CARBON PHOTOASSIMILATION BY SHARPLY STRATIFIED PHOTOTROPHIC COMMUNITIES AT THE CHEMOCLINE OF LAKE-ARCAS (SPAIN)

Three populations of phototrophic microorganisms were found closely st ratified in the chemocline of the holomictic Lake Areas. Cryptomonas s pp. 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 photoassimil ation rates were found during the stratification period at the chemocl ine (up to 197.63 mg C m(-3) h(-1)), especially at depths at which max imal densities of prokaryotic phototrophs were located, whereas much l ower values were observed in the mixed zone of the lake. Despite these high rates, the contribution of carbon photoassimilation at the chemo cline level accounted for only 16.2-18.3% of the photosynthetic assimi lation of the whole lake in the stratification period, due to the narr owness of the depth range in which these populations were active. More over, in situ oxygenic photosynthesis took place at sulfide concentrat ions of about 0.1 mM. The photosynthesis vs. irradiance curve shows th at these microorganisms were light limited, but their light-harvesting complexes allowed them to utilize the wavelengths which are relativel y more concentrated in the dim light reaching the chemocline. (C) 1998 Federation of European Microbiological Societies. Published by Elsevi er Science B.V.