1. ¹Department of Microbiology and Cell Science, Space Life Sciences Laboratory, University of Florida, Kennedy Space Center, FL, USA
2. ²Department of Oceanography, Florida State University, Tallahassee, FL, USA
3. ³Department of Biology, Boston University, Boston, MA, USA
4. ⁴Rosenstiel School of Marine and Atmospheric Science, University of Miami, Miami, FL, USA
5. ⁵Exobiology Branch, NASA Ames Research Center, Moffett Field, CA, USA

Correspondence: Dr JS Foster, Department of Microbiology and Cell Science, Space Life Sciences Laboratory, University of Florida, Building M6-1025, Room 234, Kennedy Space Center, FL 32899, USA. E-mail: jfoster@ufl.edu

⁶Current address: Department of Biological Sciences, Purdue University, West Lafayette, IN 47906, USA.

Received 28 October 2008; Revised 28 November 2008; Accepted 28 November 2008; Published online 15 January 2009.

Abstract

Stromatolites are sedimentary deposits that are the direct result of interactions between microbes and their surrounding environment. Once dominant on ancient Earth, actively forming stromatolites now occur in just a few remote locations around the globe, such as the island of Highborne Cay, Bahamas. Although the stromatolites of Highborne Cay contain a wide range of metabolically diverse organisms, photosynthetic cyanobacteria are the driving force for stromatolite development. In this study, we complement previous morphological data by examining the cyanobacterial phylogenetic and physiological diversity of Highborne Cay stromatolites. Molecular analysis of both clone and culture libraries identified 33 distinct phylotypes within the stromatolites. Culture libraries exhibited several morphologically similar but genetically distinct ecotypes, which may contribute to ecosystem stability within the stromatolites. Several of the cultured isolates exhibited both a positive phototactic response and light-dependent extracellular polymeric secretions production, both of which are critical phenotypes for stromatolite accretion and development. The results of this study reveal that the genetic diversity of the cyanobacterial populations within the Highborne Cay stromatolites is far greater than previous estimates, indicating that the mechanisms of stromatolite formation and accretion may be more complex than had been previously assumed.