It has been hypothesized that the abundant heterotrophic ocean bacterioplankton in the SAR202 clade of the phylum Chloroflexi evolved specialized metabolisms for the oxidation of organic compounds that are resistant to microbial degradation via common metabolic pathways. Expansions of paralogous enzymes were reported and implicated in hypothetical metabolism involving monooxygenase and dioxygenase enzymes. In the proposed metabolic schemes, the paralogs serve the purpose of diversifying the range of organic molecules that cells can utilize. To further explore SAR202 evolution and metabolism, we reconstructed single amplified genomes and metagenome-assembled genomes from locations around the world that included the deepest ocean trenches. In an analysis of 122 SAR202 genomes that included seven subclades spanning SAR202 diversity, we observed additional evidence of paralog expansions that correlated with evolutionary history, as well as further evidence of metabolic specialization. Consistent with previous reports, families of flavin-dependent monooxygenases were observed mainly in the group III SAR202 genomes, and expansions of dioxygenase enzymes were prevalent in those of group VII. We found that group I SAR202 genomes encode expansions of racemases in the enolase superfamily, which we propose evolved for the degradation of compounds that resist biological oxidation because of chiral complexity. Supporting the conclusion that the paralog expansions indicate metabolic specialization, fragment recruitment and fluorescent in situ hybridization (FISH) with phylogenetic probes showed that SAR202 subclades are indigenous to different ocean depths and geographical regions. Surprisingly, some of the subclades were abundant in surface waters and contained rhodopsin genes, altering our understanding of the ecological role of SAR202 species in stratified water columns.IMPORTANCE The oceans contain an estimated 662 Pg C in the form of dissolved organic matter (DOM). Information about microbial interactions with this vast resource is limited, despite broad recognition that DOM turnover has a major impact on the global carbon cycle. To explain patterns in the genomes of marine bacteria, we propose hypothetical metabolic pathways for the oxidation of organic molecules that are resistant to oxidation via common pathways. The hypothetical schemes we propose suggest new metabolic pathways and classes of compounds that could be important for understanding the distribution of organic carbon throughout the biosphere. These genome-based schemes will remain hypothetical until evidence from experimental cell biology can be gathered to test them. Our findings also fundamentally change our understanding of the ecology of SAR202 bacteria, showing that metabolically diverse variants of these cells occupy niches spanning all depths and are not relegated to the dark ocean.
Keywords: SAR202; biological carbon pump; carbon sequestration; dissolved organic matter; enolase; marine carbon cycle; recalcitrant organic matter.
Copyright © 2020 Saw et al.