The microbial carbonate factory of Hamelin Pool, Shark Bay, Western Australia.

Microbialites and peloids are commonly associated throughout the geologic record. Proterozoic carbonate megafacies are composed predominantly of micritic and peloidal limestones often interbedded with stromatolitic textures. The association is also common throughout carbonate ramps and platforms during the Phanerozoic. Recent investigations reveal that Hamelin Pool, located in Shark Bay, Western Australia, is a microbial carbonate factory that provides a modern analog for the microbialite-micritic sediment facies associations that are so prevalent in the geologic record. Hamelin Pool contains the largest known living marine stromatolite system in the world. Although best known for the constructive microbial processes that lead to formation of these stromatolites, our comprehensive mapping has revealed that erosion and degradation of weakly lithified microbial mats in Hamelin Pool leads to the extensive production and accumulation of sand-sized micritic grains. Over 40 km2 of upper intertidal shoreline in the pool contain unlithified to weakly lithified microbial pustular sheet mats, which erode to release irregular peloidal grains. In addition, over 20 km2 of gelatinous microbial mats, with thin brittle layers of micrite, colonize subtidal pavements. When these gelatinous mats erode, the micritic layers break down to form platey, micritic intraclasts with irregular boundaries. Together, the irregular micritic grains from pustular sheet mats and gelatinous pavement mats make up nearly 26% of the total sediment in the pool, plausibly producing ~ 24,000 metric tons of microbial sediment per year. As such, Hamelin Pool can be seen as a microbial carbonate factory, with construction by lithifying microbial mats forming microbialites, and erosion and degradation of weakly lithified microbial mats resulting in extensive production of sand-sized micritic sediments. Insight from these modern examples may have direct applicability for recognition of sedimentary deposits of microbial origin in the geologic record.

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.