Water chemistry, bacterial abundance and anoxygenic photosynthetic bacteria in an estuarine blue hole, Andros Island, The Bahamas

Blue Holes of Andros Island, The Bahamas are chemically extreme, highly reducing environments with hydrogen sulfide (H₂S) concentrations reaching 18 mM, the highest reported for my marine-salts system. The karstic topography, created by moving seawater and slightly acidic rainfall, resulted in sinkholes filled with seawater at depth and surface fresh water. Numerous blue holes on Andros Island include completely marine forms as well as estuarine and inland systems. This research focused on Tarpon Blue Hole (TBH), 7m deep and 40 m in diameter, located in the shallow, near-shore zone of Stafford Creek estuary. Despite limited depth, TBH was usually highly stratified with a salt driven pycnocline varying from about 3.5 to 5.0 m deep. Beneath the pycnocline the water column was anoxic, with (H₂S) ranging up to 3.5 mM, and temperatures up to 41°C. TBH, which is affected by tidal and wind forces, was physically and chemically much more dynamic than inland blue holes, even to the extent that it completely destratified twice, May 1992 and January 1998, during the study. It restratified in less than six months in each case and returned to its anoxic, highly sulfidic condition. Bacteria were very abundant (100 × 106 ml−1) in the deep water. A red layer of presumptive purple sulfur bacteria (PSB) was always present at depths of about 4.5 to 5.5 m when TBH was stratified. PSB dominate planktonic, primary production with photosynthetic rates ten times higher in the “red layer” then in the epilimnion. Enrichment cultures from this “red layer” were established, and extinction dilutions resulted in a pure culture that was microscopically and physiologically identified as a (presumptive) purple sulfur bacterium. Using refined protocols for extraction of molecular 16S rDNA, PCR and DNA sequencing resulted in its identification as a novel strain of Marichromatium purpuratum TAR (http://www.ncbi.nlm.nih.gov: GenBank Accession Number AF294030). This is the first report of M. purpuratum from an anoxic, estuarine water column, and of an anoxygenic, photosynthetic bacterium from Bahamian blue holes. Biolog® Anaerobe Microplates indicate that this strain metabolizes: fructose, fucose, galaturonic acid, palatinose, ketobutyric acid, pyruvic acid, ramnose, glyoxylic acid, and succinic acid.