Characterization of the prokaryotic community associated with the giant barrel sponge, Xestospongia muta across the Caribbean

Sponges have long been known to be ecologically important members of the benthic fauna on coral reefs. Recently, it has been shown that sponges, and their symbiotic microbes, are also important contributors to the nitrogen biogeochemistry of coral reefs. Here, I investigate the ecology and physiology of the microbial community associated the ecologically dominant sponge, Xestospongia muta. A natural experiment was conducted with X. muta form three different locations (Florida Keys, USA; Lee Stocking Island, Bahamas, and Little Cayman, Cayman Islands) to compare nitrogen cycling and prokaryotic community composition. The dissolved inorganic nitrogen (DIN) fluxes of sponges were studied using nutrient analysis, stable isotope ratios, and isotope tracer experiments. Results showed that the fluxes of DIN were variable between locations but clearly showed that X. muta can be either a source or sink of DIN. Stable isotope values of sponge and symbiotic bacterial fractions indicate that the prokaryotic community is capable of taking up both NH4+ and NO3 --, and there is potential for translocation of labeled N from the symbiotic bacteria to the host. The prokaryotic community composition of X. muta, and the variability of this community across the Caribbean were quantified using 454 pyrosequencing of the 16S rRNA gene. Phlyogenetic analysis showed differences between the sponge prokaryotic community and the surrounding bacterioplankton. Additionally, both symbiont and bacterioplankton populations were different between locations. In addition to the recovery of many sequences from bacterial phyla commonly found in sponges, a diverse archaeal community was also recovered from X. muta including sequences representing the phyla Euryarchaeota and Thaumarchaeota. Transcriptomic analysis for X. muta and its symbionts revealed a similar prokaryotic community composition to the metagenetic analyses indicating an active and diverse symbiotic community. Additionally, gene specific analyses combined with preliminary metatranscriptome data indicate the presence of genes involved in nitrogen cycling including nifH (nitrogen fixation), amoA (ammonia oxidation), norB (denitrification), and nirK (denitrification). Nitrogen cycling in X. muta appears to be more complex than previous studies have shown. These results have important ecological implications for the understanding of host-microbe associations, and provide a foundation for future studies addressing the functional roles these symbiotic prokaryotes have in the biology of the host sponge and the nutrient biogeochemistry of coral reefs