UNRAVELING THE METABOLIC AND TAXONOMIC COMPLEXITY OF MARINE STROMATOLITES USING HIGH-THROUGHPUT METAGENOMIC SEQUENCING

Western Australian stromatolites provide the “golden spike” of evidence for the origin of life dating back 3.45 Gyr. Modern analogs exist within Western Australia's Hamelin Pool, consisting of various morphologies including columnar stromatolites, and the pre-stromatolitic mats called pustular and smooth. Modern marine stromatolites are rather rare but are complex microbial assemblages that function in lamination, carbonate precipitation, and organosedimentation. We assembled metagenomic next-generation sequencing data in order to classify the taxonomic and metabolic potential across diverse morphologies of marine microbialites including thrombolites, stromatolites, and pre-microbialithic mats. The microbial community across taxonomic classifications using protein-coding (i.e. RefSeq database) and small subunit rRNA (i.e. Greengene database) genes directly extracted from the metagenomics suggest that three phyla Proteobacteria, Cyanobacteria and Bacteriodetes dominate all marine microbialites. However, the microbial community structure between Shark Bay and Highbourne Cay marine microbialites appear to be distinct from each other. The metabolic potential based on SEED subsystem database based classifications for Shark Bay and Highbourne Cay were also distinct from each other. Shark Bay metagenomes have a balanced metabolic pathway profile consisting of both heterotrophic and phototrophic pathways whereas Highbourne Cay appears to dominated exclusively by photosynthetic pathways. Shark Bay metagenomes appear to encode genes relating to salinity regulation and stress through betaine - choline biosynthesis and uptake which is likely an adaptation to hypersaline conditions. Alternative non-rubisco based carbon metabolism including reductive TCA cycle and 3-hydroxypropionate/4-hydroxybutyrate pathways were highly represented in Shark Bay metagenomes which were not represented in Highbourne Cay microbialites, which appear mainly to encode rubisco based carbon fixation pathways from cyanobacteria. In all, marine microbialite metagenomic analysis provided here invokes further hypotheses into the complex abyss of the microbial communities leading to modern analogs of the oldest continuous biochemical process on the planet.