Ocean acidification and nitrate enrichment can mitigate negative effects of soft coral (Xenia) competition on hard coral (Stylophora pistillata) endosymbionts

The combination of ocean acidification (OA) and eutrophication can undermine the physiological performance of reef-building corals during competition for benthic space, leading to shifts towards non-accreting organisms like soft corals. We conducted a 28-day laboratory orthogonal experiment to test if acidification (950 µatm pCO₂) and moderate to high nitrate enrichment (4 and 8 µmolL^-1) negatively affect the hard coral Stylophora pistillata while physically competing with the soft coral Xenia spp. We measured photosynthetic efficiency (PE) in hard corals and growth rate, Symbiodiniaceae density, and chlorophyll-a concentration in both hard and soft corals as proxies for their condition and responses to competition. Competition with the soft coral reduced PE, Symbiodiniaceae and chlorophyll-a contents of S. pistillata, while acidification alone and coupled with nitrate enrichment mitigated endosymbiont responses. The growth and chlorophyll-a concentrations of Xenia spp. were decreased by competition, but the soft coral was consistently benefited under nitrate enrichment. These results highlight that competition alone has a stronger negative impact on hard corals than on soft corals. Our study provides experimental evidence on how OA and eutrophication interact and shape coral dynamics, an overlooked but urgent topic in predicting reef futures under environmental change.