Interactive effects of increased availability of dissolved inorganic carbon and nitrogen on the photosynthesis of the tropical coral Stylophora pistillata

Reef-building corals present a high productivity despite thriving in nutrient-poor environments. The symbiosis between the coral host and dinoflagellate algae enables an effective retention of nutrients within the symbiotic association and explains the success of corals in oligotrophic environments. Anthropogenic activities may disrupt this association, by providing dissolved inorganic nitrogen in excess, leading to the overgrowth of the symbionts and to inorganic carbon limitation for their photosynthesis. In this study, we thus tested, during three weeks incubation, the individual and combined effects of ammonium (+iN) and bicarbonate (+iC) enrichment on the physiology and photosynthetic capacities of the scleractinian coral Stylophora pistillata originating from the Red Sea. We observed a significant increase in symbiont density in the +iN or +iN+iC treatments compared to the control, suggesting that symbionts were nitrogen-limited in the coral tissue. The relative electron transport rates (rETR) as well as the rates of gross photosynthesis were however enhanced in the +iN+iC treatment, suggesting that this process was both limited by inorganic nitrogen and inorganic carbon. Other physiological parameters were significantly strongly influenced by the double enrichment such as rETR max, which increased by 150% and the non-photochemical quenching (NPQ), which decreased by 30% respectively. Areal concentrations of pigments such as carotenoids, peridinin, diadinoxanthin, and DPS xanthophylls were enhanced by nitrogen or nitrogen+inorganic carbon addition, while diatoxanthin was reduced. These results therefore highlight the interactions between nitrogen and carbon availability for the physiology of coral symbionts. Further experimental studies are however required to better understand the effects of nutrient enrichment on corals and to develop adapted management strategies for the future of reef corals.