Carbon photoassimilation by sharply stratified phototrophic communities at the chemocline of Lake Arcas (Spain)

Three populations of phototrophic microorganisms were found closely stratified in the chemocline of the holomictic Lake Arcas. Cryptomonas spp. reached a maximum population density in microaerobic waters above dense plates of Oscillatoria cf. ornata and Chromatium weissei, whose maxima were found in the deeper sulfide-rich waters. High photoassimilation rates were found during the stratification period at the chemocline (up to 197.63 mg C m⁻³ h⁻¹), especially at depths at which maximal densities of prokaryotic phototrophs were located, whereas much lower values were observed in the mixed zone of the lake. Despite these high rates, the contribution of carbon photoassimilation at the chemocline level accounted for only 16.2–18.3% of the photosynthetic assimilation of the whole lake in the stratification period, due to the narrowness of the depth range in which these populations were active. Moreover, in situ oxygenic photosynthesis took place at sulfide concentrations of about 0.1 mM. The photosynthesis vs. irradiance curve shows that these microorganisms were light limited, but their light-harvesting complexes allowed them to utilize the wavelengths which are relatively more concentrated in the dim light reaching the chemocline.