Sponges impacts on coral reef nitrogen cycling, Key Largo, Florida

Sponges are potentially important drivers of nitrogen cycling on Caribbean coral reefs due to their capacity to filter large volumes of water, their dense microbial communities, and their large population size. Given the potential role that nitrogen plays in controlling primary production and reef health, it is important to understand and quantify the nitrogen fluxes between sponges, their associated microbial communities, and the surrounding water column. The main goal of this dissertation is to characterize the role of sponges and their microbial assiciates in reef N cycling, specifically: 1) to quantify the flux of dissolved inorganic nitrogen (DIN) to the water column, 2) to investigate the process of nitrification, and 3) to measure nitrogen fixation rates in sponges. Nitrogen fixation rates in sponges were found to be very low relative to the ambient water column; therefore, sponge-hosted nitrogen fixation probably does not contribute significantly to sponge nutrition or to inputs of new nitrogen for the reef. However, sponges were found to be a large source of DIN. The DIN flux from the sponge community was measured using a combination of incubation experiments and a novel in situ method and found to be 660 ± 130 mol m-2 h-1, which is approximately ten times higher than other reported benthic nutrient fluxes. Most of this DIN is released in the form of nitrate due to active communities of ammonia oxidizers and nitrifiers hosted by many of the most abundant sponge species. At present it is unknown whether the ammoniaoxidizing community is dominated by archaea or bacteria, but the results of this study show that the isotopic fractionation and sensitivity to chemical inhibition are consistent with bacterial ammonia oxidizers. Because nitrogen fixation rates were negligible compared to DIN flux rates, spongeiii produced DIN is likely derived from the remineralization of organic matter. Therefore, although sponges do not appear to facilitate fluxes of new nitrogen for the reef, they do appear to regenerate large quantities of inorganic nutrients, and to facilitate the oxidation of ammonium to nitrate