Microbial Signatures in Ooids from the Bahamas

Microbes are abundant in sedimentary systems where their metabolic capabilities can exert a profound impact on carbonate precipitation processes by altering the alkalinity of their immediate surrounding. Using a combination of clone analysis of 16SrRNA, functional gene analysis and both inorganic and organic stable isotopic analyses, we characterized the microbial community structure of ooids and their potential functional capabilities that could lead to precipitation of carbonates. Oolitic bacterial communities were highly diverse, representing 12 different prokaryotic lineages, among which Alphaproteobacteria, Gammaproteobacteria, Actinobacteria/Bacteroidetes and Deltaproteobacteria were the most abundant. Based on functional gene analysis, a large number of genes were associated with redox dependent microbial communities with putative functional capability for mineral precipitation such as aerobic/anoxygenic photosynthesis, denitrification, ammonification, and sulfate reduction. In addition, a broad diversity of genes related to organic carbon degradation and nitrogen fixation were present, implying metabolic plasticity that enables survival under oligotrophic conditions. Carbon and nitrogen isotopic analyses, which were conducted on both bulk and intracrystalline organic matter as well as in leachate sediments, identified geochemical signatures of microbial activity. δ13C values for organic C in the bulk (-11.94 to -16.71) and intracrystalline organic matter (-12.37 to -17.66), were similar and within the range of fractionation patterns associated with cyanobacteria, algae and photosynthesizers that employ the C4 carbon fixation pathway. Nitrogen isotopic values for both bulk (δ15N: -0.314 to - 0.706) and intracrystalline organic matter (δ15N: -0.343 -1.70) also showed fractionation patterns consistent with nitrogen fixation. In addition, positive δ15N and δ18O values of the NO3- leached from the ooids provided evidence of denitrification. These findings indicate that the genesis of ooids, marine cements, and carbonate muds in the supersaturated carbonate waters in the Bahamas are likely the product of intertwined abiotic and biotic factors.