Discriminating the Role of Photosynthetic and Heterotrophic Microbes Triggering Low-Mg Calcite Precipitation in Freshwater Biofilms (Lake Geneva, Switzerland)

Abstract

Field and laboratory experiments in freshwater Lake Geneva (Switzerland) have recently demonstrated a substantial role of biofilms triggering the formation of ooids. However, the precise function and specificity of the microbes involved in this process remained elusive. In this study we compared biofilms lining ooid depressions in the natural environment with those grown under laboratory conditions in a BG11 media. Confocal microscopy, SEM and TEM as well as PCR-DGGE results have been used to accomplish this comparison at both microscopical and geochemical levels, respectively. Carbonate precipitates in the natural environment show that cyanobacteria dominate the microbial community in association with green algae and diatoms all embedded within the EPS. Analogously, low-Mg calcite crystals were formed in the laboratory experiments under light conditions. These crystals are found within the EPS and always associated with photosynthetic microorganisms. The latter in concert with the lack of evidence for the presence of sulfate-reducers, allow us to postulate that photosynthetic activity through increasing pH in EPS is the main factor triggering low-Mg calcite precipitation in Lake Geneva.