EXOPOLYMERIC SUBSTANCES IN HYPERSALINE MICROBIAL MATS: THEIR POTENTIAL ROLE IN CALCIUM CARBONATE MINERALS PRECIPITATION

Exopolymeric substances (EPS) that constitute biofilms, likely represent the habitat for most living microorganisms. These EPS are mainly comprised of polysaccharides, but also include non-carbohydrate moieties such as pyruvate and succinate, as well as inorganic moieties such as sulfate or phosphate. In microbial mats, which are highly productive layered sedimentary microbial communities, EPS are particularly important. In such communities, EPS provide a cohesive matrix where many biogeochemical reactions take place. Studies on lithifying microbial mats have emphasized the role of EPS in the precipitation of calcium carbonate minerals.

Three main processes characteristic of EPS matrices have been shown to control the precipitation of carbonate minerals: (1) if calcium concentration exceeds the EPS binding capacity, precipitation will occur inside the EPS matrix due to local super-saturation; (2) self (re-)arrangement of acidic functional groups in the EPS matrix may create a template that favors the nucleation of carbonate minerals; (3) degradation of EPS by heterotrophic bacteria will contribute to the release of calcium, thus increasing the saturation index of carbonate minerals.

This study investigates the amount, the chemical properties, and the degradation of natural EPS. Depth profiles through different microbial mats reveal decreasing amounts of EPS in the sediment. In addition, potentiometric and calcium titration show that EPS buffering properties and calcium binding capacities are altered with depth. Finally, slurry experiments show that EPS stimulates aerobic and anaerobic respirations. Slurry experiments and EPS depth profiles suggest that substantial amounts of EPS produced at the surface of the mat are degraded in the underlying sediment, potentially releasing Ca and enhancing carbonate minerals formation. The titration data support this hypothesis showing a decreasing buffering capacity of EPS from deeper sediment layers.