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Marine Ecology Progress Series

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MEPS 645:55-66 (2020)  -  DOI:

In situ eutrophication stimulates dinitrogen fixation, denitrification, and productivity in Red Sea coral reefs

Yusuf C. El-Khaled1,*, Florian Roth2,3,4, Arjen Tilstra1, Nils Rädecker2,5,6, Denis B. Karcher1, Benjamin Kürten2,7, Burton H. Jones2, Christian R. Voolstra2,5, Christian Wild1

1Marine Ecology Department, Faculty of Biology and Chemistry, University of Bremen, 28359 Bremen, Germany
2Red Sea Research Center, King Abdullah University of Science and Technology (KAUST), 23995 Thuwal, Saudi-Arabia
3Baltic Sea Centre, Stockholm University, 10691 Stockholm, Sweden
4Faculty of Biological and Environmental Sciences, Tvärminne Zoological Station, University of Helsinki, 00014 Helsinki, Finland
5Department of Biology, University of Konstanz, 78464 Konstanz, Germany
6Laboratory for Biological Geochemistry, School of Architecture, Civil and Environmental Engineering, École Polytechnique Fédérale de Lausanne (EPFL), 1015 Lausanne, Switzerland
7Jülich Research Centre GmbH, Project Management Jülich, 18069 Rostock, Germany
*Corresponding author:

ABSTRACT: Eutrophication (i.e. the increase of [in-]organic nutrients) may affect the functioning of coral reefs, but knowledge about the effects on nitrogen (N) cycling and its relationship to productivity within benthic reef communities is scarce. Thus, we investigated how in situ manipulated eutrophication impacted productivity along with 2 counteracting N-cycling pathways (dinitrogen [N2]-fixation, denitrification), using a combined acetylene assay. We hypothesised that N2-fixation would decrease and denitrification increase in response to eutrophication. N fluxes and productivity (measured as dark and light oxygen fluxes assessed in incubation experiments) were determined for 3 dominant coral reef functional groups (reef sediments, turf algae, and the scleractinian coral Pocillopora verrucosa) after 8 wk of in situ nutrient enrichment in the central Red Sea. Using slow-release fertiliser, we increased the dissolved inorganic N concentration by up to 7-fold compared to ambient concentrations. Experimental nutrient enrichment stimulated both N2-fixation and denitrification across all functional groups 2- to 7-fold and 2- to 4-fold, respectively. Productivity doubled in reef sediments and remained stable for turf algae and P. verrucosa. Our data therefore suggest that (1) turf algae are major N2-fixers in coral reefs, while denitrification is widespread among all investigated groups; (2) surprisingly, and contrary to our hypothesis, both N2-fixation and denitrification are involved in the response to moderate N eutrophication, and (3) stimulated N2-fixation and denitrification are not directly influenced by productivity. Our findings underline the importance and ubiquity of microbial N cycling in (Red Sea) coral reefs along with its sensitivity to eutrophication.

KEY WORDS: Nitrogen cycle · Climate change · Pollution · Red Sea · Acetylene reduction assay · Acetylene inhibition assay

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Cite this article as: El-Khaled YC, Roth F, Tilstra A, Rädecker N and others (2020) In situ eutrophication stimulates dinitrogen fixation, denitrification, and productivity in Red Sea coral reefs. Mar Ecol Prog Ser 645:55-66.

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