Species-specific processing of fluorescent dissolved organic matter by Caribbean sponges

Coral reefs are biodiverse and productive ecosystems that are found in typically oligotrophic environments. Many studies have attempted to explain this paradox by demonstrating that microbial symbionts expand metabolic capabilities of host animals. However, few studies have determined if and how coexisting organisms and their symbiotic microbial communities partition available dissolved organic matter. To address this question, we built an artificial reef near the Florida Keys using dominant emergent sponges found in the Caribbean. Fluorescent dissolved organic matter (fDOM) components were measured in incurrent and excurrent water samples of 5–7 biological replicates for each of 10 sponge species. Sponges generally consumed more fDOM as incurrent concentrations increased, but there were also species-level differences in their responses. We found that microbial abundance (i.e., high and low microbial abundance HMA/LMA classification) explained less variation in fDOM processing than sponge species. Additionally, fDOM analysis revealed both a separation of sponges into two groups based on whether they were net producers or consumers of fDOM, as well as their responses to fDOM concentrations in ambient seawater across species. Differential consumption of fDOM components also yielded species-specific fluxes of fDOM. This observation contrasts with broad uptake of all fDOM components, which would be expected if net consumption of fDOM was simply concentration dependent and the same for all species. These results provide the first support for species-level resource partitioning of fluorescent dissolved organic matter across coexisting sponge species and provide additional support for the evolutionary importance of microbial communities in sponges.