Deoxygenation over coral reefs triggers bleaching and microbial community shift

The threat of ocean deoxygenation to coastal ecosystems is increasing globally, but its impacts are primary recognized in temperate regions. Recent evidence suggests that low-oxygen (hypoxic) events can have catastrophic effects in tropical ecosystems as well, including mass mortality on coral reefs, and are likely more widespread than previously thought. The ecological response of coral ecosystems to these events are poorly documented, but are needed to develop a predictive understanding of community dynamics and the potential for reef resilience. Here, we present results from detailed sampling from two sites spanning a hypoxic gradient during an event in Caribbean Panama. We sampled indicators of coral physiological performance and found that photosynthetic performance, zooxanthellae density, and photosynthetic pigment concentrations were all depressed at the hypoxic site relative to the site with normal oxygen (normoxic) conditions. This indicates a bleaching response more commonly associated with thermal stress, and represents a sublethal decreased in performance as corals absorb the stress of hypoxia. During the same event, we also sampled the microbial community in the water directly over the reef. The community at the hypoxic site was notably different than the microbial assemblage at the normoxic site. Notably, the microbial community at the hypoxic site rebounded within weeks and converged on that found at the normoxic site. This potential for rapid recovery has important implications for key ecological processes that are regulated by microbes on coral reefs including metabolic rates, nutrient cycling, andpathogen virulence. Our study provides some of the first evidence of ecological shifts on coral reefs in response to hypoxia, and reveals the capacity for resilience that will be important for predicting how the structure and function of reefs will respond to increasingly common low-oxygen events.