Dramatic expansion of microbial groups that shape the global sulfur cycle
The biogeochemical cycle of sulfur on Earth is driven by microbial sulfate reduction, yet organisms from relatively few lineages have been implicated in this process. Recent studies using functional marker genes have detected abundant, novel dissimilatory sulfite reductases that confer the capacity for microbial sulfate reduction and could do not be affiliated with known organisms. Thus, the identity of a significant fraction of sulfate reducing microbes has remained elusive. Here we report the discovery of the capacity for sulfate reduction in the genomes of organisms from twelve bacterial and archaeal phyla, thereby doubling the number of microbial phyla associated with this process. Eight of the twelve newly identified groups are candidate phyla that lack isolated representatives, a finding only possible given genomes from metagenomes. Two candidate phyla, Candidatus Rokubacteria and Candidatus Hydrothermarchaeota contain the earliest evolved genes. The capacity for sulfate reduction has been laterally transferred in multiple events within some phyla, and a key gene potentially capable of switching sulfur oxidation to sulfate reduction in associated cells has been acquired by putatively symbiotic bacteria. We conclude that functional predictions based on phylogeny will significantly underestimate the extent of sulfate reduction across Earths ecosystems. Understanding the prevalence of this capacity is integral to interpreting the carbon cycle because sulfate reduction is often coupled to turnover of buried organic carbon. Our findings expand the diversity of microbial groups associated with sulfur transformations in the environment and motivate revision of biogeochemical process models based on microbial community composition.