Sugar enrichment provides evidence for a role of nitrogen fixation in coral bleaching

Abstract
The disruption of the coral-algae symbiosis (coral bleaching) due to rising sea surface temperatures has become an unprecedented global threat to coral reefs. Despite decades of research, our ability to manage mass bleaching events remains hampered by an incomplete mechanistic understanding of the processes involved. In this study, we induced a coral bleaching phenotype in the absence of heat and light stress by adding sugars. The sugar addition resulted in coral symbiotic breakdown accompanied by a fourfold increase of coral-associated microbial nitrogen fixation. Concomitantly, increased N:P ratios by the coral host and algal symbionts suggest excess availability of nitrogen and a disruption of the nitrogen limitation within the coral holobiont. As nitrogen fixation is similarly stimulated in ocean warming scenarios, here we propose a refined coral bleaching model integrating the cascading effects of stimulated microbial nitrogen fixation. This model highlights the putative role of nitrogen-fixing microbes in coral holobiont functioning and breakdown.

Citation
Pogoreutz C, Rädecker N, Cárdenas A, Gärdes A, Voolstra CR, et al. (2017) Sugar enrichment provides evidence for a role of nitrogen fixation in coral bleaching. Global Change Biology. Available: http://dx.doi.org/10.1111/gcb.13695.

Acknowledgements
The authors thank Abdulaziz Al-Suwailem and Zenon Batang for allocation of working space at the Coastal and Marine Resources Core Lab (CMOR); Paul Muller for technical support with coral maintenance; and Ramzi Al-Jadahli, Haitham Al-Jadahli, and David Pallett for support with diving operations. We further thank Matthias Birkicht, Dorothee Dasbach, Katherine A. Krogslund, Craig Michell, and Dieter Peterke for support during sample analysis. Finally, we would like to thank the editor and three anonymous reviewers, whose comments greatly improved the manuscript. The contribution of C.P. was supported by GLOMAR - Bremen International Graduate School for Marine Sciences. N.R. acknowledges financial support by the MARUM Research Award for Marine Science and a KAUST Center Partnership Fund with AIMS. C.R.V. acknowledges funding by the King Abdullah University of Science and Technology (KAUST). This work was also supported by German Research Foundation (DFG) grant Wi 2677/9-1 to C.W.

Publisher
Wiley

Journal
Global Change Biology

DOI
10.1111/gcb.13695

Additional Links
http://onlinelibrary.wiley.com/doi/10.1111/gcb.13695/full

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