Physical, chemical, and microbiological characteristics of microbial mats (KOPARA) in the South Pacific atolls of French Polynesia

Microbial mats that develop in shallow brackish and hyposaline ponds in the rims of two French polynesian atolls (Rangiroa and Tetiaroa) were intensiv ely investigated during the past three years. Comparative assessment of the se mats (called kopara in polynesian language) showed remarkable similariti es in their composition and structure. Due to the lack of iron, the color o f the cyanobacterial pigments produced remained visible through the entire depth of the mats (20-40 cm depth), with alternate green, purple, and pink layers. Profiles of oxygen, sulfide, pH, and redox showed the anoxia of all mats from a depth of 2-3 mm. Analyses of bacterial pigments and bacterial lipids showed that all mats consisted of stratified layers of cyanobacteria (mainly Phormidium, Schizothrix, Scytonema) and purple and green phototrop hic bacteria. The purple and green phototrophic bacteria cohabit with sulfa te reducers (Desulfovibrio and Desulfobacter) and other heterotrophic bacte ria. The microscopic bacterial determination emphasized the influence of sa linity on the bacterial diversity, with higher diversity at low salinity, m ainly for purple nonsulfur bacteria. Analyses of organic material and of ex opolymers were also undertaken. Difference and similarities between mats fr om geomorphological, microbiological, and chemical points of view are discu ssed to provide multicriteria of classification of mats.