Microbial Diversity in Modern Stromatolites

Poised at the biosphere–lithosphere interface, the microbial consortia associated with stromatolites have a profound impact on the evolution of Earth’s environment. In this chapter, we review the current state of knowledge of microbial diversity in extant stromatolites by examining data generated using cultivationindependent molecular techniques. Specifically, we compare natural stromatolitic mat systems of three distinctive habitats: the hypersaline pools of Shark Bay, Australia; the open ocean stromatolites of Highborne Cay, Bahamas; and the lacustrine lagoons of Ruidera Pools, Spain. We compare these natural systems to an experimental artificial microbialite, looking for fundamental differences and similarities within the microbial communities. Of the 21 bacterial phyla or sub-phyla detected in the various stromatolite ecosystems, only Cyanobacteria were found dominant in all habitats. Within the phylum, few cyanobacterial ecotypes were common to all ecosystems. The marine and hypersaline stromatolite ecosystems had significantly higher bacterial diversity than did the artificial microbialite or the freshwater stromatolite, and the diversity approached that observed in non-lithifying hypersaline microbial mats. Finally, we consider the ecological insights provided by the acquisition of metagenomic sequence data for understanding stromatolite diversity and function. These high-throughput metagenomic sequencing approaches have been applied to modern stromatolitic and microbialitic mat communities and have facilitated a higher resolution characterization of microbial diversity at the molecular-level, thus providing an initial glimpse into the functional complexity of these dynamic ecosystems.