Linking the modern to the ancient with a comprehensive geobiological understanding of biosignature preservation in microbial mats

Our understanding of early life on Earth relies, in part, on morphological and geochemical signatures preserved in microbialites. Nuanced evaluation of such records requires that physical and chemical patterns in Archean and Proterozoic rocks be interpreted in terms of biological processes in play at the time of formation. Expansive microbial mats from a vast tidal marsh in the interior of Little Ambergris Cay, Turks and Caicos Islands, BWI, provide a model system for examining how local topography, tidal water flow, water chemistry, microbiology, and sediment supply interact to influence the preservation of geochemical and morphological mat signatures. Little Ambergris mats range from 20 cm-thick growths with numerous variegated microbial strata to desiccated, cm-scale mats with simple architecture, distributed in a broadly zoned pattern across the island. Mats vary among three basic surface textures: EPS-covered flat mats, tufted polygonal mats, and knobbly pustular mats. Mats consist primarily of EPS and evacuated sheaths of cyanobacteria, with upper parts of the mats displaying three principal cyanobacterial morphotypes. Below and admixed with the cyanobacteria are abundant mm-scale pink consortia comprising aerobic and phototrophic Proteobacteria. Additional diversity was recovered by iTAG metagenomic sequencing, with further taxonomic insights provided by fluorescence in-situ hybridization (FISH) imaging, microbial membrane lipids, and electron microscopy imaging of viruses. To evaluate the preservation potential of geobiological signatures we coupled these observations with analyses of extractable, sulfur-bound and kerogen-bound biomarkers, fine-scale and bulk carbon and sulfur isotopes, and microfossil distributions. Taken together, biological and geochemical analyses of Little Ambergris mats underscore the geobiological richness preservable in ancient microbialites, while highlighting aspects of microbial diversity that do not preserve readily.