Inter-domain horizontal gene transfer of nickel-binding superoxide dismutase.

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The ability of aerobic microorganisms to regulate internal and external concentrations of the reactive oxygen species (ROS) superoxide directly influences the health and viability of cells. Superoxide dismutases (SODs) are the primary regulatory enzymes that are used by microorganisms to degrade superoxide. SOD is not one, but three separate, non-homologous enzymes that perform the same function. Thus, the evolutionary history of genes encoding for different SOD enzymes is one of convergent evolution, which reflects environmental selection brought about by an oxygenated atmosphere, changes in metal availability, and opportunistic horizontal gene transfer (HGT). In this study, we examine the phylogenetic history of the protein sequence encoding for the nickel-binding metalloform of the SOD enzyme (SodN). The genomic potential to produce SodN is widespread among bacteria, including Actinobacteriota (Actinobacteria), Chloroflexota (Chloroflexi), Cyanobacteria, Proteobacteria, Patescibacteria, and others. The gene is also present in many archaea, with Thermoplasmatota and Nanoarchaeota representing the vast majority of archaeal sodN diversity. A comparison of organismal and SodN protein phylogenetic trees reveals several instances of HGT, including multiple inter-domain transfers of the sodN gene from the bacterial domain to the archaeal domain. Nearly half of the archaeal members with sodN live in the photic zone of the marine water column. The sodN gene is widespread and characterized by apparent vertical gene transfer in some sediment- or soil-associated lineages within the Actinobacteriota and Chloroflexota phyla, suggesting the ancestral sodN likely originated in one of these clades before expanding its taxonomic and biogeographic distribution to additional microbial groups in the surface ocean in response to decreasing iron availability. In addition to decreasing iron quotas, nickel-binding SOD has the added benefit of withstanding high reactant and product ROS concentrations without damaging the enzyme, making it particularly well suited for the modern surface ocean.