Budget of Primary Production and Dinitrogen Fixation in a Highly Seasonal Red Sea Coral Reef

Ecosystems, Mar 2016

Biological dinitrogen (N2) fixation (diazotrophy, BNF) relieves marine primary producers of nitrogen (N) limitation in a large part of the world oceans. N concentrations are particularly low in tropical regions where coral reefs are located, and N is therefore a key limiting nutrient for these productive ecosystems. In this context, the importance of diazotrophy for reef productivity is still not resolved, with studies up to now lacking organismal and seasonal resolution. Here, we present a budget of gross primary production (GPP) and BNF for a highly seasonal Red Sea fringing reef, based on ecophysiological and benthic cover measurements combined with geospatial analyses. Benthic GPP varied from 215 to 262 mmol C m−2 reef d−1, with hard corals making the largest contribution (41–76%). Diazotrophy was omnipresent in space and time, and benthic BNF varied from 0.16 to 0.92 mmol N m−2 reef d−1. Planktonic GPP and BNF rates were respectively approximately 60- and 20-fold lower than those of the benthos, emphasizing the importance of the benthic compartment in reef biogeochemical cycling. BNF showed higher sensitivity to seasonality than GPP, implying greater climatic control on reef BNF. Up to about 20% of net reef primary production could be supported by BNF during summer, suggesting a strong biogeochemical coupling between diazotrophy and the reef carbon cycle.

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Budget of Primary Production and Dinitrogen Fixation in a Highly Seasonal Red Sea Coral Reef

Ecosystems Budget of Primary Production and Dinitrogen Fixation in a Highly Seasonal Red Sea Coral Reef Ulisse Cardini 1 2 Vanessa N. Bednarz 2 Nanne van Hoytema 2 Alessio Rovere 0 Malik S. Naumann 2 Mamoon M. D. Al-Rshaidat 4 5 Christian Wild 2 3 0 Sea Level and Coastal Changes Group, Leibniz Center for Tropical Marine Ecology (ZMT) and MARUM - Center for Marine Environmental Sciences and Faculty of Geosciences, University of Bremen , 28334 Bremen , Germany 1 Division of Microbial Ecology, Department of Microbiology and Ecosystem Science, Research Network Chemistry meets Microbiology, University of Vienna , Althanstrasse 14, 1090 Vienna , Austria 2 Coral Reef Ecology Group, Leibniz Center for Tropical Marine Ecology (ZMT) , Fahrenheitstr. 6, 28359 Bremen , Germany 3 Faculty of Biology and Chemistry (FB 2), University of Bremen , 28359 Bremen , Germany 4 Department of Marine Biology, The University of Jordan - Aqaba Branch , Aqaba 77110 , Jordan 5 Laboratory for Molecular Microbial Ecology (LaMME), Marine Science Station , Aqaba 77110 , Jordan Biological dinitrogen (N2) fixation (diazotrophy, BNF) relieves marine primary producers of nitrogen (N) limitation in a large part of the world oceans. N concentrations are particularly low in tropical regions where coral reefs are located, and N is therefore a key limiting nutrient for these productive ecosystems. In this context, the importance of diazotrophy for reef productivity is still not resolved, with studies up to now lacking organismal and seasonal resolution. Here, we present a budget of gross primary production (GPP) and BNF for a highly seasonal Red Sea fringing reef, based on ecophysiological and benthic cover measurements combined with geospatial analyses. Benthic GPP varied from 215 to 262 mmol C m-2 reef d-1, with hard corals making the largest contribution (41-76%). Diazotrophy was Diazotrophy; Photosynthesis; Productivity; Nutrient budget; Biogeochemical cycling; Gulf of Aqaba - omnipresent in space and time, and benthic BNF varied from 0.16 to 0.92 mmol N m-2 reef d-1. Planktonic GPP and BNF rates were respectively approximately 60- and 20-fold lower than those of the benthos, emphasizing the importance of the benthic compartment in reef biogeochemical cycling. BNF showed higher sensitivity to seasonality than GPP, implying greater climatic control on reef BNF. Up to about 20% of net reef primary production could be supported by BNF during summer, suggesting a strong biogeochemical coupling between diazotrophy and the reef carbon cycle. INTRODUCTION Nitrogen (N) is a fundamental component of all living organisms. In particular, N is needed by primary producers in capturing energy through photosynthesis and building biomass, leading to a tight coupling of the N and carbon (C) cycles (Gruber and Galloway 2008) and dictating constraints to the flexibility of the ecosystem C:N stoichiometry (Geider and La Roche 2002) . In unperturbed and oligotrophic marine systems, primary productivity is often limited by bioavailable forms of N, which are scarce due to low atmospheric inputs and N loss pathways (Vitousek and Howarth 1991) . This limitation is of particular significance in coral reef ecosystems, as these are among the ecosystems displaying the highest rates of gross primary production (GPP), yet experiencing very low ambient concentrations of dissolved nutrients. Here, biological dinitrogen (N2) fixation (diazotrophy, BNF) is thought to play an important role in replenishing the N pool, maintaining the ecosystem productivity and its biological storage of C (D’Elia and Wiebe 1990; Capone 1996; O’Neil and Capone 2008) . Since the early work by Wiebe and others (1975) on BNF in algal reef flats of the Marshall Islands, many studies have found high rates of BNF associated with several benthic substrates, ranging from sediments and cyanobacterial mats to macroalgae and scleractinian corals (for reviews, see O’Neil and Capone 2008; Cardini and others 2014) . Moreover, after first evidence of a diazotroph–coral association by Shashar and others (1994a) and the subsequent discovery of diazotrophic cyanobacteria in the tissue of scleractinian corals of the genus Montastraea (Lesser and others 2004) , there has been emergent recognition of the potential contribution of N2fixing symbioses in coral reefs (Fiore and others 2010; Cardini and others 2014) . Although reef sediments and cyanobacterial mats show high rates of BNF (O’Neil and Capone 1989; Capone and others 1992; Shashar and others 1994b; CharpyRoubaud and others 2001; Bednarz and others 2015b) , N2-fixing coral symbioses may also be responsible for significant inputs of N on the ecosystem level, particularly in reef habitats with high live coral coverage. BNF in coral reef habitats is highly variable (O’Neil and Capone 2008) and potentially affected by global and local anthropogenic disturbances (Cardini and others 2014). Therefore, it is increasingly important to quantify B (...truncated)


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Ulisse Cardini, Vanessa N. Bednarz, Nanne van Hoytema, Alessio Rovere, Malik S. Naumann, Mamoon M. D. Al-Rshaidat, Christian Wild. Budget of Primary Production and Dinitrogen Fixation in a Highly Seasonal Red Sea Coral Reef, Ecosystems, 2016, pp. 771-785, Volume 19, Issue 5, DOI: 10.1007/s10021-016-9966-1