Phototrophic sulfur bacteria and sulfate-reducing bacteria causing red waters in a shallow brackish coastal lagoon (Prévost Lagoon, France)

Abstract In the shallow lagoon of Prévost (43°30'N, 3°54'E; French Mediterranean coast), red waters occurring periodically during warm summers are a consequence of a succession of ecological events beginning in the early spring with a bloom of algae ( Ulva lactuca ). In summer 1977, a red water was analyzed; in the early summer, the water turned anoxic and became rich in sulfide which originated from sulfate reduction in the first 10 cm of the sediment. Numbers of both phototrophic and sulfate-reducing bacteria (SRB) increased during spring and summer, and the genera in the prevailing populations did not change: Thiocapsa (80%) among the phototrophic bacteria and Desulfovibrio and Desulfobacter among the SRB. They were also dominant during the period of red waters. A few Chromatium and Thiocystis species were also identified. During red water periods, these bacteria grew very actively, removing all the sulfide produced by SRB, and accumulated in the whole water column. Consequently, the sulfate level increased to 5 mmol·1⁻¹ higher than the theoretical sulfate level calculated from salinity, showing the active oxidation of sulfide by phototrophic bacteria. After the dystrophic crisis, oxic conditions were reestablished and the phototrophic bacterial biomass was partly grazed by zoobenthos organisms which densely populated the sediment surface.