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.

Keywords:

• cyanobacteria
• exopolymeric substances
• freshwater ooid
• green algae
• microbial carbonate

We would like to thank Sophie Lavigne from the “Service Cantonal de l’Écologie de l’Eau”, Geneva, for microbes’ identification. We are grateful to Tomaso Bontognali of the Laboratory of Geomicrobiology, ETH-Zürich, for help with the confocal microscope. Yann Floris and Rossana Martini from the University of Geneva are kindly acknowledged for chlorophyll-a measurements and assistance for using the scanning electron microscope, respectively. The authors thank Danielle Jaillard (Service laboratory for electron microscopy, department of cellular biology, Paris-Sud University, France) and Maria Dittrich (ETH Zürich, EAWAG, Kastanienbaum) for TEM analyses. We thank Elias Samankassou for his suggestions concerning English expression and written style. We thank two anonymous reviewers for helpful comments that have improved an early version of the manuscript. This research was partially funded by the Augustin Lombard fellowship to the leading author.