Bacterially mediated precipitation in marine stromatolites

Stromatolites are laminated, lithified (CaCO3) sedimentary deposits formed by precipitation and/or sediment accretion by cyanobacterial-bacterial mat communities. Strometolites have been associated with these communities as f ar back as the Precambrian era some 2+ billion years ago. The means by whic h microbial communities mediate the precipitation processes have remained u nclear, and are the subject of considerable debate and speculation. Two alt ernative explanations for microbially mediated precipitation include: (i) c yanobacterial photosynthesis increases pH in a system supersaturated in res pect of CaCO3, resulting in CaCO3 precipitation and then laminated lithific ation, and (ii) decomposition of cyanobacterial extracellular organic matte r (e.g. sheaths, mucilage and organic acids) by microheterotrophs leads to release of organic-bound Ca2+ ions and CaCO3 precipitation. We evaluated th ese explanations by examining metabolically active, lithifying stromatoliti c mat communities from Highborne Gay, Bahamas, using microautoradiography. Micro-autoradiographic detection of (CO2)-C-14 fixation and H-3 organic mat ter (D-glucose and an amino acid mixture) utilization by photosynthetically active cyanobacteria and microheterotrophs, combined with community-level uptake experiments, indicate that bacteria, rather than cyanobacteria are t he dominant sites of CaCO3 deposition. In the oligotrophic waters in which stromatolites exist, microheterotrophs are reliant on the photosynthetic co mmunity as a main source of organic matter. Therefore, autotrophic producti on indirectly controls microbially mediated precipitation and stromatolite formation in these shallow marine environments.