Microbial mats in the hypersaline ponds of Mediterranean salterns (Salins-de-Giraud, France)

Abstract

Microbial mats that develop in the gypsum crust of the hypersaline ponds of Salins‐de‐Giraud (Camargue, France) were carefully investigated between 1989 and 1991. During the warm seasons, when these mats were fully 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 μmol cm−3 h−1 during the maximum daylight whereas the oxidation of sulfide in the light was calculated to be 12.7 μmol cm−3 h−1, i.e. 300 to 180 mmol m−2 day−1 assuming 8–10 hours of constant daylight and a sulfide oxidation zone of 3 mm in thickness. 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−1 originating from sulfate reduction which takes place in the 6 cm depth horizon of sediment plus mat. According to the amounts of sulfate precipitated at the sediment surface in the form of gypsum, sulfate reduction is never limited and was found to be among the highest values reported 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 from the mats. These latter mats were composed of 2 to 3 laminated layers of phototrophic organisms: an upper brown layer of the cyanobacterium Aphanothece, an intermediate green layer of the cyanobacterium Phormidium and an underlying red layer of purple sulfur‐oxidizing bacteria from which two new halophilic species were isolated (Chromatium salexigens and Thiocapsa halophila). It has been found that the accumulated sulfide is oxidized not only by the 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.

Diversity of cyanobacteria in man-made solar saltern, Petchaburi Province, Thailand - a pilot study.

“…Caumette et al (1994) report the species compositions of microbial mats from a Mediterranean saltern in France during the warm season. The top brown layer was composed of the unicellular cyanobacterium Aphanothece spp.…”

Saltern evaporation ponds as model systems for the study of primary production processes under hypersaline conditions

“…Such microelectrodes have been extensively used in photosynthesis studies of hypersaline microbial mats in saltern evaporation ponds (Canfield & Des Marais 1993, Caumette et al 1994, Des Marais 1995. No special problems are reported to have been caused by the hypersaline nature of the medium that may necessitate special modifications in the construction or calibration of such electrodes.…”

“…Below the cyanobacterial layers, a purple layer of anoxygenic phototrophic sulfur bacteria (Halochromatium, Ectothiorhodospira and related organisms) is often found. Especially beautiful are the layered communities of orange and dark-green Cyanobacteria and purple sulfur bacteria found embedded within the gypsum layers that precipitate at intermediate salinities (Caumette et al 1994). Due to the elevated salinity, grazing is reduced, enabling the development of stratified benthic microbial mats that may often reach several centimeters in thickness.…”

Effects of Elevated Carbon Dioxide and Salinity on the Microbial Diversity in Lithifying Microbial Mats

“…Even when individual phyla and subphyla (i.e., Actinobacteria, Bacteroidetes, Cyanobacteria, Deltaproteobacteria) were removed from the Fast UniFrac dataset (data not shown) the overall shift in the community structure in response to salinity was present indicating that salinity imposes a widespread affect on lithifying mat organisms. Of the many taxa to be impacted by the increase in salinity the Alphaproteobacteria showed a change in relative abundance of key taxa including an enrichment of Rhodospirillales (Figure 3), a group of purple-sulfur phototrophs that has been shown to be enriched in many hypersaline habitats [79,80]. Salinity has also been previously shown to significantly impact the metabolic activity of hypersaline mats under gradients and flux conditions [23,77,81].…”