A BIOGEOCHEMICAL INVESTIGATION OF THE ECOSYSTEM AND THE MICROBIALITES IN STORR'S LAKE, SAN SALVADOR ISLAND, BAHAMAS

Storr’s Lake in San Salvador Island, Bahamas, is an inland, hypersaline lake, harboring lithified, organo-sedimentary structures called microbialites. Salinity, pH, redox chemistry, water level and chemical composition of the lake fluctuate due to rain events and high rates of evaporation. The sulfur concentration is around 3.3 times that of seawater suggesting that sulfur might be actively produced in this environment. Suspended planktonic material including algae, bacteria, dinoflagellates and organic material contribute to the turbidity of the lake, while also restricting light penetration. Five different microbialite types from different depths in the lake and ooze material from the lake bottom were collected during two visits to the lake. The microbialites were described morphologically as calcerous knobs, plateau, pinnacle mound, sharpy and mushroom shaped, respectively. Pinnacle mound type was found at the deepest point transected. A well laminated, thick, leathery, ‘cheesecake’ microbial mat was found distributed on top of the ooze in most locations. Scanning electron microscopy investigations revealed the presence of angular, blocky and needle shaped Mg-calcite and possibly aragonite, with re-crystallized cyanobacterial filaments and remains of exopolymeric substances. X-ray diffraction analysis of four of the microbialites indicated the presence of Mg-calcite as the dominant mineral constituent, while aragonite was present only in the pinnacle mound and plateau types. A depth-wise profile of these two microbialite types indicated that the aragonite/calcite ratio increased from the top of these microbialites to the bottom.

Carbonate precipitation and lithification process in microbialites is generally driven by the photosynthetic activity of cyanobacteria, along with other microbial metabolisms. We initially speculated that cyanobacterial growth would be restricted in the deeper microbialites of Storr’s Lake due to the absence or limited availability of light at those depths. On the contrary, light intensity measurements revealed that sunlight, though at very low intensities, does penetrate to the bottom of the lake, indirectly indicating that cyanobacterial growth in the deeper microbialites is plausible.