INCORALS · Influence of nutrient starvation on corals' susceptibility to bleaching

INCORALS will establish a novel conceptual model that introduces a transition of symbiotic algae from a nutrient limited to a nutrient starved state as a process that renders reef building corals more susceptible to heat stress. Elevated temperatures have been identified as the key driver for coral bleaching, which is the often fatal loss of corals’ symbiotic algae. Thus, studies have estimated that reefs will be lost within the next one hundred years as a result of global warming. High temperatures undoubtedly play a major role in triggering coral bleaching. However, observations made for instance during the 1998 bleaching event, suggest also a connection between the susceptibility of corals to heat stress and anthropogenically elevated nutrient levels. Here, I present evidence that unbalanced ratios of dissolved inorganic nitrogen to phosphorus in the water column perturb the lipid composition of photosynthetic membranes of zooxanthellae and result in an increased susceptibility to thermal bleaching. I have developed a novel conceptual model of coral bleaching that introduces nutrient starvation as a cause for increased heat stress susceptibility. The model clarifies the previously unexplained correlation between the reduction of the thermal bleaching threshold of corals and their exposure to coastal run-off with elevated concentrations of dissolved inorganic nitrogen. INCORALS will conduct an in-depth study of nutrient starvation of reef corals, comparing the impact of nitrogen, phosphorus and iron. INCORALS will combine physiological experiments under tightly controlled laboratory conditions and field-based studies, using a suite of optical methods and cutting-edge molecular techniques to study this yet unexplored cause of coral bleaching and define its relevance for coral ecosystems. The improved understanding of coral bleaching gained during this project is urgently required to develop knowledge-based management strategies to support coral reef resilience.