Ooid Accreting Diatom Communities from the Modern Marine Stromatolites at Highborne Cay, Bahamas

Abstract

The modern marine stromatolites at Highborne Cay, Bahamas are inhabited by diverse surface microbial communities. Although these communities are most often dominated by cyanobacteria (e.g., Schizothrix gebeleinii, Solentia sp., Oscillatoria sp. etc.), diatoms can be abundant and have been implicated in stromatolite biogenesis. We have identified two distinct surface diatom communities involved in sediment deposition: (1) a thick (0.5–1 cm) yellow surface layer (yellow fur) of the stalked diatoms Licmophora remulus, L. paradoxa, and Striatella unipunctata, and (2) a colorless cohesive surface layer (“pustular blanket”) formed by a tube-forming diatom with individual cells resembling Navicula. The stalks could be labeled with a lectin–FITC conjugate and observed with confocal scanning laser microscopy (CSLM). Ooids were seen trapped within a well-developed network of stalks at the surface and clusters of stalks oriented vertical to the surface were found to penetrate the subsurface to a depth of several centimeters. Interestingly, the tubular structures formed by the naviculid species did not stain with the lectin–FITC conjugate. Nevertheless, manual manipulations of the sediment indicated that this diatom community also trapped ooids. Rapid sediment accretion could be attributed to these two diatom surface communities. Observations over the past decade as well as intensive monitoring over a 3-year period (2005–2007) indicate, however, that these surface diatom communities occur only at particular times of the year and do not survive burial. In addition, the surface layer of diatom-trapped ooids readily erodes unless stabilized by subsequent infiltration of extracellular polymeric substances (EPS) secreting cyanobacteria (e.g., S. gebeleinii). These observations suggest a limited contribution of diatoms to stromatolite biogenesis.