Discovery of new marine species Stentor hondawara and its whole-genome reveal their unique ecology in comparison with freshwater stentors

Stentor is a genus of large ciliates that can be found in ponds, lakes, rivers, and fresh waters all over the world. Since their initial discovery in 1744, Stentor strains have been isolated from all populated continents. To date, over 50 individual strains have been identified, yet not a single isolate from a marine environment has been verified. Over 200 years since the initial description of the Stentor genus, our study entails the first concrete discovery of a fully marine Stentor species, as evidenced by its morphological, ecological, and phylogenetic positioning amongst Stentor. This new marine organism, which we have named Stentor hondawara, was verified to be a new species of the Stentor genus that appears to have fully adapted to a uniquely marine lifestyle in a high-salinity environment. Using comparative genomics analysis between the whole-genome sequences of Stentor hondawara and two freshwater species of Stentor, we further detected several intriguing differences in the enrichment of gene orthologs between the marine Stentor hondawara and the freshwater species, Stentor coeruleus and Stentor pyriformis. The gene groups specifically enriched in Stentor hondawara encode a variety of proteins, including ion channels, pH-responsive proteins, osmoprotectants, amino acid biosynthesis enzymes, and signaling receptors. Additionally, using metagenomics, we detected and isolated, from within our initial genome assembly, the genome of a novel marine bacteria, which we propose is an endosymbiont of Stentor hondawara. This bacterial species is an uncharacterized member of the order Rhodospirillales and appears to be a nutritional factory for the host Stentor hondawara. Taken together, our study provides insight into how Stentor hondawara adapted to a marine environment distinct from the habitats of all the other currently known Stentor species living in freshwater.