Bahamian giant stromatolites: microbial composition of surface mats

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Abstract
Subtidal columnar stromatolites up to 2.5 m high near Lee Stocking Island in the Exuma Cays, Bahamas, have surface mats approximately equally composed of algae and cyanobacteria. The stromatolites are composed of fine–medium oöid and peloid sand. This sediment is supplied to the growing stromatolite surfaces by strong tidal currents which lift grains into suspension and sweep migrating dunes over the columns. The algae include an unidentified filamentous chlorophyte, and numerous diatom species mostly belonging to Mastogloia, Nitzschia and Navicula. The dominant cyanobacteria are two oscillatoriacean species, both probably belonging to Schizothrix. Trapping of sediment is mainly effected by the unidentified chlorophyte which is veneered by epiphytic diatoms. Grains are bound into a mucilaginous mat composed of diatoms and cyanobacteria. Cyanobacteria alone would not be able to trap and bind coarse sediment so effectively in this environment. In being coarse-grained and having a significant eualgal component to their mats, these stromatolites are similar to subtidal columnar stromatolites at Shark Bay, Western Australia. The Lee Stocking stromatolites are physically stressed by high velocity tidal currents and mobile sediment. The Shark Bay stromatolites are stressed by hypersalinity. In both cases stress deters grazers, encrusters and bioeroders. These coarse-grained eualgal stromatolites contrast with micritic and predominantly prokaryotic stromatolites of most Recent marine environments, and are not analogues for most pre-Phanerozoic stromatolites. They appear to be a response to changing stromatolitic mat components in the Cenozoic.