Modern Microbialites Provide a Window onto the Microbial Communities That Have Been Major Drivers of the Global Carbon Cycle Over Geological Time

Background: Modern microbialites are thriving complex ecosystems that have been present on Earth for ~3.6 billion years and are the oldest record of life on the planet. These microbialites are provide a window into how early complex ecosystems assembled and functioned, as well as being sentinels of modern environmental change. Microbialites are found on all continents in a wide array of ecosystems from the high Canadian arctic to freshwater lakes of Antarctica and tropical and subtropical marine and freshwater systems. Microbialites provide long-term storage of carbon dioxide (a potent greenhouse gas) in the form of calcium carbonate. The extent of microbial biodiversity found within unique microbialite structures (i.e. morphotypes) is not known, nor is their functional relationship well linked to microbialite formation. Methods: Microbialites from Pavilion lake, BC, Canada, where collected from 10, 20, 25 and 45 meter depths corresponding to different microbialite morphotypes found at each of these depths. The taxonomic structure of the microbal communities from each of these samples was determined by amplifying and 454 pyrosequencing ribosomal gene fragments. Results: Comparing the ribosomal gene sequences to several databases revealed unique communities associated with each morphotype, both in terms of relative abundance and sequence divergence. Alpha-proteobacteria appears to be numerically the most abundant bacterial taxa followed by Firmicutes and Cyanobacteria. From the 18s rRNA sequences the Chromalveolates appear to increase in genetic diversity as a function of depth, while other sequences reveal the presence of Chara, an algal which is known to precipitate calcium carbonate. Conclusions: Modern microbialites encompass a vast diversity of microbial life that varies as a function of depth-dependent morphotype. Firmicutes and Cyanobacteria, both potent precipitators of calcium carbonate are also significant components of these microbial communities. These results are beginning to unravel the complex communities associated with microbialite formation, and carbon sequestration, and provide new insights into these mechanisms that have allowed microbialite formation to be a major player in the Earth’s carbon cycle over geological time.