Linking abundance and diversity of sponge-associated microbial communities to metabolic differences in host sponges

Many sponge species contain large and diverse communities of microorganisms. Some of these microbes are suggested to be in a mutualistic interaction with their host sponges, but there is little evidence to support these hypotheses. Stable nitrogen isotope ratios of sponges in the Key Largo, Florida ( USA) area grouped sponges into species with relatively low delta N-15 ratios and species with relatively high delta N-15 ratios. Using samples collected in June 2002 from Three Sisters Reef and Conch Reef in the Key Largo, Florida area, transmission electron microscopy ( TEM) and denaturing gradient gel electrophoresis were performed on tissues of the sponges Ircinia felix and Aplysina cauliformis, which are in the low delta N-15 group, and on tissue of the sponge Niphates erecta, which is in the high delta N-15 group. Results showed that I. felix and A. cauliformis have large and diverse microbial communities, while N. erecta has a low biomass of one bacterial strain. As the low delta N-15 ratios indicated a microbial input of nitrogen, these results suggested that I. felix and A. cauliformis were receiving nitrogen from their associated microbial community, while N. erecta was obtaining nitrogen solely from external sources. Sequence analysis of the microbial communities showed a diversity of metabolic capabilities among the microbes of the low delta N-15 group, which are lacking in the high delta N-15 group, further supporting metabolic differences between the two groups. This research provides support for hypotheses of mutualisms between sponges and their associated microbial communities.