Pseudoalteromonas is a symbiont of marine invertebrates that exhibits broad patterns of phylosymbiosis.

Despite growing insights into the composition of marine invertebrate microbiomes, our understanding of their ecological and evolutionary patterns remains poor, owing to limited sampling depth and low-resolution datasets. Previous studies have provided conflicting results that both confirm and deny the existence of phylosymbiosis between marine invertebrates and marine bacteria. Here, we investigated potential animal-microbe symbioses in Pseudoalteromonas, a bacterial genus consistently identified as a core microbiome taxon in diverse invertebrates. Using a pangenomic analysis of 236 free-living and invertebrate-associated bacterial strains (including two new nematode-associated isolates generated in this study), we confirm that Pseudoalteromonas is a symbiont with substantial evidence of phylosymbiosis across at least three marine invertebrate phyla (e.g. Nematoda, Mollusca, and Cnidaria). Patterns of symbiosis were consistent irrespective of geography (including in Antarctica), with fluorescence in situ hybridization images from nematodes indicating that bacterial symbionts form biofilms in the mouth and esophagus and are sometimes present in female nematode ovaries exhibiting stunted development. The evolutionary history of Pseudoalteromonas is marked by substantial host-switching and lifestyle transitions, and host-associated genomes suggest that these bacteria are facultative symbionts involved in nutritional symbioses. In marine environments, we hypothesize that horizontally acquired symbionts may have co-evolved with invertebrates, using host mucus as a physical niche and food source, while providing their animal hosts with vitamin B, amino acids, and bioavailable carbon compounds in return.