Ooids represent more than 90% of modern shallow water sediments in the western part of Lake Geneva, Switzerland. Previous investigations showed the presence of biofilms lining depressions on ooidal surfaces and further appointed them as starting sites for low Mg calcite cortex formation. The factual processes behind the formation of these endogenic carbonates were still unknown. This thesis assembles the results of a detailed microbiological study combining field and laboratory experiments that confirm a substantial role of biological versus purely physicochemical processes triggering ooid formation. An experimental device was designed to harvest and study in laboratory the biofilms involved in ooids cortex formation. Biofilms lining ooids depressions and in situ harvested biofilms communities were compared using microscopical tools (natural, ultraviolet light, confocal microscope and TEM) and a PCR-DGGE approach. Microscopical observations show that these biofilms are mainly composed of photosynthetic microorganisms associated to low- Mg calcite precipitation. The microbial community is composed of different green algae, coccoid and filamentous cyanobacteria and diatoms which are embedded in EPS produced by the living bacteria. Low Mg calcite crystals are found mostly within EPS. Results obtained by PCR/DGGE are in agreement with microscopical investigations validating the in situ experiment and confirming a similar microbial diversity between the ooids and the harvested biofilms from the glass slides.