Metabolic flexibility of enigmatic SAR324 revealed through metagenomics and metatranscriptomics

Chemolithotrophy is a pervasive metabolic lifestyle for microorganisms in the dark ocean. The SAR324 group of Deltaproteobacteria is ubiquitous in the ocean and has been implicated in sulfur oxidation and carbon fixation, but also contains genomic signatures of C1 utilization and heterotrophy. Here, we reconstructed the metagenome and metatranscriptome of a population of SAR324 from a hydrothermal plume and surrounding waters in the deep Gulf of California to gain insight into the genetic capability and transcriptional dynamics of this enigmatic group. SAR324's metabolism is signified by genes that encode a novel particulate hydrocarbon monooxygenase (pHMO), degradation pathways for corresponding alcohols and short-chain fatty acids, dissimilatory sulfur oxidation, formate dehydrogenase (FDH) and a nitrite reductase (NirK). Transcripts of the pHMO, NirK, FDH and transporters for exogenous carbon and amino acid uptake were highly abundant in plume waters. Sulfur oxidation genes were also abundant in the plume metatranscriptome, indicating SAR324 may also utilize reduced sulfur species in hydrothermal fluids. These results suggest that aspects of SAR324's versatile metabolism (lithotrophy, heterotrophy and alkane oxidation) operate simultaneously, and may explain SAR324's ubiquity in the deep Gulf of California and in the global marine biosphere.