The curated database of full length DsrAB sequences (n=274) was used to construct a concatenated DsrA and DsrB phylogeny, as described previously (11), that was congruent with previous phylogenetic reconstructions of DsrAB (11, 23, 24). As previously documented, recently discovered Euryarchaeote DsrAB formed a group with those from Crenarchaeota, that together formed a basal-branching group among all DsrAB (Figure 1). All organisms within this first group were recovered from hydrothermal environments (largely hot springs) and their DsrAB are inferred to be involved in HSO3- or SO42- reduction (11). An additional basal-branching second group comprised DsrAB recovered from uncultured metagenome-assembled-genomes (MAGs) from various organisms and environments, along with duplicate DsrAB copies within Moorellasp. genomes.
The remainder of DsrAB comprised a large group inclusive of both reductive- and oxidative-type DsrAB that primarily includes bacterial DsrAB. Within the “bacterial” group, homologs from Archaeoglobales (Archaea) form a relatively early-evolving group, although the presence of DsrAB in Archaeoglobales is thought to derive from a horizontal gene transfer (HGT) event from a bacterial donor (22, 23). Lastly, theD. vulgaris homologs were present within a large cluster comprising homologs from other putative and characterized SO42- reducing Deltaproteobacteria.
Mapping of taxonomic information on to the DsrAB phylogenetic tree revealed general concordance of DsrAB clades with their respective taxonomic groups, consistent with previous analyses (23, 24). This indicates that DsrAB are generally vertically inherited, although several exceptions to this rule are evident including the example of Archaeoglobales above. The mapping also revealed the broad range of ecological contexts for SRO and their DsrAB. As documented previously (11), DsrAB from organisms with subsurface and hydrothermal environmental origins are particularly prominent near the root of the tree, suggesting that the earliest DsrAB may derive from oxidant limited and/or high temperature environments (45). However, general patterns of ecological distributions beyond these early groups were not readily apparent. This is likely attributable to the coarseness by which these original environmental designations were assigned (i.e., by site of isolation/sequence generation).