Spatio-temporal distribution and growth dynamics of phototrophic sulfur bacteria populations in the sulfide-rich Lake Arcas

Lake Areas exhibits a thermal stratification from April to October. A sulfi de-rich anoxic hypolimnion is then formed between the deeper part of the th ermocline and the lake bottom, and high population densities of phototrophi c microorganisms are found at the oxic-anoxic interface. Chromatium weissei , a large rod, 8 x 4 mum in size, was the dominant phototrophic bacterium, reaching densities of up to 1.84 x 10(6) cells ml(-1). Other phototrophic s ulfur bacteria, such as Amoebobacter cf. purpureus, Thiocapsa sp., and Pelo dictyon clathratiforme were also present in the anoxic hypolimnion, but the ir cell size and population densities were much lower. Net growth rates (0. 125 to -0.123 d(-1)) and frequency of dividing cells, indicated that C. wei ssei grew most rapidly in the upper part of the phototrophic bacterial laye r. The highest growth rates were found during the first half of the stratif ication period, with a marked decrease in population density as mixing appr oached. Our results suggest that purple sulfur bacteria in Lake Areas are l ight limited, even though they possess okenone, which can efficiently harve st Light at the wavelengths penetrating to the chemocline. High rates of ca rbon photoassimilation by phototrophic bacteria were measured (up to 200 mg C m(-3) h(-1)), but because of the narrow depth range in which anoxygenic photosynthesis occur, bacterial contribution to overall primary production during summer was estimated to he only 12-13%.