MYSTERIOUS MICROBES IN MOUNDS: CYANOBACTERIAL COMMUNITIES AND TEXTURE IN MODERN BAHAMIAN STROMATOLITES

Stromatolites were the dominant community in the fossil record for most of Earth history, and are thus our keys to understanding the interactions between life and the environment on early Earth. The inherent difficulty in making this connection is the lack of information regarding the complex microbial community that once existed within the living stromatolite. The discovery of modern structures resembling stromatolites has provided the opportunity to investigate the relationship between the modern microbial community, the environment, and the potentially preservable morphology and original texture of the stromatolite mound. With this understood, we will be closer to resolving whether modern stromatolites are analogs for ancient ones, as well as better understanding stromatolites as paleoenvironmental indicators.

Through a combination of field observations and lab experiments, this project begins to address the relationship between the microbial communities and textures within stromatolites off of Lee Stocking Island, Bahamas. Surficial soft cores and internal rock cores were collected from stromatolites in the tidal channel between Exuma Sound and the shallow Great Bahama Bank. Location of cores relative to water flow was noted, so correlation between flow and either texture or microbial community could be made. Textures of the soft cores were described as to cohesiveness, color, cement, sand grain condition, and density and apparent health of bacterial sheaths, providing a look at initial lithification. Textures of the hard rock cores are described within the geologic framework used to examine fossil stromatolites to elucidate the history of these mounds as stromatolites, thrombolites, or otherwise. Cyanobacteria, the primary producers of the mounds' ecosystem, were subsampled from the soft surficial cores and are being isolated and identified in culture. Cultures are used in lab simulations in which salinity, light, and depth of burial are varied to determine differing optimal growing conditions and motility capabilities of the various types of cyanobacteria. This will provide information regarding the vertical stratification of the cyanobacteria, which may have consequences for laminated textures, and will help constrain the preferred environmental conditions for these mounds.