensive, while microalgal cells are energy-producing.The MMH can convert highquality light energy into low-quality chemical energy through photosynthesis, and the chemical energy can spontaneously convert into bioenergy in tumors and inflammatory tissue.Therefore, the MMH for biochemical energy conversion offers a new strategy for biomedical therapy that is based on energy conversion.It should also be noted that current research mostly involves the improvement of existing therapies through MMHs, but they are not yet able to fundamentally transform diseased tissue.Recently, Tang and co-workers developed a therapeutic strategy for degenerative diseases based on nanothylakoid units (NTUs), which can enter chondrocytes and enhance cell anabolism by providing ATP and NADPH independently [ 103 ].To enable crossspecies, a specific mature cell membrane (the chondrocyte membrane (CM)) was used for camouflage encapsulation, which can overcome the pre-existing immune mechanism in the human body.Moreover, the CM-NTUs can protect against pathological progression of osteoarthritis by systemically correcting energy imbalance and restoring cellular metabolism.What is distinctive in the study is that the therapeutic strategy for degenerative diseases is based on the natural photosynthetic system that can enter the disease cell as a transplanted organel le, whi le the other studies are mainly based on tissue microenvironment regulation by microalgae.This study further provides an enhanced understanding of the crossspecies photosynthetic energy conversion for the treatment of disease, which may boost the application of MMHs in biotherapy.However, the biosafety and biocompatibility of microalgae in human bodies should be solved before the ultimate clinical application of microalgae-based in situ therapy.

CHEMICAL MECHANISM OF MICROALGAE-MATERIAL INTERACTION
There are two levels of meaning in microalgaematerial interaction: one is the material-induced construction of MMHs and the other is the material-endowed improvement of microalgal

Cell
H + Figure 9. Schematic illustration of chemical mechanisms for microalgae-material interaction.Materials combine with microalgae through intermolecular forces, covalent bonds or coordinate bonds, which interfere with the exchange of matter and energy between the microalgae and the extracellular environment.The microalgae-material interface affects the photosynthetic electron transport chain through three principal pathways: (i) material hinders light energy from entering the photosystems to produce photosynthetic electrons, (ii) extracellular electrons enter the cell and are involved in photosynthetic electron transport, (iii) extracts electrons from the inner surface of the cell.
function.From a perspective of the construction of MMHs, the chemical mechanism is microalgae combined with material through intermolecular forces, covalent bonds or coordinated bonds to form the biotic-abiotic interface, and the materials affect microalgae functions by interfering with matter and energy transfer between the microalgae and the extracellular environment (Fig. 9 ).Inorganic materials combine with microalgal cells mainly through coordination bonds and intermolecular forces, while organic materials combine with microalgal cells mainly through covalent bonds and intermolecular forces.In terms of interaction strength, a covalent bond is stronger than coordination, stronger than intermolecular forces.However, covalent binding is not the most common method, mainly due to its effect on cell activity.Coordination bond binding can not only provide stable cell-material contact, but also keep good cell activity.Therefore, in situ formation of inorganic materials on the cell surface under mild conditions is a feasible and common approach to constructing MMHs.
Apart from MMH construction, microalgaematerial interaction in the hybrid structure is the most critical problem.Based on the above studies, we propose two mechanisms of materialendowed improvement of microalgal function.One is microalgae-material electron transfer and the other is material-induced cell microenvironment transformation.In the mechanism of microalgae-material electron transfer, the essence of the microalgae-material interaction is that microalgae-material interface directly is involved in the PETC (Fig. 9 ) whereas in the mechanism of material-induced cell microenvironment transformation, the essence of the microalgae-material interaction is that the microalgae-material interface is indirectly involved in the PETC (Fig. 9 ).In addition, the microalgae-material interaction may also affect the respiratory electron transport chain.The microalgae-material interface not only affects cell communication with the extracellular environment, but also has a significant effect on the function of MMHs.The interface should allow efficient signal transduction (e.g.optical and electrical signals) for bidirectional cell-material communication.Therefore, it is important to develop materials with unique physicochemical properties and biological compatibility.Take BPV devices for example.Charge-transfer rates and energy-conversion efficiency have been improved by manipulating the surface chemistry of the semiconductor.However, materials are not limited to binding to the cell surface; they may also enter the microalgae cell to become artificial organelles in the future.Investigation and understanding of the chemical mechanism of the microalgal-material interaction are the key for this research field to move from the kingdom of necessity to the kingdom of freedom.
The property and function of the material must be coupled with the property and function of microalgal cells, otherwise the cell-material hybridization has no practical significance and may not be successfully achieved.

CONCLUSIONS AND OUTLOOK
The MMH has been studied for > 10 years, aiming to preserve biological f unction in sy nthetic systems for diverse applications in energy, the environment and medicine.MMHs have made considerable achievements including CO 2 fixation, H 2 production, bioelectrochemical energy conversion and biomedical therapy (Table 1 ).These achievements highlight microalgal functionalization with synthetic materials, which shows the important roles of the materials in biological evolution, providing innovative views about materials in biology.In a sense, MMHs can be considered as an artificial organism evolution strategy by using a material technique.Although great progress has been made in this emerging research field, the ideal integration of microalgae and material remains in exploration.
Since microalgae and materials are two key components in constructing MMHs, the functional modification should be based on the inherent properties of microalgae.Until now, cyanobacteria ( Synechocystis sp.strain PCC 6803, Synechocystis sp.strain PCC 7002 and Synechococcus elongatus PCC 7942) and green algae ( Chlorella pyrenoidosa , Chlorella vulgaris and Chlamydomonas reinhardtii ) have been the most used in the construction of MMHs and are two representative categories in the microalgae family.The cyanobacteria-material hybrid has been applied to CO 2 fixation, bioelectrochemical energy conversion and biomedical therapy, while the green algae-material hybrid has been applied to hydrogen production and biomedical therapy (Table 1 ).However, there are many other kinds of microalgae to be studied for material modification.Different cells not only have different inherent properties, but also have different response mechanisms to materials, which may induce different functions of the hybrid.Optimizing the inherent properties of microalgae is also important for the hybrid to achieve better performance.For example, synthetic biology has been playing an increasingly important role in bioelectrochemical energy conversion [ 78 ].While synthetic biology is mainly used to improve the in vivo function of cells [ 83 ], the tools of synthetic biology can in principle enhance the ability of microalgal cells to interact with electrodes.Future opportunities also lie in integrating synthetic materials with engineered microalgae to open up new pathways in energy conversion.
Apart from the inherent properties of microalgae, a suitable material strategy is the most critical factor to affect the function of MMHs.Here, the material strategy includes material selection and the construction method.Currently, silica is one of the most suitable materials for microalgal modification due to good biocompatibility and light transmittance, and bioinspired cel l si licification is an emerging approach to induce the combination of microalgae and silica.Until now, cellular silicification has been demonstrated not only to protect cel l viabi lity against environmental stresses, such as heat, high light and UV-C irradiation, but also to induce photosynthetic H 2 production [ 21 , 3 6 , 104 ].A part from silica, hydroxyapatite, Fe 3 O 4 , rare earth materials and hydrogel can also be used for microalgae modification.In situ biomimetic material formation on microalgae cell surfaces is a feasible approach to constructing MMHs, but there remain many open questions and challenges.First, the centrifugal operation and combination process inevitably cause cell damage, hence biocompatibility should continue to be improved.Second, a sophisticated structure of the materials on the cell has not been achieved.Third, most modifications by materials can prevent cell replication, but the improvement usually disappears as the cells divide, which means a lack of recyclability.Fourth, the lifespan of MMHs is limited and depends on the intrinsic lifetime of the microalgae and the protective capability of the hybrid.In addition, the construction method of the cell-material is another aspect of the materials strategy that has a great influence on the microalgal function.According to the current material technology, there are three construction methods for MMHs, including cell immobilization, unicellular shellization and multicellular aggregation.Among the three construction methods, cel l immobi lization of microalgae was mainly applied in CO 2 fixation, bioelectrochemical energy conversion and biomedical therapy; unicellular shellization of microalgae was mainly applied in CO 2 fixation, hydrogen production and biomedical therapy; whi le multicel lular aggregation of green algae was mainly applied in hydrogen production.Except for microalgae-electrode hybrids, current modification by materials is mainly based on the changes in the cellular microenvironment induced by the materialmicroalgae interaction, although the material cannot confer new functions to the microalgae.Therefore, a new material strategy is critical to exploring new functions and new applications.What calls for special attention is that the MMH general ly fol lows a principle: the selection of materials and the hybridization methods should depend on the expected function.Besides biocompatibility, stability under light conditions also needs to be considered.The advancement of nanomaterial technology is an important driving force for the development of this field.If the controllable formation and combination of nanomaterials at the biological interface can be realized, it wi l l be helpful to realize the precise transformation of biological bodies.
Looking back on the development of MMHs in the past 10 years, we can find that the initial applications were focused on photosynthetic energy conversion.At present, the main limitation of MMHs for photosynthetic energy conversion is the low energyconversion efficiency, which makes practical application in carbon neutrality difficult.Besides, the high cost of the microalgae culture and the lack of material technology suitable for the large-scale engineering of microalgae are also important limiting factors for practical application.In the future, improving the energy-conversion efficiency is the general aim for fundamental research.Before achieving the general aim, some fundamental questions surrounding the biological functionalization should be investigated, such as the effect of synthetic materials on microalgae at the genetic level.Besides the studies in the biological field, more work is needed in developing new techniques for integrating synthetic materials with biological systems.The important consideration in constructing the hybrid is the interface between the synthetic materials and the biological systems, not only from the perspective of biocompatibility, but also from the perspective of functional compatibility.Exploring new functions of MMHs and new application scenarios of the established hybrid systems is also a future trend of the emerging area.In some special circumstances, cost is not an issue, so appropriate application scenarios wi l l promote the practical application of MMHs.For example, cross-species energy conversion for therapy is becoming an emerging area that mainly utilizes photosynthetic energy carriers.Meanwhile, exploring new functions and applications wi l l spur the study of the interaction mechanism between organisms and materials.
Generally, MMHs for photosynthetic energy conversion is an emerging field and the bioticabiotic interface is the fundamental problem in the field.The investigation of microalgae-material interaction mechanisms can not only enhance the understanding of the biotic-abiotic interface, but also promote the development of the applied research of MMHs.The MMH is not only to demonstrate material-based microalgae modification, but also to achieve functional synergy between materials and microalgae.The original intention of the MMH was to expand the application of microalgae by materials, but the material-based organism improvement has also blurred the boundary between biotic and abiotic systems, which follows the significant roles of materials in biological evolution.Furthermore, research on the biotic-abiotic hybrid can combine biological and material sciences together for an inseparable integration.Once the photosynthetic energyconversion efficiency is achieved, MMHs can greatly promote the applications of photosynthetic CO 2 fixation and H 2 production.Finally, there is sufficient reason to believe that application of MMHs wi l l greatly contribute to the achievement of carbon neutrality.

•• 2 BiofuelFigure 1 .
Figure 1.Timeline of the development of a microalgae-material hybrid for enhanced photosynthetic energy conversion.

Figure 2 .
Figure 2. Strengths and limitations of microalgal immobilization methods.Each method features inherent unique functionalities and tradeoffs (a-e) in photosynthetic energy conversion.(a) Viability represents the cell survival state, which can be evaluated by green fluorescence intensity by fluorescein diacetate (FDA)-stained cells under confocal laser scanning microscopy.(b) Stability represents the durability of the microalgaematerial interaction, which can be evaluated by the maintenance time of the hybrid structure.(c) Cytotoxicity represents the effect of the microalgae-material interaction on cell viability, which can be evaluated by MTT assay or a fluorescein luminescence method.(d) Scalability indicates the potential for large-scale microalgae immobilization.(e) Applicability indicates the application prospect of the immobilized microalgae in a comprehensive consideration of the above factors.Achieving the optimal outcome relies on the rational matching of material properties and microalgal functions.Furthermore, different levels of microalgae-material hybridization may be necessary, depending on the desired outcome.

C
e ll d iv is io n S p o n ta n e o u s a g g r e g a ti o

IFigure 8 .
Figure 8. Microalgae-material hybrid for biochemical energy conversion.(a) Schematic illustration of microalgae-tumor biochemical energy conversion for biomedical therapy.(b) Schematic illustration of cyanobacteria-UCNP-Ce6 for enhanced photodynamic therapy of hypoxic tumors.Reproduced with permission from [ 95 ].Copyright 2021 Springer Nature.(c) Schematic illustration of the Chlorella -SiO 2 hybrid for cascade radio-photodynamic therapy.Reproduced with permission from[ 97 ].Copyright 2020 American Chemical Society.(d) Schematic illustration of microalgae-hydrogel patch for chronic wound healing by light response dissolved oxygen release[ 25 ].(e) Schematic illustration of the algae −bacteria gel patch for producing H 2 to modulate oxidative stress in diabetic wound healing.Reproduced with permission from[ 24 ].Copyright 2022 American Chemical Society.

Table 1 .
Microalgae-material hybrid for functional improvement.