Atmospheric dispersal shapes rapid bacterial colonization of Icelandic Lava Rocks

Abstract Microorganisms released into the atmosphere by various disturbances can travel significant distances before depositing, yet their impact on community assembly remains unclear. To address this, we examined atmospheric and lithospheric bacterial communities in 179 samples collected at two distinct Icelandic volcanic sites: a small volcanic island Surtsey, and a volcanic highland Fimmvörðuháls using 16S rRNA amplicon sequencing. Airborne microbial communities were similar between sites while significant differences emerged in the communities on lava rocks after 1-year exposure. SourceTracker analysis revealed distinct bacterial populations in the atmosphere and the lava rocks with surrounding soil contributed more significantly to lava rock microbial composition. Nevertheless, shared genera among air, rocks, and local sources, suggested potential exchange between these environments. The prevalent genera shared between rocks and potential sources exhibited stress-resistant properties, likely helping their survival during air transportation and facilitating their colonization of the rocks. We hypothesize that the atmosphere serves as a conduit for locally sourced microbes and stress-resistant distant-sourced microbes. Additionally, bacterial communities on the lava rocks of Fimmvörðuháls showed remarkable similarity after 1 and 9 years of exposure, suggesting rapid establishment. Our study reveals that atmospheric deposition significantly influences bacterial community formation, potentially influencing ecosystem dynamics and microbial communities’ resilience.


Introduction
Particles of biological origin suspended in the atmosphere are referred to as primary biological aerosol particles (Emetere et al. 2021 ).These particles are released from the Earth's surface into the air by wind or physical disturbances such as agricultural activity or wildfires (F röhlich-No w oisk y et al. 2016, Šantl-Temkiv et al. 2022 ) and ar e ubiquitousl y r epr esented.The density and composition of airborne microbes and communities primarily rely on the source of their emissions, which can be either natural sources like water bodies , soils , and vegetated areas, or anthropogenic sources (Xie et al. 2021 ).
Micr oor ganisms ar e usuall y the first to settle on ne w land or surfaces, influencing the growth of communities over time (Šantl-Temkiv et al. 2022 ).The process of colonization is influenced by mineral properties, soil pH, and microbial traits.Notably, bacterial binding to minerals, facilitated by extracellular pol ysacc harides and proteins, is crucial for stable colonization (Vieira et al. 2020 ), but is notably reduced in acidic and alkaline soils (Malard and Pearce 2022 ).Famous for its active volcanoes, Iceland offers an optimal setting to investigate the colo-nization and evolution of micr oor ganisms on r ecentl y cr eated volcanic r oc ks.On a mor e localized scale, the volcanic island Surstey, formed in 1963Surstey, formed in -1967 by an underw ater eruption, w as one of the first places where researchers began studying this .T hey noted the presence of phototrophs by 1968 (Sigurdsson 1970 ).Howe v er, the study of the inland Fimmvör ð uhals lava field, formed in 2010, sho w ed contr asting r esults, with the earl y dominance of diazotr ophs, c hemolithotr ophs, and heter otr ophs r ather than phototr ophs (K ell y et al. 2014 ).A recent study comparing the atmospheric and lithospheric culturable bacterial communities from Surtsey and Fimmvör ð uháls (Daussin et al. 2023b ) also revealed that Proteobacteria and Actinobacteria were the dominating phyla.Furthermore, the study discov er ed notable differ ences between the microbial communities present in the atmosphere and lithosphere, and the proportion of bacterial taxa isolated from each site was influenced by the location's environment and characteristics.
Research indicates that the atmosphere is the primary contributor to the dispersion of microbial cells in r ecentl y formed environments .Despite this , there is a notable lack of comprehensive studies on the processes of dispersal, colonization, and succession in these ar eas.Additionall y, the r ole of atmospheric deposition in community assembly remains to be understood.Studying these aspects is crucial for comprehending ecosystem dynamics and contributing to scientific knowledge in the fields of extreme microbiology , ecology , and environmental science.
This study had three main goals: (i) to examine and compare the microbial diversity of air and 1-year-old lava rock samples of tw o Icelandic v olcanic sites-Surtsey Island and Fimmvör ð uháls Highland, (ii) to compare the microbial composition of lava rocks at Fimmvör ð uháls after 1 and 9 years of exposure to the air, and (iii) to investigate the origin of lava rocks microbes in Surtsey after 1-year exposure to the air.From the 179 samples, around 50 000 filtered amplicon sequence variants (ASVs) or single DNA sequences were used for assessing and comparing the microbial diversity on both sites, using statistical tools.The comparison of microbial communities was used to investigate the origin of the lava rocks in Surtsey.

Site description and sample collection
To compare environmental influences on microbial composition and draw hypotheses on the origin and dispersal of microbes, samples were collected from two distinct volcanic sites in Iceland (Daussin et al. 2023b ).Briefly, the Surtsey site (63 • 18 11 N, 20 • 36 11 W) is a ne wl y formed volcanic island situated off the southern coast of Iceland.Its formation resulted from an underw ater v olcanic eruption lasting 4 y ears, from 1963 to 1967.UNESCO declared the island a World Heritage Site and it has been protected since its formation, remaining closed to the general public ( UNESCO-World Heritage Con vention ).T he Fimmvör ð uháls site (63 • 37 53 N, 19 • 26 50 W, altitude 1100 m) is positioned between the Eyjafjallajökull and Mýrdalsjökull glaciers in the southern region of Iceland.This lava field originated from the eruption of the Eyjafjallajökull volcano in the spring of 2010.Each site was divided into different sampling stations, with five stations in Surtsey and three stations in Fimmvör ð uháls (Fig. 1 ).Samples from Surtsey were recovered on 19-22 July 2018 and 18-22 July 2019, and from Fimmvör ð uháls on 12-14 September 2018 and 27-28 September 2019.Different types of samples were obtained at each site during each sampling period.
We used air and lava rock samples previously collected as described in our prior publication (Daussin et al. 2023b ).Briefly, w e emplo y ed high-flo w-rate impingers (Fig. 2 ) for air sample collection, and r ecov er ed on-site autoclav ed lav a r oc ks after 1 y ear, denoted as "1-y ear-old lav a r oc ks." Additionall y, nonautocla ved 9-year-old la va rocks from Fimmvör ð uháls were designated as "9-years-old lav a r oc ks."The decision to sample old lav a r oc ks fr om Fimmvör ð uháls was made after the first expedition to Surtsey, where the focus was on collecting samples .T his choice was influenced by the greater accessibility of Fimmvör ð uháls compared to the remote and challenging access to Surtsey, allowing to study older volcanic r oc ks in a more feasible manner.The 1-year-old lava rocks were fragmented using a sterilized hammer to facilitate analyses of the top, middle, and bottom parts.Based on the first results and to enhance clarity and align with our goals, the microbial communities from all three sections will be combined and analyzed collectiv el y.
Soil was collected as a potential source of the 1-year-old lava r oc k micr obes in Surtsey in 2019 and k e pt at −20 • C before be-ing stored at −80 • C back in the laboratory.It was collected within 10 cm of the sampled r oc k.Additionall y, 16S rRNA sequences from fumar ole, drill cor e, and seawater samples collected on Surtsey by Bergsten et al. ( 2021 ) were incorporated into this study to explore potential sources for the 1-year-old lav a r oc ks of Surtsey.The total number of samples, comprising duplicates of air samples with contr ols and r oc k samples fr om v arious sections, a ges, and locations (Surtsey and Fimmvör ð uháls), added up to 179.The coordinates, c har acteristics, number, and type of sample r ecov er ed fr om the different stations can be found in Supplementary Material -Table 1 .

DN A extr action, 16S rRN A gene amplifica tion, and Illumina sequencing
The DNA of the air samples was extr acted fr om the Steriv ex filters using a modified version of the pr otocol fr om Le v er and al 2015 ((Le v er et al. 2015 ), Supplementary Material -Protocol ).A sterile Sterivex filter was used as a negative control.The rock samples were crushed into powder aseptically using a homemade tool designed for this purpose ( Supplementary Material -Figure 1 ) The DNA of the different parts of the r oc ks and the soil samples were extracted using the DNeasy Po w erMax Soil Kit (Qiagen, Hilden, German y).DNA concentr ations wer e c hec ked for all samples using Qubit for high-sensitivity dsDNA (Thermo Fisher Scientific, Waltham, MA, USA).
The 16S rRNA gene sequencing method was used to effectiv el y c har acterize the taxonomic composition of micr obial comm unities, whic h is a primary focus of this study.The V4 region of the bacterial and archaeal 16S rRNA genes were amplified in triplicates from each DNA using the 515F (5'-GTGCCAGCMGCCGCGGTAA-3') and 806R (5'-GGA CTA CHVHHHTWTCTAAT-3') primers (Skirnisdottir et al. 2000 ) and the Q5 DNA pol ymer ase (Ne w England Biolabs , Ips wich, USA).Amplification was carried out according to Daussin et al. ( 2023b ) with 2 ng/ μl of DNA template for the r oc ks and soil, or 2-5 μl of the r aw DNA solution extr acted fr om the air samples.The master mix r eceiv ed an addition of bovine serum albumin (BSA) at a concentration of 1 μg/ μl to facilitate amplification of the challenging samples .T hermal cycling conditions consisted of 98 • C for 30 s; 30 cycles of 98 • C for 10 s, 52 • C for 30 s, and 72 • C for 30 s; and 72 • C for 2 min.Triplicate reactions were pooled, and amplicons were size selected from 1% (w/v) agarose gels using the Monar ch DN A Gel Extr action Kit (Ne w England Biolabs).The pr epar ation of sequencing libraries and sequencing was done according to Bergsten et al. ( 2021 ).

Comparison of the ASVs recovered with amplicon sequencing and cultiv a tion
To compare the ASVs r ecov er ed fr om both cultiv ation (Daussin et al. 2023b ) and amplicon sequencing, only the samples utilized in both methods were considered (Daussin et al. 2023b ).A megablast w as performed betw een the ASVs obtained with both methods befor e an y decontamination pr ocess.Similar to the Daussin et al. ( 2023b ) study, only matches with a minimum identity of 98.65% wer e r etained.

Da ta anal ysis
RStudio was used for bioinformatics and statistical analyses, employing R (v 3.6.0)and according to the D AD A2 pipeline tutorial (1.8) (Callahan et al. 2016 ) for the lav a r oc k and the soil samples, and the air samples separ atel y.The two resulting Phyloseq objects wer e mer ged after the r espectiv e decontamination steps.2021) with the Decontam (v 1.6.0)pac ka ge .T he total number of reads per sample in the merged Phyloseq was equalized to a rarefaction depth corresponding to 90% of the minimum sample depth in the dataset, which was 5602 reads.All analyses were performed on the merged phyloseq object, without the controls, and all coding lines used on the phyloseq objects on R can be found in the Supplementary Material -Codes .

Sta tistical anal ysis
Alpha div ersity anal ysis was conducted in R using the Vegan package [17].To assess the influence of different factors and confirm the alpha diversity results, an analysis of variance (ANOVA) was applied to the Shannon Indexes.
For beta div ersity e v aluation, data normalization was performed using the "r ar efy_e v en_depth" method befor e conducting a Bray −Curtis-based nonmetric multidimensional scaling (NMDS).The statistical significance of factors such as the sampling site and year was determined using permutational multiv ariate anal ysis of v ariance (PERMANOVA).
Before executing the PERMANOVA analysis, we ensured the prer equisites wer e met by conducting preassessments using PER-MDISP and Tuk e y's honest significant differences tests .T hese e v aluations confirmed significant statistical disparities in both dispersion between groups and groups means, thereby supporting the validity of our findings ( Supplementary material -Table 2 ).

Source prediction for the 1-year-old lava rock microbes in Surtsey
The Venn dia gr am cr eated in Excel shows the common ASVs identified through the microbiome package (Lahti and Shetty 2019 ) among the air, the 1-year-old lava rock, and the soil of Surtsey.The SourceTr ac ker pac ka ge (Knights et al. 2011 ), utilizing Bayesian methods (McGhee et al. 2020 ), assessed the contributions of air and soil to the lava rocks, with each rock individually analyzed, considering air and soil from the same location as potential sources.To quantify the hypothetical contribution of each source to the micr obial div ersity of the 1-year-old lava rock, we performed a supplementary investigation in which we compared the presence of genera across the lava rocks and different environments (soil, air, fumarole , drill core , and sea water), suggesting potential dispersal between them.The speculative nature of this source prediction method arises from the occurrence of certain genera in multiple en vironments .

Overview of the microbial diversity in the air and lava rock samples recovered from Surtsey and Fimmvör ð uháls
A total of 48 581 ASVs (archaea: 447, bacteria: 47 999, and NA: 135) wer e r ecov er ed fr om the 179 samples anal yzed ( Supplementary Material -Table 3 ).The r ar efaction curv es sho w ed that all the libr aries r eac hed a point of saturation, indicating that the sequencing generated sufficient reads to capture the complete diversity of the samples ( Supplementary Material -Figure 2 ).After performing a megablast, 174 ASVs r epr esenting the cultivable bacterial community obtained in a pr e vious study (Daussin et al. 2023b ) matched the amplicon results, corresponding to 1.25% of the total ASVs in these samples (Fig. 3 ).
Statistical analysis of the separate rock sections revealed only a minor difference in microbial communities between the top and the bottom of the r oc ks (T able 1 ).T o enhance clarity and align with our goals, the microbial communities from all three sections will be combined and analyzed collectively.
T he Bra y-Curtis-based NMDS exhibited a str ess v alue of 0.14, indicating the need for caution in inter pr eting the results.Howe v er, notable tr ends ar e clearl y a ppar ent and sho w ed distinct micr obial comm unities between the air and the 1-year-old lav a r oc k samples (Fig. 4 ).Ho w e v er, micr obial comm unities of the air samples r ecov er ed fr om Surtsey and Fimmvör ð uháls wer e similar to eac h other wher eas the 1-year-old lav a r oc k comm unities exhibited differences between Surstey and Fimmvör ð uháls.Furthermore , at Fimmvör ð uháls , the micr obial comm unities r ecov er ed from the 1-year-old lava rocks were similar to the communities on the 9-year-old lava rocks .T he ANOVA for Shannon's index sho w ed significant differences between the microbial communities at the different sites, for both the air and the 1-year-old lava rocks.No significant differences were found between the communities recov er ed fr om the differ ent sampling years, and with or without using BSA (Table 1 ) .The alpha diversity was higher in the 1-year-old Figure 3. AVS as Phyla proportion of the prokaryotic cells obtained using culture-dependent (A) (Daussin et al. 2023a ) and culture-independent (B) (this study) methods.A total of 1.25% of the ASVs obtained by amplicon sequencing were also detected using cultivation and 16S rRNA sequencing.The five major phyla detected by cultivation were also found by amplicon sequencing or 85% of the total ASVs.Proteobacteria and Actinobacteria were the dominant phyla, 97% and 54%, r espectiv el y, using both methods.lav a r oc k samples than in the air samples, at both sites .Moreo ver, the highest diversity was found in the rocks of Surtsey whereas the lo w est diversity w as found in the air of the same site (Fig. 4 ).
The alpha diversity was similar for the 1-year-old and the 9-yearold lava rocks of Fimmvör ð uháls.

Community composition in the different samples of Surtsey and Fimmvör ð uháls
In the air samples, cells affiliated with Proteobacteria represented the majority of the community (Fig. 4 ).The most abundant gener a wer e similar for the two sites with a majority of Pseudoalteromonas and Meth ylobacterium-Meth ylorubrum , follo w ed b y Nocardioides , Massilia , and Conexibacter .The air of Fimmvör ð uháls was dominated by Methylobacterium-Methylorubrum whereas the air of Surtsey was dominated by either Pseudoalteromonas or Meth ylobacterium-Meth ylorubrum , depending on the sampling station (Fig. 4 ).Re presentati ves of the Meth ylobacterium-Meth ylorubrum genus w ere found in majority in the air above the Máva varp, Tangi, and Viti stations , wher eas r epr esentativ es of the Pseudoalteromonas genus dominated the air above the Austur, Borhola, and Pálsbaer stations ( Supplementary Material -Figure 3 ).
In the 1-year-old lav a r oc k samples, the majority of sequences were affiliated with Proteobacteria and Actinobacteria (Fig. 4 ), and clear differences were observed between the two sites.Circles in the NMDS plot are statistical ellipses for the different types of samples, the left one represents the air samples whereas the right circle r epr esents the 1-year-old lav a r oc k samples .T he figur e illustr ates that the micr obial div ersity in the air samples fr om both sites is similar, while the communities inhabiting the 1-year-old lava rocks differ between the two sites.
The 1-year-old lava rocks of Fimmvör ð uháls were dominated by Massilia and Pseudarthrobacter , followed by Methylobacterium-Methylorubrum, whereas the l-year-old lava rocks of Surtsey were dominated by Nocardiodes and Psychroglaciecola , followed by Conexibacter and Massilia (Fig. 4 ).No difference was noticed between stations on each site ( Supplementary Material -Figure 3 ).
The micr obial comm unities r ecov er ed fr om the 9-year-old lav a r oc k samples of Fimmvör ð uháls were dominated by r epr esentatives of the Massilia and Pseudarthrobacter genera (Fig. 5 ).

Hypothetical prediction of the microbial origin in 1-y ear-old la v a rock deposit a t Surtse y
The Venn dia gr ams of the differ ent 1-year-old lav a r oc ks r ecover ed fr om Surtsey along with the corr esponding air and soil samples sho w ed that on av er a ge, 12.6% of the ASVs found in the 1year-old lav a r oc ks samples wer e also found in the underlying soil samples whereas only 2.6% of these ASVs were also found in the air samples (Fig. 6 A).The SourceTr ac ker anal ysis confirms this r esult with on av er a ge 12.9% of the micr obes r ecov er ed fr om the 1year-old lava rock in Surtsey coming from the underlying soil and 1.8% from the air (Fig. 6 B).Ho w ever, different proportions were observed in the different investigated sampling stations with a maximum of 40% of the microbes in the 1-year-old lava rock originating from the underlying soil at Pálsbaer, and a minimum of 1% at Viti.
Additionally, 30% of the genera in the lava rocks after 1 year of exposure at Surtsey were also found in the air, and 28% in the soil (Fig. 7 ).Mor eov er, the contribution of the fumarole, drill core , and sea water samples led up to 25% of the total genera in the lava rock, leaving 17% of unkno wn-sour ced genera.No corr elation was observ ed between the drill cor e depth and the common genera with the 1-year-old lava rock at the surface  ( Supplementary Material -Figure 5 ).The 10 most abundant genera found in the lava rock were also the most abundant in the other potential sources, except for the seawater samples that only contained Pseudoalteromonas and Alteromonas .No Psychroglaciecola was found in common between the lava rocks and the drill core samples ( Supplementary Material -Figure 4 ).

Discussion
T he air samples , whic h wer e taken during two consecutive summers, wer e c har acterized by a similar alpha and beta diversity regardless of the sampling year (Table 1 ).As demonstrated in a pr e vious study (Daussin et al. 2023b ), the air masses were coming r oughl y fr om the same dir ection in both years and at all sites.Based on the NMDS analysis (Fig. 4 ) of the atmospheric microbial communities, we conclude that these communities were relativ el y similar, e v en though ther e wer e significant differ ences identified by the ANOVA analysis (Table 1 ).These variations could be attributed to the presence of outliers, which are apparent on the NMDS plot, and to the dissimilar microbial compositions between the stations on Surtsey .Specifically , Pálbaer, Austur, and Borhola exhibit the dominance of Pseudoalteromonas , while Máv av ar p, Viti, and Tangi are characterized by a predominance of Meth ylobacterium-Meth ylorubrum.
T he o v er all similarities between the differ ent sites can be attributed to the fact that (i) the local surroundings and ther efor e sources of airborne cells at both sites share common traits, (ii) the backw ar d trajectories indicate that the same areas contributed to the aerosols engaged in the long-distance dispersal, and (iii) the samples were collected during the summer period.Based on the similarity observed between the bacterial communities in aerosol samples collected during two consequent summers, we conclude that despite the r elativ el y low tempor al cov er a ge, we could describe the taxonomy of depositing airborne cells during the summer periods well.
In the 1-year-old lava rock samples, the beta diversity analysis sho w ed that the micr obial comm unities differ ed between the sampling sites, which can be related to the differences in rock composition between the two sites .La v a r oc ks at Fimmvör ð uháls contained a ppr oximatel y eight times more Fe 2 O 3 than the lava r oc ks of Surtsey (Table 2 ).Mor eov er, it is essential to consider other potential contributors to this diversity and dynamics.Elements like physical structure and broader environmental and meteorological conditions, including spatial heterogeneity, external rain precipitation, and wind patterns, could also play significant roles in shaping these differences (Byloos et al. 2018 ).Collecting these additional data in a further study would be crucial for a more compr ehensiv e understanding of the factors influencing microbial dynamics and diversity.While both air and lava rock environments host dominant gener a with r a pid ada ptation ca pabilities, it is e vident fr om the NMDS analysis and the culture-de pendent stud y conducted on the same samples (Daussin et al. 2023b ) that there are substantial differences between the micr obial comm unities in these en vironments .Although the survival and initial establishment of cells on volcanic r oc ks may r el y on similar ada ptation pr operties as the ones needed for survival in the atmospher e, subsequent gr owth and success on lav a r oc ks demand different characteristics .T his disparity arises from both specific properties exhibited by the rockdwelling microbes and the fundamental differences between the atmosphere and the rock environment itself.Unlike the atmosphere , la va rocks provide a wide range of microenvironments for microbial colonization, with cracks and fissures offering microorganisms se v er al adv anta ges, including pr otection fr om harsh envir onmental conditions suc h as extr eme temper atur es and UV r adiation, water retention in pore spaces, limited competition for r esources, limited pr edation fr om macr oscopic or ganisms, and a stable physical structure for long-term colonization and adaptation (Antony et al. 2012 ).Moreover, the air samples represent the microbes found at the instant of the sampling, whereas the r oc k micr obes r eflect a longer colonization period.Additionall y, it is important to note that the air samples r epr esent the microbial composition at the time of sampling, while the r oc k samples reflect a longer period of colonization.These inherent distinctions may account for the observation that alpha diversity was highest in the r oc ks of Surtsey but lo w est in the air at the same site.
We observed the transfer of bacterial cells between the soil and 1-year-old lava rocks, with four times more lava rock ASVs originating from soils compared to lava rock ASVs originating from the air.This could be attributed to the direct physical contact between r oc k and soils, which facilitates the exchange of microbial cells but could also be attributed to indirect contributions of soil ASVs via atmospheric dispersal.The latter is in line with studies that found that soils are the major source of atmospheric bacterial communities (Šantl-Temkiv et al. 2018, Archer et al. 2023 ).Moreov er, our SourceTr ac ker anal ysis highlighted spatial v ariations in microbial contributions from the soil across different sampling stations, indicating a site-dependent relationship.In Pálsbaer and Austur, we observed a higher number of shared ASVs between the soil and the lava rocks .T his can be attributed to the presence of similar volcanic r oc ks cov ering the soil in these areas, which is a result of their proximity to the respective west and east craters.
Ho w e v er, despite the significant exchange between soil and r oc k, most ASVs inhabiting the 1-year-old lava rocks came from unkno wn sour ces, distinct from both air and soil.This observation led us to hypothesize that these microbes likely settled over an extended period, originating from a diverse range of sources, contributing to the unique r oc k composition.
Mor eov er, we discov er ed that a significant proportion (83%) of the genera identified on the 1-year-old lav a r oc ks at Surtsey were also found in the other investigated environments, suggesting potential exchange dynamics between them.Notably, an average of 71% of the gener a pr esent in the 1-year-old lav a r oc k and other investigated sources were also detected in the air samples (as shown in Supplementary Material -Figure 6 ).
Additionally, the most abundant genera found in common between the 1-year-old lava rocks and the potential sources were Nocardioides , Blastocatella , Hymenobacter , Spirosomona , and Deinococcus ( Supplementary Material -Figure 4 ).Most of them possess properties that might have helped them colonize a wide range of environments and/or survive atmospheric transportation (Daussin et al. 2023a ).Indeed, r epr esentativ es of the genus Nocardiodes have the capability to use various carbon and nitrogen sources (Geshe v a and Vasile v a-Tonk ov a 2012 ) Hymenobacter has pr e viousl y been isolated fr om Icelandic envir onments and is a cold-and r adiation-toler ant genus ( Hilmisson 2020 ).Some Spirosoma species are also radiation-resistant (Park et al. 2021 ) and Deinococcus is w ell-kno wn for its m ultir esistance ca pacity (Hirsc h et al. 2004 ).
These observations suggest that the atmosphere could potentiall y serv e as a v ehicle for the tr ansportation of a div erse r ange of str ess-r esistant micr obial taxa fr om distant sources ov er longer periods of time .Furthermore , the significant exchange observed between soil and r oc k, alongside the shar ed gener a between lava r oc ks and other en vironments , including air samples, indicates that habitat selection may be occurring from the air populations of microbes.To determine whether this is indeed the case, a long-term sampling strategy for atmospheric communities should be emplo y ed that w ould allo w for a better understanding of its tempor al heter ogeneity.
Similar microbial communities were found after 1 year of exposure and after 9 years of exposure in Fimmvör ð uháls with repr esentativ es of the genera Massilia and Pseudarthrobacter dominating (Fig. 5 ).In line with our findings, an earlier study also concluded that after a certain duration of exposure, the age of Icelandic basaltic r oc k does not have a significant influence on micr obial comm unities (Byloos et al. 2018 ).One explanation could be that these communities were already well-established after 1 year, making it difficult for new depositing cells to colonize the r oc ks.It is also possible that the properties that helped these comm unities surviv e atmospheric str essors allo w ed them to settle and thriv e sustainabl y on the r oc ks, thus facilitating their pr eferential colonization even after 9 years.For instance, certain Massilia species are known for possessing m ultir esistance pr operties that allow them to withstand various stresses present in both the air and r oc k envir onments (Sedlá ček et al. 2022 ).It is important to recognize the limitations of our method, especially the existence of certain genera in multiple en vironments .Ho w ever, it allo ws us to ca ptur e br oad tr ends among phylogeneticall y r elated gr oups that fr equentl y shar e similar pr operties.
One limitation of this study is that the origin of the microbes in the 1-year-old lav a r oc ks was onl y inv estigated at one sampling site.While the results provide valuable information about microbial communities at that location, it is possible that the results would differ if the analysis was applied to other sites.Another limitation is that the different potential sources were not sampled at the same time.To establish the link between the atmospheric and volcanic communities, as well as predict the sources of the micr oor ganisms, micr oor ganisms fr om the differ ent envir onments would need to be sampled continuously over a long time while sim ultaneousl y the succession on volcanic r oc ks would be follo w ed.Unfortunatel y, this is tec hnicall y extr emel y c hallenging in the remote locations that we targeted.While acknowledging the limitations of the 16S rRN A sequencing, w e opted for it due to its practicality for our research goals.We are aware of potential bias introduced by primer selection, as it can influence pr efer ential amplification of certain micr obial gr oups and r esult in taxonomic disparities.Ad ditionally, our stud y did not measur e the pr efer ential settling of certain taxa through dry and wet processes, which could have influenced the microbial communities' composition and distribution.

Conclusion
In this study, we examined and compared the microbial communities from two different active volcanic sites in Iceland: Surtsey and Fimmvör ð uháls.Airborne microbial communities were similar between sites while significant differences were found between the communities established on the lava rocks after 1-year exposur e. Micr obial comm unities did not c hange significantl y between 1 and 9 years of exposure at Fimmvör ð uháls, which implies that the bacterial comm unities ar e alr eady well-established earl y in the process.Based on a comprehensive examination of bacterial communities that developed after 1 year of exposure on the lav a r oc ks of Surtsey, we hypothesized that a significant portion of these communities could have potentially originated from local surrounding sources and been dispersed to the r oc ks thr ough the atmosphere over time.
Ov er all, we conclude that atmospheric dispersal is a k e y factor involved in the colonization of volcanic r oc ks, and the cells that settle in these ne wl y formed environments are responsible for establishing unique and diverse communities in less than 1 year.
Understanding the link between atmospheric and lithospheric micr obial comm unities has m ultifaceted implications.It aids in assessing the resilience of microbial communities in response to environmental changes and disturbances.Additionally, it advances our scientific comprehension of microbial ecology in extr eme envir onments, with potential implications for fields like geomicr obiology, astr obiology, and biogeogr a phy.

Figure 4 .
Figure 4. Alpha diversity, most abundant genera, and Br ay-Curtis-based NMDS plot of the differ ent samples r ecov er ed in this study (str ess v alue = 0.139091).Circles in the NMDS plot are statistical ellipses for the different types of samples, the left one represents the air samples whereas the right circle r epr esents the 1-year-old lav a r oc k samples .T he figur e illustr ates that the micr obial div ersity in the air samples fr om both sites is similar, while the communities inhabiting the 1-year-old lava rocks differ between the two sites.

Figure 5 .
Figure5.PCoA analysis and most abundant genera found in the different sample types r ecov er ed fr om Fimmvör ð uháls .T he micr obial comm unities in the lava rocks exhibit a similar diversity both after 1 year and 9 years of exposur e. Howe v er, these lav a r oc k comm unities differ significantl y fr om the micr obial comm unities found in the air samples collected on the same site.

Figure 6 .
Figure 6.Venn dia gr am of the ASVs r ecov er ed fr om the differ ent sample types in Surtse y (A, n umbers re present ASVs) and SourceTracker result for 1-year-old lava rocks recovered from Surtsey (B) (each circle represents one rock, and the different sources are indicated with colors.V: Viti; P: Pálsbaer; A: Austur; and B: Borhola).

Figure 7 .
Figure 7. Sc hematic vie w of the potential sources and their r elativ e contribution to the micr obial div ersity found in the 1-year-old lav a r oc k of Surtsey.Numbers indicate the proportion of shared genera between the 1-year-old lava rocks and the different potential sources.(Sequences from drill core; SW: seawater and fumarole were acquired from Bergsten et al. 2021 ).

Table 1 .
Statistical analysis of the microbial diversity.Statistically different parameters are underlined.