N2 Fixation in Marine Sponges: A New Nitrogen Source for Coastal Ecosystems

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ABSTRACTOCE- 0351893 Microbial transformations of nitrogen (N) strongly influence the dynamics and diversity of marine communities and ecosystems. Inputs of new N from N2 fixation are of particular interest. 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. In recent studies of stable isotopes of multiple sponge species in the FL Keys two basic groups of sponges were identified and differentiated by their d15N values. In addition to low d15N values, which are indicative of N2 fixation, one group had a 15N2 incorporation assay. In this study, researchers from the University of North Carolina at Chapel Hill will focus on quantifying rates of microbial N2 fixation together with photosynthesis and respiration in low and high d15N sponges. Further comparative studies with low and high d15N sponges will use molecular biology techniques (e.g., 16S, nifH, DGGE, FISH analyses) to examine their microbial communities and to identify active N2-fixing microbes within test sponges. The team of scientists will also examine the dynamics of N2-fixing symbiont-sponge interactions throughout the sponge life cycle and estimate levels of new N export from sponges. After exploring a natural environmental gradient in the Florida Keys the study will be expanded to oligotrophic waters off San Salvador Island, Bahamas, in order to investigate how other environmental parameters, such as light levels, affect d15N values of sponges and rates of N2 fixation. This research will not only address the longstanding questions regarding N2 fixation in sponges and importance of sponges as a source of new N for coral-algal reefs and other coastal ecosystems but also 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.