Phylogenetic, genomic, and biogeographic characterization of a novel and ubiquitous marine invertebrate-associated Rickettsiales parasite, Candidatus Aquarickettsia rohweri, gen. nov., sp. nov

Bacterial symbionts are integral to the health and homeostasis of invertebrate hosts. Notably, members of the Rickettsiales genus Wolbachia influence several aspects of the fitness and evolution of their terrestrial hosts, but few analogous partnerships have been found in marine systems. We report here the genome, phylogenetics, and biogeography of a ubiquitous and novel Rickettsiales species that primarily associates with marine organisms. We previously showed that this bacterium was found in scleractinian corals, responds to nutrient exposure, and is associated with reduced host growth and increased mortality. This bacterium, like other Rickettsiales, has a reduced genome indicative of a parasitic lifestyle. Phylogenetic analysis places this Rickettsiales within a new genus we define as Candidatus Aquarickettsia. Using data from the Earth Microbiome Project and SRA databases, we also demonstrate that members of Ca. Aquarickettsia are found globally in dozens of invertebrate lineages. The coral-associated Candidatus A. rohweri is the first finished genome in this new Glade. Ca. A. rohweri lacks genes to synthesize most sugars and amino acids but possesses several genes linked to pathogenicity including Tlc, an antiporter that exchanges host ATP for ADP, and a complete Type IV secretion system. Despite its inability to metabolize nitrogen, Ca. A. rohweri possesses the NtrY-NtrX two-component system involved in sensing and responding to extracellular nitrogen. Given these data, along with visualization of the parasite in host tissues, we hypothesize that Ca. A. rohweri reduces coral health by consuming host nutrients and energy, thus weakening and eventually killing host cells. Last, we hypothesize that nutrient enrichment, which is increasingly common on coral reefs, encourages unrestricted growth of Ca. A. rohweri in its host by providing abundant N-rich metabolites to be scavenged.