Heat stress reduces the contribution of diazotrophs to coral holobiont nitrogen cycling

Abstract
Efficient nutrient cycling in the coral-algal symbiosis requires constant but limited nitrogen availability. Coral-associated diazotrophs, i.e., prokaryotes capable of fixing dinitrogen, may thus support productivity in a stable coral-algal symbiosis but could contribute to its breakdown when overstimulated. However, the effects of environmental conditions on diazotroph communities and their interaction with other members of the coral holobiont remain poorly understood. Here we assessed the effects of heat stress on diazotroph diversity and their contribution to holobiont nutrient cycling in the reef-building coral from the central Red Sea. In a stable symbiotic state, we found that nitrogen fixation by coral-associated diazotrophs constitutes a source of nitrogen to the algal symbionts. Heat stress caused an increase in nitrogen fixation concomitant with a change in diazotroph communities. Yet, this additional fixed nitrogen was not assimilated by the coral tissue or the algal symbionts. We conclude that although diazotrophs may support coral holobiont functioning under low nitrogen availability, altered nutrient cycling during heat stress abates the dependence of the coral host and its algal symbionts on diazotroph-derived nitrogen. Consequently, the role of nitrogen fixation in the coral holobiont is strongly dependent on its nutritional status and varies dynamically with environmental conditions.

Citation
Rädecker, N., Pogoreutz, C., Gegner, H. M., Cárdenas, A., Perna, G., Geißler, L., … Voolstra, C. R. (2021). Heat stress reduces the contribution of diazotrophs to coral holobiont nitrogen cycling. The ISME Journal. doi:10.1038/s41396-021-01158-8

Acknowledgements
We thank Dr Zenon B. Batang and Dr Nabeel M. Alikunhi for their continuous support and assistance with aquaria maintenance. Further, Ioannis Georgakakis, Mustafa Altunkaya, and Professor Matt Kilburn are acknowledged for their help and support with sample processing and data analysis. We thank the editor and the three anonymous reviewers for their constructive feedback and suggestions. NR, CP, AC, MP, J-BR, and CRV were supported by the KAUST competitive research grant URF/1/ 3400-01-01. CRV also acknowledges funding from the German Research Foundation (DFG), grant 433042944. NR and AM are supported by the Swiss National Science Foundation, grant 200021_179092.
FUNDING Open Access funding enabled and organized by Projekt DEAL.

Publisher
Springer Science and Business Media LLC

Journal
The ISME Journal

DOI
10.1038/s41396-021-01158-8

Additional Links
https://www.nature.com/articles/s41396-021-01158-8

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