LACUSTRINE NOSTOC (NOSTOCALES) AND ASSOCIATED MICROBIOME GENERATE A NEW TYPE OF MODERN CLOTTED MICROBIALITE RESEMBLING MESOPROTEROZOIC THROMBOLITES

Some of the oldest fossils on Earth are microbialites, which are mineral formations formed by microbial communities that are often dominated by cyanobacteria. Carbonate microbialites have existed from Proterozoic times, and they are recognized for sequestering globally significant amounts of inorganic carbon. The best-studied microbialites are laminate structures formed by microbial mats (stromatolites). There exists another type of microbialite dominated by distinctive clotted structures (thrombolites), and the taxonomic composition and functions of associated microbial communities are less well understood. We used microscopy and deep shotgun pyrosequencing to characterize the microbiome (microbial taxa and their genomes) associated with a single cyanobacterial host linked by 16S sequences to Nostoc commune Vaucher ex Bornet & Flahault, which dominates abundant littoral clotted microbialites discovered in a shallow, subpolar, freshwater lake in southern Chile. Clot dimensions resembled those of known Mesoproterozoic microbialites. Microscopy and energy-dispersive X-ray spectroscopy suggested that adherent hollow carbonate spheres typical of thrombolites begin forming on the rigid curved outer surface of the Nostoc balls. The surface biofilm included over 50 non-oxygenic bacterial genera that indicate diverse ecological functions, including sulfate reducers and nitrogen fixers. Among the genes discovered in the metagenome are all known genes coding for proteins specific to sulfate reduction, a number of different types of nifH including a nostocalean version, nostocalean sulfide:ferredoxin oxidoreductase (indicating anoxygenic photosynthesis), and biosynthetic pathways for the secondary metabolites scytonemin, mycosporine, and microviridin. These results allow for comparisons to microbiota and microbiomes of other algal and microbial communities and illuminate biogeochemical roles of ancient microbialites. Graham et al. 2014. Lacustrine Nostoc (Nostocales) and associated microbiome generate a new type of modern clotted microbialite. Journal of Phycology 50:280-91.