Project Summary for 2005–15B: Sponge production and recycling of new nitrogen in coral reef ecosystems
This proposal focuses on quantifying in situ rates of sponge-hosted N2 fixation and dissolved inorganic (DIN) uptake and release in sponges common to the FL Keys reef tract. Additional studies will be directed toward determining the potential impact of sub–thermocline NO3– on reef tract sponges and examining N export routes from sponges. We seek to begin rigorously quantifying the contributions of sponge processed N to the overall N cycle of coral–reef ecosystems. Nitrogen–rich sponges represent a relatively large proportion of the total biomass and thus represent a major pool of organic N within coral–reef ecosystems. They are known to both assimilate and release DIN as well as dissolved and particulate organic N. We seek to quantify the role of sponges in the overall N cycle of coral–reef ecosystems. During year 2 of the project, we propose to build upon the result of our 2004 AQUARIUS mission and shore–based 15N–tracer experiments to more rigorously examine the role of upwelled DIN on N cycling within sponges by using the advanced technical diving capabilities (Alternative Inert Gas) available through the UNCW/NURC to sample sponges on deep (200-300 ft) continental slope sites frequently exposed to high concentrations of upwelled DIN. The stable N isotopic composition of the continental slope sponges will be compared to those of the same species found on at shallower reef tract sites. The stable N isotopic composition of nitrate the sub-thermocline water vs. reef tract waters will also be compared. N export studies will also continue in 2005, along with additional in situ and shore–based 15N–tracer incubation experiments. Sources and sinks of new N from N2 fixation and upwelled DIN are of particular interest in coral–reef ecosystems where excessive nutrient fluxes are thought to represent a threat to ecosystem's stability. Further, many species of abundant marine sponges are known to host large and diverse microbial communities thought to fix N2, although few data support this assumption. Our recent stable isotope studies of multiple sponge species across a natural environmental gradient in the FL Keys have identified two basic groups of sponges differentiated by their d15N values. One group has low values indicative of N2 fixation, which is supported by the results of our other preliminary studies, including a 15N2 incorporation assay. Rates of N2 fixation will be determined through in situ and shore-based 15N–tracer experiments. Sponges are also known to assimilate and release DIN. The proposed research addresses longstanding questions regarding N2 fixation in sponges and the importance of sponges as a source of new N for coral–algal reefs and other coastal ecosystems. Further, our studies will broaden our basic understanding of the biogeochemical significance of marine microbial-invertebrate associations and how environmental factors control the rates of biogeochemical processes occurring within these systems.
Our stable isotope studies with common Caribbean sponges potentially provide a unique means of addressing important environmental priorities for coastal regions, such as south Florida, where there is a critical need to understand water quality linkages among the Everglades, Florida Bay and coral–reef ecosystems. The health of these ecosystems has declined precipitously in recent years and our proposed project could lead to the use of marine sponges as indicator organisms to assess natural vs. anthropogenic N inputs at critical locations where significant impacts may occur. Additionally, coral-reef management strategies would benefit from the knowledge that sponges are a significant source of new N, particularly because sponge biomass can increase on stressed reefs and nutrient loading is a major stress on coastal coral reefs. The breadth of the proposed research has been enhanced by building important collaborations with internationally recognized leaders with expertise in the biogeochemical cycling of N and in the molecular characterization of sponge-microbe symbioses. Thus, the proposed project will continue to build bridges between different scientific disciplines and foster the exchange of ideas and project personnel for interdisciplinary training. Another important aspect of the proposed research will be the inclusion of undergraduates and individuals of ethnic groups, including African Americans and Native Americans, underrepresented in the basic sciences. The Marine Sciences Program at UNC–CH has a long history of supporting minorities and undergraduates in research, including through UNCW/NURC–funded research.
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