MICROBIAL MATS IN THE HYPERSALINE PONDS OF MEDITERRANEAN SALTERNS (SALINS-DE-GIRAUD, FRANCE)

Microbial mats that develop in the gypsum crust of the hypersaline pon ds of Salins-de-Giraud (Camargue, France) were carefully investigated between 1989 and 1991. During the warm seasons, when these mats were f ully developed, analyses of microbial activities and microprofiles of oxygen and sulfide have shown a great activity of the different kinds of bacteria found in the mat below the gypsum crust. Oxygen production could amount to 2 mu mol cm(-3) h(-1) during the maximum daylight whe reas the oxidation of sulfide in the light was calculated to be 12.7 m u mol cm(-3) h(-1), i.e. 300 to 180 mmol m(-2) day-l assuming 8-10 hou rs of constant daylight and a sulfide oxidation zone of 3 mm in thickn ess. This sulfide oxidation consumes about 65-95% of the diel sulfide production which has been estimated to be 400 to 450 mmol m(-2) day-l originating from sulfate reduction which takes place in the 6 cm depth horizon of sediment plus mat. According to the amounts of sulfate pre cipitated at the sediment surface in the form of gypsum, sulfate reduc tion is never limited and was found to be among the highest values rep orted in the literature (average value of 8200 nmols cm(-3) day(-1)). Completely covered by the gypsum crust, this ecosystem has been found to react as a closed system. Consequently, the sulfide does not escape and accumulate below the crust. It was detected up to the top of the mat after a few hours of darkness. It is reoxidized during the day by the photosynthetic organisms that form the mats. These latter mats wer e composed of 2 to 3 laminated layers of phototrophic organisms: an up per brown layer of the cyanobacterium Aphanothece, an intermediate gre en layer of the cyanobacterium Phormidium and an underlying red layer of purple sulfur-oxidizing bacteria from which two new halophilic spec ies were isolated (Chromatium salexigens and Thiocapsa halophila). It has been found that the accumulated sulfide is oxidized not only by th e phototrophic bacteria in the sulfide oxidation zone but also by the oxygen produced by the cyanobacteria which are able to photosynthesize in the presence of sulfide.