Algal symbionts of the freshwater sponge Ephydatia muelleri

The freshwater sponge, Ephydatia muelleri , is an emerging model system for studying animal:microbe symbioses. Intracellular green microalgae are one of the more common symbionts that live in a facultative mutualism with E. muelleri . While these symbioses have long been known, the identity of the algal symbionts in E. muelleri cells has not been studied in detail. Here, we isolate and characterize endosymbiotic algae from E. muelleri collected from different geographic locations. We find that the algae can be transmitted through asexually produced gemmules and importantly that they can form symbioses with different, differentiated sponge cell types in the adult sponge. Our findings indicate that at least two algal lineages form endosymbioses with E. muelleri . One of the lineages includes species commonly found in samples from two locations in Canada and one in the United States (clade 1: closely related to Auxenochlorella pyrenoidosa ). The other clade includes algae found in sponges from one site in Maine, USA, and Lewiniosphaera symbiontica , which is a strain isolated in 1956 from the freshwater sponge Spongilla . We compared microbiomes found in cultures of microalgae as well as the original sponge hosts, and found that very similar bacterial microbiomes associate with both clades (91 orders of Bacteria are shared among the samples we compared). The microbiomes found in the cultures resemble, with a high degree of overlap, the microbiome associated with the sponge host.