Diel cycle of metabolism of phototrophic purple sulfur bacteria in Lake Cisó (Spain)

During a bloom of purple sulfur bacteria in Lake Cisó (Spain) data were collected on diel changes in the concentrations of sulfide, sulfur, glycogen, and poly‐β‐hydroxybutyrate (PHB), the rates of CO2 fixation and H2S oxidation, and the viability of cells along the vertical profile. During the day, sulfide oxidation resulted in the intracellular accumulation of sulfur and glycogen. At night, the concentrations of glycogen and sulfur decreased concomitant with the production of sulfide and PHB. Throughout the day, CO2 fixation and H2S oxidation were maximal in the zone of maximal population density (peak of the layer); however, in the top of the layer the specific rates were 2–6 times higher. The ratio of CO2 fixation to H2S oxidation, the sulfur content of the cells, and their specific density all indicated that sulfide was predominantly oxidized to sulfur. In the top and the peak of the layer cell viability was high but decreased rapidly with increasing depth. It was concluded that the existence of the bloom is the combined result of slow growth at the peak of the layer and a much faster rate of reproduction at the top of the layer. The cells at the top stored sulfur extensively and then sank to deeper layers where they could not oxidize the sulfur because of insufficient light.