Relationships between microbial activity, mineralogy, and water chemistry in the development of stromatolites of Lake Dziani Dzaha (Mayotte, Western Indian Ocean)

Lake Dziani Dzaha is a halo-alkaline tropical crater lake located on the Mayotte Island (Comoros archipelago, Western Indian Ocean). The lake water presents a permanent green color due to the high concentration and dominance of Cyanobacteria, which mainly belong to the Arthrospira genus. Stromatolites are actively growing near the lake shores. These stromatolites are mainly composed of aragonite with lesser proportions of hydromagnesite, calcite, dolomite and phyllosilicates. They are morphologically and texturally diverse ranging from tabular with a globular surface to conical ones with a smooth surface. In order to evaluate to which extent the microorganisms of Lake Dziani Dzaha may exert an influence on the shape and mineralogy of the stromatolites, a multidisciplinary study has been undertaken which combined microscopic observations with molecular biology and aqueous geochemistry methods. High-throughput sequencing of bacterial and archaeal 16S rRNA gene combined with confocal laser scanning microscopy (CLSM) analysis revealed that the microbial diversity of the mats associated with the stromatolites was clearly distinct from that of the Arthrospira-dominated lake water. These Cyanobacteria belonged to the Pleurocapsales order in the mat associated with the globular crust and to the Oscillatoriales order in the mats associated with the smooth surface. Observations using CLSM, scanning electronic microscopy (SEM) and Raman spectroscopy indicated that the globular texture consists of laminations of aragonite, a magnesium-silicate phase and hydromagnesite. The associated microbial mat, as confirmed by laser microdissection and whole-genome amplification (WGA), is composed of Pleurocapsales coated by abundant filamentous and coccoid Alphaproteobacteria. These potentially phototrophic Alphaproteobacteria promote the formation of aragonite in which they become incrusted. In contrast, the Pleurocapsales are not calcifying but instead accumulate silicon and magnesium in their sheaths, which may be responsible for the formation of the Mg-silicate phase found in the globular crust. We therefore propose that Pleurocapsales and Alphaproteobacteria are involved in the formation of two distinct mineral phases present in the globular texture: Mg-silicate and aragonite, respectively. These results point out the role of phototrophic Alphaproteobacteria in the formation of stromatolites, which may open new perspective for the analysis of the fossil record.