DEVELOPMENT OF MODERN MARINE STROMATOLITES IN ECOLOGIC REFUGIA ON THE MARGINS OF EXUMA SOUND, BAHAMAS

Modern marine stromatolites are uncommonly common on the margins of Exuma Sound, Bahamas. They develop in intertidal and subtidal settings, forming columnar structures up to two meters high in tidal channels and fringing reefs. Growth of these stromatolites represents a dynamic balance between sediment accretion and lithification of microbial mats. Lithification is linked to two important microbial processes 1) photosynthetic production by cyanobacteria and 2) heterotrophic respiration by bacteria.

A laminated microstructure is formed by repeated transitions between three distinct prokaryotic surface communities, each of which forms a distinct microstructure. A community of filamentous cyanobacteria forms unlithified layers of trapped and bound sand grains. A biofilm community dominated by heterotropic bacteria forms micritic crusts, 20-50 µm thick. A community of coccoid endolithic cyanobacteria forms cemented layers of fused micritized grains. Preservation of alternating microstructures in the subsurface creates a laminated microfabric.

Colonization of the stromatolite surface by eukaryotic algae, such as Batophora, Gracilaria, and heterogeneous algal turf, disrupts the cyclic pattern of layers and inhibits formation of a laminated microstructure. As a result, stromatolite growth in these modern marine settings is confined to ecologic refugia where eukaryotic colonization is limited by frequent episodes of sediment inundation.

These findings indicate that competition with plants is of critical importance in the development and persistence of Phanerozoic stromatolites. Stromatolite-forming microbes are common in many modern reef environments, but form laminated buildups only where abiotic stresses reduce competition with algae and metazoans. Ongoing research by the Research Initiative on Bahamian Stromatolites (RIBS) project is investigating interactions between environmental parameters, microbial population dynamics, and mineral formation in the three billion-year-old stromatolite ecosystem.