REVEALING THE GLOBAL METABOLIC COMPLEXITY OF MARINE AND FRESHWATER MODERN MICROBIALITES THROUGH METAGENOMICS

Modern microbialites are built from carbonate-rich organosedimentary structures formed by complex microbial communities in interface with abiotic factors, their predecessors being the earliest evidence of life on Earth. Despite their importance as documents of evolution of life and present global distribution, metagenomic surveys of microbialites have not determined which microbes and metabolic pathways are specific to microbialite-forming communities. Using high-throughput metagenomic sequencing and comparative metagenomics, we conducted a comprehensive study to reveal microbialite specific pathways in eight freshwater (Clinton Creek, Pozas Azules II, Ríos Mesquites, Kelly, Pavilion, and Pátzcuaro lakes) and marine microbialite systems (Shark Bay, Highbourne Cay), and of nonlithifying microbial mats. Proteobacteria and Cyanobacteria dominate both microbialite-forming and nonlithifying microbial mats. However, marine and freshwater microbialites were taxonomically and metabolically distinct as to communities being either primarily photosynthetic, or performing a mix of heterotrophic and photosynthetic processes. Marine microbialites had significantly higher relative abundances of pathways related to dormancy (sporulation and potassium metabolism), whereas pathways related to metabolism of aromatic compounds and secondary small molecules were relatively more frequent in freshwater microbialites. But despite these distinct metabolic potentials, freshwater and marine microbialites share >50% of their metabolic pathways, mainly related to “housekeeping functions” (biosynthesis and degradation of amino acids, nucleic acids, and proteins). Sulfur oxidation pathways, known to be involved in microbialite dissolution, were found amongst mixed heterotrophic and photosynthetic microbialite ecosystems (e.g., Kelly, Pavilion, Shark Bay), suggesting these are in higher dissolution stress than primarily photosynthetic systems (e.g., Pátzcuaro, Highbourne Cay, Clinton Creek). Our comparative metagenomics approach significantly enhanced our understanding of these evolutionary and ecologically significant systems.