Physiological ecology of a metalimnetic Cryptomonas population: relationships to light, sulfide and nutrients

Abstract

The possible mechanisms by which Cryptomonas phaseolus maintains a large metalimnetic population during stratification were studied in Lake Cisó. The population was shown to always remain at the oxygen-sulfide interface despite the fact that sometimes the amount of light reaching was <1% of incident light. This meant that the amount of light reaching the peak was sometimes much lower than the optimum light intensity for photosynthesis of the population, which was ∼150 μE m⁻² s⁻¹. Sulfide inhibited carbon fixation by Cryptomonas, but, not other metabolic processes, because the cells metabolized accumulated carbohydrate during the night while in sulfiderich waters. Cryptomonas phaseolus was shown to be phosphorus limited at certain times. Phosphorus, however, could not be derived from the hypolimnion. Cryptomonas phaseolus did not ingest bacteria and the amount of carbon fixed, as measured by ¹⁴CO₂ incorporation, was sufficient to meet all the carbon requirements of the population. Altogether, none of the proposed mechanisms explained the development of the cryptomonad by itself. Cryptomonas phaseolus seemed especially well adapted to the metalimnetic habitat and few other species can compete with it. We hypothesize that predation is also reduced in the surroundings of the anoxic metalimnion. Reduction of these two processes would explain why some algal species can attain high biomasses and persist throughout stratification in the metalimnion of lakes.