Biogeochemistry of Microbialites

Microbialites are organosedimentary structures formed by microbial communities through binding and trapping and/or in situ precipitation. They have a wide array of distinctive morphologies and a geologic record extending back 3.45 billion years. The origin of morphological variability is hotly debated and elucidating the origins of microfabric differences provides insights into ecosystem functioning and biogeochemistry during much of Earth’s history. We have been studying the role of eukaryotes, and particularly the foraminifera, in modifying microbialites fabrics and impacts on modern microbialites. We propose that the microbialite fossil record, and the demise of stromatolites in the Neoproterozoic may reflect the impact of the concurrent radiation of these protists.

While studying the microbialites of Highborne Cay, in the Bahamas, we became intrigued by the carbonate component which is mostly comprised of ooids. These are small, spherical to ellipsoidal grains characterized by concentric layers of calcium carbonate and organic matter. These sand-sized particles are incorporated with the aid of extra-cellular polymeric substances (EPS), into the matrix of laminated stromatolites and clotted thrombolite mats.

Fossil ooids, oolites, are enigmatic components of the carbonate fossil record including those of the Neoproterozoic. Of particular note, are recent observations that oolitic preserve C-isotopic records of Neoproterozoic environmental change and carbon cycle anomalies.

Based on analyses of clone libraries of small subunit ribosomal RNA gene fragments and lipid biomarkers, we found that the microbial community colonizing ooid sands was virtually indistinguishable from the community in stromatolites themselves. We also found that ooids from Cat and Andros islands in the Bahamas and from Carbla Beach in Hamelin Pool, Western Australia, preserve abundant and notably similar distributions of hydrocarbons, fatty acids (FAs) and alcohols.