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MEPS prepress abstract   -  DOI: https://doi.org/10.3354/meps13352

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

Yusuf C. El-Khaled*, Florian Roth, Arjen Tilstra, Nils Rädecker, Denis B. Karcher, Benjamin Kürten, Burton H. Jones, Christian R. Voolstra, Christian Wild

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ABSTRACT: Eutrophication (i.e. the increase of (in-)organic nutrients) may affect functioning of coral reefs, but knowledge about the effects on nitrogen (N) cycling and its relationship to productivity within benthic reef communities is scarce. We thus investigated how in situ manipulated eutrophication impacted productivity along with two 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 weeks 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 compared by 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 against hypothesis both N2-fixation and denitrification are involved in processing 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.