Assessing the role of microbial interactions in freshwater low Mg calcite ooids cortex formation
|Location||International Geological Congress,oslo 2008|
|Author||Plée, Karine۱; Ariztegui, Daniel۱; Sahan, Emel۲; Pacton, Muriel۱; Dittrich, Maria۳|
|Holding Date||27 September 2008|
More than 90% of modern shallow water sediments in the western part of Lake Geneva are composed of ooids. 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. Recently, a detailed microbiological study combining field and laboratory experiments have confirmed 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 (Plée et al., 2008). Biofilms lining ooids depressions and in situ harvested biofilms communities were compared using microscopical tools (natural, ultraviolet light, confocal microscope and TEM) and PCR-DGGE approach, including two different primers to target all bacteria, and cyanobacteria and diatoms (341F-GC/907RM and 359F-GC/781 (a+b), respectively).
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 within EPS and on the cell wall of cyanobacteria. Results obtained by PCR/DGGE method 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.
As in the natural ooids, low Mg calcite precipitates in the biofilms of both the in situ experiment and the cultures are always in close association with the photosynthetic microbial community. Furthermore, since there is no evidence for sulfate-reducing bacteria among the identified microbial community, we postulate that in freshwater Lake Geneva cyanobacteria-induced photosynthetic activity is the main factor triggering low Mg calcite precipitation during the initial phase of ooid formation.