SAR202 Genomes from the Dark Ocean Predict Pathways for the Oxidation of Recalcitrant Dissolved Organic Matter

Deep-ocean regions beyond the reach of sunlight contain an estimated615 Pg of dissolved organic matter (DOM), much of which persists for thousands ofyears. It is thought that bacteria oxidize DOM until it is too dilute or refractory tosupport microbial activity. We analyzed five single-amplified genomes (SAGs) fromthe abundant SAR202 clade of dark-ocean bacterioplankton and found they encodemultiple families of paralogous enzymes involved in carbon catabolism, includingseveral families of oxidative enzymes that we hypothesize participate inthe degradation of cyclic alkanes. The five partial genomes encoded 152 flavinmononucleotide/F420-dependent monooxygenases (FMNOs), many of which arepredicted to be type II Baeyer-Villiger monooxygenases (BVMOs) that catalyze oxygeninsertion into semilabile alicyclic alkanes. The large number of oxidative enzymes,as well as other families of enzymes that appear to play complementary rolesin catabolic pathways, suggests that SAR202 might catalyze final steps in the biologicaloxidation of relatively recalcitrant organic compounds to refractory compoundsthat persist.