ORGANIC MATTER MATTERS: PREDICTING PH, ALKALINITY AND MINERAL SATURATION FOR MICROBIAL METABOLISM OF DIFFERENT ELECTRON DONOR COMPOUNDS

In aqueous environments, microbial metabolisms can alter local biogeochemistry and potentially affect mineral precipitation and dissolution, causing sequestration of elements and leading to possible biosignature formation. For example, in the open marine stromatolites at Highborne Cay in the Bahamas, high sulfate-reduction (SR) rates in the zone of calcium carbonate precipitation indicate a possible link between sulfate-reducing bacteria (SRB) and mat lithification. The contribution of SRB to increasing porewater alkalinity in the context of overall community metabolism, the “alkalinity engine”, is often cited as a potential causative factor for precipitation. However, closer inspection of balanced redox equations indicates that while alkalinity increases, pH may actually decrease, causing mineral dissolution. We have shown that different types of electron donor for sulfate-reduction can have drastically different effects on pH and alkalinity, potentially affecting the carbonate mineral saturation index (SI) and resulting in either precipitation or dissolution depending on the specific electron donor compound. The implication is that the types of metabolisms supplying low molecular weight organic compounds to SRB in a mixed community must be considered, and that assumptions of “representative” or “general” organic matter such as (CH₂O)_n should be avoided.

Here, we extend this work beyond sulfate reduction to include other microbial metabolisms, and use theoretical calculations and geochemical modeling to examine whether similar trends might be observed, i.e. whether alkalinity and pH are similarly affected by consumption of specific electron donors. Consideration of these effects and their importance may provide insight into mineral precipitation for other environments.