Defining nutritional bottlenecks of reef corals growth and stress tolerance

Among the most charismatic and diverse ecosystems on Earth, coral reefs sustain >25% of all marine biodiversity in only <1% of ocean surface area and the livelihoods of >0.5 billion people. Yet the habitat-building corals are acutely vulnerable to anthropogenic change. Best studied are the threats posed to coral reefs by ocean warming. Perturbations to the natural nutrient environment also represent an existential danger, but why is this the case? This question has old origins. More than 175 years after Charles Darwin's seminal observation, we still lack a convincing mechanistic explanation as to why reef corals thrive, sustaining exceptionally high rates of biological production, in nutrient-poor oceanic regions. The answer is certainly rooted in the symbiotic association between endocellular microalgae (zooxanthellae) and the coral animal host but the physiological processes involved are far from understood. Recent field studies fundamentally contradict existing models for symbiosis functioning by suggesting that corals source most of their nutrients from the water column as dissolved inorganic nitrogen (DIN) and phosphorus (DIP) but there is no known mechanism for such a process in coral animals. Our work (pilot study results herein) suggests that corals graze on their symbiont population, providing a novel mechanism for DIN and DIP transfer to the host. We propose to transform our understanding of nutrient cycling and symbiosis in corals worldwide by testing the grazing hypothesis in ten representative reef-building species. We will: (i) Document DIN and DIP uptake and utilisation by symbiont-grazing through (15N) isotopic labelling and elemental analysis in controlled long-term experiments. ii) Quantify the efficiency of nutrient incorporation by symbiont-grazing versus zooplankton-feeding and compare the resulting effects on coral growth and stress tolerance. iii) Determine how present-day and projected future ocean temperature and pH influence the utilisation of dissolved inorganic nutrients and shape coral growth and symbiosis functioning. We will also undertake a broad range of impact-promoting activities including a knowledge transfer workshop. For this event, we will bring stakeholders in coral reef management and policy development together with leading specialists in coral reef nutrient biology, spatial ecology and biogeochemical modelling to translate our findings into advice for policy makers and coral reef management.