Phototrophic sulfur bacteria in two Spanish lakes: Vertical distribution and limiting factors1

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Abstract
The anaerobic zones of Lakes Cisó and Vilar (Banyoles karstic area, NE Spain) had mass developments of purple sulfur bacteria during summer 1982. In Lake Vilar, Chromatium spp. was dominant (up to 92% of the microbial biovolume). In Lake Cisó, the predominant microorganisms were Chromatium spp. (up to 71%) and another purple sulfur bacterium forming aggregates (20%).The bacterial layer could be divided according to the physiological state of the cells into a top part of maximal specific activity, a peak of maximal abundance, and a bottom part of inactive cells. The bacteria in the peak were predominantly limited by light; sulfide, phosphate, and acetate were not limiting in the middle of the day. The light limitation started at the depth having the maximal concentration of cells; the top of the layer appeared to be sulfide‐limited. Specific contents of photopigments, elemental sulfur, and reserve polymers decreased from the top to the bottom of the bacterial layer. These phenomena point to the crucial role of light in the development of layers of phototrophic bacteria in stratified lakes.