Project: RAPID: Ecosystem impact of a coral bleaching event: The role of coral exudates in shifting oligotrophic biogeochemistry and reef microbiomes.

NSF Award Abstract:
Shallow tropical reefs are biodiversity hotspots. Their ecosystem services make them key areas of economic, ecological, and cultural importance. Yet coral reefs are under significant threat due to both local and global stressors which can lead to coral bleaching, disease, and eventually coral death. When corals bleach, they release materials such as dead tissue, mucus, bacteria, and viruses that may affect the entire ecosystem. This project uses a wide-spread bleaching event at a Long Term Ecological Research site in French Polynesia to explore how these released materials impact the reef ecosystem The water chemistry and microbes associated with the corals and surrounding water are examined. This research aims to better understand how corals interact with their environment and how this interaction changes when corals are stressed. Throughout this project two female graduate students are being trained and interactive programs are used to communicate results to elementary, high school, and undergraduate students in Oregon. A 20-minute documentary for web release focusing on the reef scale impact of coral bleaching is in preparation together with film students.

Coral exudates include particulate and dissolved material (sloughed tissue, mucus, bacteria, viruses) that together add limiting nutrients and carbon compounds to the reef, fueling auto- and heterotrophic bacterial production. In recent experiments, coral-bleaching derived exudates were observed to themselves cause rapid bleaching, and often mortality, of previously healthy corals. Importantly, these negative impacts of coral exudate exposure were far greater than thermal stress. These experiments provided insight into a novel mechanism in which bleaching corals can adversely affect the health of adjacent corals. This project leverages these data and an extensive and ongoing bleaching event on the island of Mo?orea to quantify the cascading effects of coral exudates on reef ecosystems. It is hypothesized that during widespread bleaching: (1) DOC is significantly elevated across the reefscape; (2) coral holobiont components become enriched in the pelagic microbiome; (3) the water column microbial community shifts in function to increased heterotrophy and pathogenicity; and (4) coral holobionts diverge from their previous stable state leading to coral reef dysbiosis and/or disease and mortality. Sampling throughout the course of an ongoing bleaching event in Mo?orea is used to quantify the effects of this bleaching event on DOM dynamics and reef health. Mo'orea is situated in the MCR Long Term Ecological Research (MCR LTER) site. The rare reef-scale bleaching event at this well-studied location provides the unusual opportunity to quantify the impact of coral exudates on reef health and to better understand the temporal and spatial impacts of an island-wide bleaching event in an oligotrophic ecosystem. Measurement of the amount of organic matter released per unit area of coral on an island where the reef ecosystem is well parameterized over time and space allows development of a model of the impact of bleaching events on the island carbon budget.

This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.