A Study of the Microbial Spatial Heterogeneity of Bahamian Thrombolites Using Molecular, Biochemical, and Stable Isotope Analyses.

Thrombolites are buildups of carbonate that exhibit a clotted internal structure formed through the interactions of microbial mats and their environment. Despite recent advances, we are only beginning to understand the microbial and molecular processes associated with their formation. In this study, a spatial profile of the microbial and metabolic diversity of thrombolite-forming mats of Highborne Cay, The Bahamas, was generated by using 16S rRNA gene sequencing and predictive metagenomic analyses. These molecular-based approaches were complemented with microelectrode profiling and in situ stable isotope analysis to examine the dominant taxa and metabolic activities within the thrombolite-forming communities. Analyses revealed three distinctive zones within the thrombolite-forming mats that exhibited stratified populations of bacteria and archaea. Predictive metagenomics also revealed vertical profiles of metabolic capabilities, such as photosynthesis and carboxylic and fatty acid synthesis within the mats that had not been previously observed. The carbonate precipitates within the thrombolite-forming mats exhibited isotopic geochemical signatures suggesting that the precipitation within the Bahamian thrombolites is photosynthetically induced. Together, this study provides the first look at the spatial organization of the microbial populations within Bahamian thrombolites and enables the distribution of microbes to be correlated with their activities within modern thrombolite systems. Key Words: Thrombolites-Microbial diversity-Metagenome-Stable isotopes-Microbialites. Astrobiology 17, 413-430.

Characterization of the stromatolite microbiome from Little Darby Island, The Bahamas using predictive and whole shotgun metagenomic analysis.

Modern stromatolites represent ideal ecosystems to understand the biological processes required for the precipitation of carbonate due to their long evolutionary history and occurrence in a wide range of habitats. However, most of the prior molecular work on stromatolites has focused on understanding the taxonomic complexity and not fully elucidating the functional capabilities of these systems. Here, we begin to characterize the microbiome associated with stromatolites of Little Darby Island, The Bahamas using predictive metagenomics of the 16S rRNA gene coupled with direct whole shotgun sequencing. The metagenomic analysis of the Little Darby stromatolites revealed many shared taxa and core pathways associated with biologically induced carbonate precipitation, suggesting functional convergence within Bahamian stromatolites. A comparison of the Little Darby stromatolites with other lithifying microbial ecosystems also revealed that although factors, such as geographic location and salinity, do drive some differences within the population, there are extensive similarities within the microbial populations. These results suggest that for stromatolite formation, 'who' is in the community is not as critical as metabolic activities and environmental interactions. Together, these analyses help improve our understanding of the similarities among lithifying ecosystems and provide an important first step in characterizing the shared microbiome of modern stromatolites.