ESEM-EDS AND XRD STUDY OF MICROMINERALOGICAL LAYERING IN A MICROBIAL MAT FROM A HYPERSALINE POND ON LEE STOCKING ISLAND, BAHAMAS: FORMATION OF CELESTITE IN MICROBIAL EXOPOLYMERS

Lee Stocking island is located on the westward bank of Exuma Sound in the Bahamas. It bears a small hypersaline pond which hosts an extensive microbial mat which covers the bottom and is up to 7 centimetres thick. Its clearly defined coloured layers, each 1-3 mm in thickness, are made up of dominant microbial types and their associated minerals. Both the dominant cell types and the minerals occur as specific layers with depth in the mat. It is the layering of mineral types which was the focus of this study. Microscale (layer by layer) examination of the mat by environmental scanning electron microscopy (ESEM), energy dispersive x-ray spectroscopy (EDS) and XRD, coupled with light microscopy illustrated the trends in mineralogy with depth in the mat. The top of the mat, as it is exposed on the pond bottom, commonly consists of a gypsum crust, immediately underlain by a layer of diverse cyanobacteria, the most easily identifiable being Microcoleus chthonthoplastes with its multiple filaments within a common sheath. Below are different types of unicellular cyanobacteria showing Mg-calcite mineral formation. The Mg-calcite layers, corresponding to the presence of unicellular cyanobacteria, alternate with gypsum-rich layers where filamentous cyanobacteria are the most obvious microbial type. Deeper in the mat, at depths of approximately 2 cm, 3.5 cm, and 5.5 cm are layers rich in celestite (SrSO4) which has formed in the exopolymers surrounding large spherical purple sulfur bacteria present as dense microcolonies. Like the celestite layers, the Mg-calcite and the gypsum layers are repeated, each with their associated microbial type so that, within the 6.5 cm thick mat examined, there is a repeated set of layers, possibly due to seasonal evaporation of the pond water and subsequent regrowth of the mat upon rehydration. This study presents a rare view of a thick well-developed microbial mat by ESEM, a technique which allows us to see the microbial mat in its natural, fully hydrated state and allows elemental analysis to be performed on selected mineral constituents within, the identity of which was confirmed by XRD. Our data shows that mineral formation in this mat is mediated by microorganisms rather than being simple trapping and binding of pre-fromed minerals.