Biogeochemical Stratification and Carbonate Dissolution-Precipitation in Hypersaline Microbial Mats (Salt Pond, San Salvador, The Bahamas)

Microbial mat communities host complex biogeochemical processes and play a role in the formation of most carbonate rocks by influencing both carbonate precipitation and dissolution. In this study, the biogeochemistry of microbial mats from the hypersaline Salt Pond, San Salvador, Bahamas are described using scanning electron microscopy, X-ray diffraction, microelectrode profiling, fatty acid methyl esters, and carbon and nitrogen analyses. These microbial mats are distinctly layered both chemically and with regard to composition of microbial community, where significant (ρ < 0.05) differences are noted between layers and cores. Furthermore, an oxic upper zone and an H 2 S-rich lower zone dominate the Salt Pond microbial mats, where H 2 S concentrations were measured approaching 8 mM. The high H 2 S concentrations along with the lacking evidence of mineral precipitation in SEM images point to the prevalence of carbonate dissolution. Moreover, the high concentrations of organics (3–9%) reveal that the mats are self-sourcing and can provide ample fuel to sustain the highly active heterotrophic (both aerobic and anaerobic) metabolism. Seasonal differences in sulfide and oxygen concentrations in Salt Pond mats indicate that the carbonate dissolution and precipitation reactions are dynamic in this hypersaline lake.