Characterization of bacteria associated with the kleptoplastic sea slug Elysia chlorotica and its algal prey Vaucheria litorea

            Microbes, both free-living and symbiotic, are ubiquitous throughout all environments and play key roles in nutrient cycling, making life on earth possible. Bacteria also associate with the gut and mucus of many animals, including humans, where they are important for host development and protection. The sea slug Elysia chlorotica forms an obligate endosymbiosis with chloroplasts (kleptoplasty) obtained from its algal prey, Vaucheria litorea. These chloroplasts allow the sea slug to forgo its algal diet and survive photosynthetically for several months. This sea slug also associates with a number of bacteria, which may contribute to the long-term success of the Elysia-Vaucheria symbiosis through the provision of nutrients as well as developmental and protective benefits.

            This study utilized two 16S rDNA-based metagenomic analyses, clone libraries and amplicon pyrosequencying (high-throughput sequencing of 16S rDNA fragments), to characterize the bacteria associated with several samples (whole-animal, digestive gut, and mucus) of E. chlorotica and V. litorea. The most prevalent classes for all sea slug and algal samples included alpha-, beta-, and gamma-proteobacteria as well as Actinobacteria, Bacilli, Flavobacteria, Mollicutes, and Sphingobacteria. However, the samples were unique based on further taxonomic classification of the bacteria, at the order and family levels, and in terms of microbial abundance. Varying environmental conditions (i.e., salinity) as well as lifestyle (i.e., starvation) greatly affected bacterial diversity amongst the samples. Putative metabolic functions of identified bacteria included polysaccharide digestion, photosynthesis, denitrification, nitrogen fixation, vitamin B₁₂ production, and natural product biosynthesis. Further studies are needed to localize symbionts and determine their exact metabolic roles in the Elysia-Vaucheria symbiosis.