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

Abstract:

Lake Arcas exhibits a thermal stratification from April to October. A sulfide-rich anoxic hypolimnion is then formed between the deeper part of the thermocline and the lake bottom, and high population densities of phototrophic microorganisms are found at the oxic-anoxic interface. Chromatium weissei, a large rod, 8 × 4 μm in size, was the dominant phototrophic bacterium, reaching densities of up to 1.84 × 10⁶ cells ml^-1. Other phototrophic sulfur bacteria, such as Amoebobacter cf. purpureus, Thiocapsa sp., and Pelodictyon clathratiforme were also present in the anoxic hypolimnion, but their 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. weissei grew most rapidly in the upper part of the phototrophic bacterial layer. The highest growth rates were found during the first half of the stratification period, with a marked decrease in population density as mixing approached. Our results suggest that purple sulfur bacteria in Lake Arcas are light limited, even though they possess okenone, which can efficiently harvest light at the wavelengths penetrating to the chemocline. High rates of carbon 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 be only 12—13%.