Nitrification controls on fluxes and isotopic composition of nitrate from Florida Keys sponges

We performed a combination of field and laboratory experiments designed to investigate sponge-hosted microbial nitrification processes, plus a survey of the prevalence of nitrification among sponge species at patch and outer reefs, Florida Keys, USA. Results from dissolved inorganic nitrogen (DIN) measurements, chemical inhibition, and 15N isotope tracer experiments with Caribbean sponges show that 9 of 12 species tested hosted nitrification. Of the species in our study that are reported to host internal microbial populations, all but one has been found to host this process. Nitrification occurred with an apparent N isotopic fractionation factor of 11 ± 2.6‰ (standard error) and was inhibited by nitrapyrin. Both of these characteristics are consistent with bacterially mediated ammonium oxidation; however, ammonium oxidation by crenarchaeota is also possible. The isotopic composition of nitrate expelled from sponges in situ had lower δ15N values than nitrate from the ambient water column, suggesting active utilization of the nitrate product after it is released. The frequency of the association between nitrifying microbial communities and sponges that harbor large microbial communities suggests that nitrification is an integral part of the metabolic function of many sponge species. Given the abundance of these species on Conch Reef and the magnitude of their nitrification rates, the majority of benthic nitrification on the Florida Keys outer reef tract probably occurs in sponges. Sponge population size and composition could therefore strongly influence the concentration and speciation of DIN in the reef water column, and possibly other ecosystems where sponges are abundant.