Comparisons of diversity of bacterial communities associated with three sessile marine eukaryotes

Host-associated bacterial communities are potentially critical components of marine microbial diversity, yet our understanding of bacterial distribution on living surfaces lags behind that for planktonic communities. We used 16S rRNA gene library analysis to compare within-host (alpha) and between-host (beta) diversity among bacterial communities on 3 co-occurring marine eukaryotes from temperate Australia: the demosponge Cymbastela concentrica, the red macroalga Delisea pulchra and the green intertidal alga Ulva a ustralis. The bacterial community on C. concentrica had high phylum-level diversity (7 phyla including 3 proteobacterial classes) but relatively low 'species' richness (estimated at 24 species). Among the algae, D. pulchra contained 7 phyla including an estimated 79 species, while the U. australis library yielded only 4 phyla with an estimated 36 species. Alpha-, Delta- and Gammaproteobacteria were well represented in all libraries, while Planctomycetes and Bacteroidetes were relatively common on the 2 algae, but absent or rarely encountered in the sponge. At the phylum level, the community of C, concentrica largely mirrored that found in other marine sponges (e.g. Proteobacteria, Actinobacteria, Nitrospira), although large numbers of diatoms and the presence of Verrucomicrobia were atypical. Overall, within-host (alpha) diversity was relatively high, at least for C. concentrica and D. pulchra, while between-host (beta) diversity depended heavily on the phylogenetic level examined. Generally, there was a remarkable lack of overlap at the species level. No species showed universal distribution across hosts, indicating high beta diversity at the species level. At the level of phyla, however, both universal (e.g. Proteobacteria) and distinct (e.g. Nitrospira) groups existed. This study is among the first to compare patterns of alpha and beta diversity for microbial communities associated with co-occurring marine eukaryotes.