2021, Pogoreutz, Claudia, Voolstra, Christian R., Rädecker, Nils, Weis, Virginia, Cárdenas, Anny, Raina, Jean-Baptiste

Coral reefs face unprecedented threats from anthropogenic environmental change. Climate change, pollution, and overfishing are affecting symbiotic interactions in the coral holobiont, which constitute the structural and functional foundation of reef ecosystems, eventually leading to the breakdown of the symbiosis and/or the onset of disease(s). The resulting dysbiosis of species relationships within the coral holobiont causes coral mortality at a global scale, accompanied by unprecedented loss of coral reef cover. In this chapter, we discuss the diversity of microbes (Symbiodiniaceae, bacteria, archaea, protists, fungi) associated with the coral host and what is known of their respective contribution to holobiont functioning. We highlight how the coral–dinoflagellate symbiosis forms the “engine” of the coral holobiont machinery, and we discuss the complexity of interactions that have shaped the ecological success of corals. We conclude that the coral holobiont is a prime example of how microbial associates shape the biology of their animal hosts and enable them to inhabit and even thrive in otherwise inhospitable environments. Given the current global decline of coral reef ecosystems, it is imperative to better understand the mechanisms governing coral holobiont function and health in order to develop strategies for mitigating the consequences of climate change and local anthropogenic stressors.

2020-07-09, El-Khaled, Yusuf C., Roth, Florian, Tilstra, Arjen, Rädecker, Nils, Karcher, Denis B., Kürten, Benjamin, Jones, Burton H., Voolstra, Christian R., Wild, Christian

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.