MICROBIAL MEDIATION OF CARBONATE PRECIPITATION: SATURATION INDEX, EXOPOLYMERIC SUBSTANCES AND COMMUNICATION
In understanding the interactions between photoautotrophs (predominantly cyanobacteria), aerobic heterotrophs, fermenters, sulfate-reducing bacteria and sulfide oxidizing bacteria and their role in forming a permanent sedimentary record, three processes emerge: 1) geochemical alteration of the sediment porewater resulting from the metabolic activity of the above functional groups; 2) production by cyanobacteria and other microbes of exopolymeric substances (EPS), though which the environment is engineered, and coupled to this, physicochemical and biological modification of EPS; 3) cell-to-cell signaling (or quorum sensing), through which the microbial community adapts the metabolic activity and physiological response to environmental conditions.
Strong diel fluctuations in geochemical gradients typical for mats result in community metabolism that favors either precipitation or dissolution of calcium carbonate. The net precipitation in lithifying mats is impacted by sulfate-reducing bacteria, which play a critical role in tipping the geochemical balance. EPS production is also important in the precipitation process: calcium binding occurs in freshly produced EPS and degradation of the EPS matrix yields nucleation sites. Finally, highly favorable conditions for growth often coincide with highly unfavorable conditions. As a result, coordination of metabolism within and between individual microbial species and possibly between species is needed. This may be accomplished by chemical signaling compounds, including acylated homoserine lactones (AHL). These AHL are subject to chemical destruction at elevated pH, which typically coincides with the high rates of photosynthesis found in mats. We will present field and lab observation in support of a conceptual model for calcium carbonate precipitation in which the microbial ecology drives the sedimentology.