The Role of EPS Properties and Turnover in Carbonate Mineral Nucleation

Calcium carbonate precipitation results from alterations in alkalinity (coupled to the saturation index) caused by microbial metabolism or physicochemical changes. In addition, extracellular substances (EPS) produced by the microbial community play an important role in carbonate precipitation: both properties and turnover are controlling carbonate nucleation and growth. EPS consist of macromolecules with various negatively-charged acidic groups, which complex strongly cations, potentially inhibiting CaCO3 precipitation. This inhibition can be overcome through EPS degradation or oversaturation of the EPS binding capacity.

We characterized EPS from bacterial cultures and natural microbial mats, focusing on sulfate-reducing bacteria (SRB), because of their demonstrated role in carbonate precipitation. Our studies indicated that EPS from SRB exhibited buffering capacities corresponding to carboxyl acids, sulfur-containing groups (thiols, sulfonic, sulfinic acids) and amino groups. The calcium-binding capacity ranged from 0.12-0.15 g Ca/g EPS. The properties and microbial turnover of EPS were measured in hypersaline microbial mats (Eleuthera, Bahamas). Depth profiles of enzyme activities and oxygen indicated that EPS turnover mainly occurred within the top 15mm. Below 15-20mm, EPS content and enzyme activity decreased drastically. Experiments using homogenized mats revealed that aerobic and anaerobic respiration was rapidly stimulated by the addition of low molecular weight organic carbon, sugar monomers and different types of EPS. The labile EPS fraction (5-8%) was consumed rapidly and the refractory EPS still contained a large amount of calcium (50% of that present in fresh EPS). This calcium was tightly bound to EPS as it remained after ten dialyses steps.

We present a carbonate nucleation model in which low-molecular weight organic carbon is complexed by the tightly-bound calcium and released upon enzyme activity. This calcium is then available for carbonate nucleation, provided suitable alkaline conditions exist. This model indicates that microbial production and consumption of EPS are key players in microbial mat lithification.