COCCOID CYANOBACTERIAL “GHOSTS”: IN SITU MICRITIC MG-CALCITE PRECIPITATION WITHIN A MICROBIAL MAT FROM A HYPERSALINE POND, LEE STOCKING ISLAND, BAHAMAS

Microbial mats are laminated organosedimentary structures formed by the growth of complex microbial communities, which trap and bind sediments and precipitate carbonate and other minerals in situ. The microbial communities within thick (>1 cm) microbial mats form interdependent layers (<1-3 mm thick) of vertically stratified photo- and heterotrophic bacteria due to the light and chemical gradients within the mat. This study entails a detailed microscopic examination (on a micrometer scale) of the vertically stratified microbial community from a hypersaline pond on Lee Stocking Island, Bahamas. We characterized the microbial layers and associated mineral precipitates in detail using light microscopy, standard and environmental scanning electron microscopy (SEM and ESEM), and transmission electron microscopy (TEM) equipped with energy dispersive X-ray spectroscopy. The data shows the mat consists of dominantly 5 stratified layers: 1) a gelatinous golden-brown diatom layer, 2) a dense bright-green filamentous cyanobacterial layer, 3) a dark-green coccoid cyanobacterial layer, 4) a red purple-sulfur bacterial layer, and 5) a brown anaerobic or sulfate-reducing bacterial layer. All layers have gradational boundaries and may not always be laterally continuous. The 5 layers repeat the sequence many times with depth in the mat. They vary, however, in their state of metabolic activity and decomposition with depth. Evidence indicates a new set of microbial layers forms either seasonally or annually as the older layers get buried. A very distinctive aspect of mineral formation in the mat occurs in association with dense aggregates of coccoid cyanobacteria. They precipitate very fine grain Mg-calcite that eventually completely entombs the cells and where the cells degrade they leave behind cell-shaped voids or “ghosts”. These “ghosts” can mostly be found deeper in the mat, where the cells have been completely degraded. We postulate a mechanism for micrite formation in this mat and suggest a connection with observations of void-filled micrite in ancient rocks back to the Archean.