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Nitrogen pollution interacts with heat stress to increase coral bleaching across the seascape.

ABSTRACT: Climate change is increasing the frequency and magnitude of temperature anomalies that cause coral bleaching, leading to widespread mortality of stony corals that can fundamentally alter reef structure and function. However, bleaching often is spatially variable for a given heat stress event, and drivers of this heterogeneity are not well resolved. While small-scale experiments have shown that excess nitrogen can increase the susceptibility of a coral colony to bleaching, we lack evidence that heterogeneity in nitrogen pollution can shape spatial patterns of coral bleaching across a seascape. Using island-wide surveys of coral bleaching and nitrogen availability within a Bayesian hierarchical modeling framework, we tested the hypothesis that excess nitrogen interacts with temperature anomalies to alter coral bleaching for the two dominant genera of branching corals in Moorea, French Polynesia. For both coral genera, Pocillopora and Acropora, heat stress primarily drove bleaching prevalence (i.e., the proportion of colonies on a reef that bleached). In contrast, the severity of bleaching (i.e., the proportion of an individual colony that bleached) was positively associated with both heat stress and nitrogen availability for both genera. Importantly, nitrogen interacted with heat stress to increase bleaching severity up to twofold when nitrogen was high and heat stress was relatively low. Our finding that excess nitrogen can trigger severe bleaching even under relatively low heat stress implies that mitigating nutrient pollution may enhance the resilience of coral communities in the face of mounting stresses from global climate change.

SUBMITTER: Donovan MK

PROVIDER: S-EPMC7071909 | biostudies-literature | 2020 Mar

REPOSITORIES: biostudies-literature

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Nitrogen pollution interacts with heat stress to increase coral bleaching across the seascape.

Donovan Mary K MK Adam Thomas C TC Shantz Andrew A AA Speare Kelly E KE Munsterman Katrina S KS Rice Mallory M MM Schmitt Russell J RJ Holbrook Sally J SJ Burkepile Deron E DE

Proceedings of the National Academy of Sciences of the United States of America 20200224 10

Climate change is increasing the frequency and magnitude of temperature anomalies that cause coral bleaching, leading to widespread mortality of stony corals that can fundamentally alter reef structure and function. However, bleaching often is spatially variable for a given heat stress event, and drivers of this heterogeneity are not well resolved. While small-scale experiments have shown that excess nitrogen can increase the susceptibility of a coral colony to bleaching, we lack evidence that h ...[more]

PMID: 32094188

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Project description:The emergence of genomic tools for reef-building corals and symbiotic anemones comes at a time when alarming losses in coral cover are being observed worldwide. These tools hold great promise in elucidating novel and unforeseen cellular processes underlying the successful mutualism between corals and their algal endosymbionts (Symbiodinium spp.). Since thermal stress triggers a breakdown in the symbiosis (coral bleaching), measuring the transcriptomic response to thermal stress-induced bleaching offers an extraordinary view of the cellular processes specific to coral-algal symbioses. In the present study, we utilized a cDNA microarray containing 2,059 genes of the Caribbean Elkhorn coral Acropora palmata to identify genes differentially expressed upon thermal stress. Fragments from four separate colonies were exposed to elevated temperature (3ËC increase) for two days, and samples were frozen for microarray analysis after 24 and 48 hours. Fragments experienced a 60% reduction in algal cell density after two days. 204 genes were differentially expressed in samples collected one day after thermal stress; in samples collected after two days, 104 genes. Annotations of the differentially expressed genes indicate a conserved cellular stress response in A. palmata involving: 1) growth arrest; 2) chaperone activity; 3) nucleic acid stabilization and repair; and 4) the removal of damaged macromolecules. Other differentially expressed processes include sensory perception, metabolite transfer between host and symbiont, nitric oxide signaling, and modifications to the actin cytoskeleton and extracellular matrix. The results are also compared to those from a previous coral microarray study of thermal stress in Montastraea faveolata. The field experiment was performed at the Smithsonian Tropical Research Instituteâs Bocas del Toro field station in PanamÃ¡ during September and October 2006. Four colonies of A. palmata were sampled from two separate reefs 21 km apart (two colonies from Isla Solarte â 9o19â56.78â N and 82o12â54.65â W, and two colonies from Cayos Zapatillas â 9o15â08.79â N and 82o02â24.63â W). Each colony was broken into six fragments using a hammer and chisel. For each colony, three fragments were placed in a control aquarium, and three fragments were placed in an experimental aquarium fitted with two 200-Watt aquarium heaters, such that each colony was represented by a pair of aquaria (total of three control and three heated aquaria, all 75-liter). The three control aquaria were placed in one large fiberglass pond with continuous water flow, and the three experimental aquaria were placed in another large pond. All aquaria were exposed to shaded ambient light, and each aquarium was a closed system (but contained a pump to generate continuous water flow). Fragments were kept at a depth of ~25cm. HOBO Pendant Temperature/Light Data Loggers (Onset Corp UA-002-64) recorded temperature and light data every three minutes. These data loggers are not designed to measure photosynthetically active radiation (PAR â 400-700nm), as only ~30% of the measured light is in the range of PAR. For this reason, relative light levels in the aquaria are reported (expressed as the percentage of the average 10am to 2pm light intensity measured on a reef ~4m deep in Bocas del Toro (9o22â68.4â N and 82o18â24.6â W) during September and October 2007). Light intensity differed slightly between the four aquaria fitted with HOBOs (control aquaria 1 â 43%; control aquaria 2 â 46%; heated aquaria 1 â 35%; and heated aquaria 2 â 34% of reef light). After an acclimation period of four days at the natural temperature of the seawater system (mean temperature = 30.29Â±0.07oC), a fragment from each control and experimental aquaria was sampled (t0C and t0H). After time zero sampling, the heaters in each of the experimental aquaria were turned on. The temperatures of the experimental aquaria increased to ca. 32oC over three hours. The mean temperature of the control aquaria during the entire experiment was 29.74Â±0.03oC, and the mean temperature of the heated aquaria was 32.72Â±0.32oC. Control and experimental fragments were sampled again one day (1dC and 1dH) and two days (2dC and 2dH) after turning on the heaters. Heated fragments from one of the colonies (col3) showed extreme bleaching after one day of thermal stress. The remaining fragment of col3 was removed at this time to avoid fouling of the water due to death. Thus, there are four replicates for t0C, t0H, 1dC, and 1dH, and three replicates for 2dC and 2dH. All samples were taken at night. Fragments were frozen in liquid nitrogen. We employed a reference design where all control and heat-stressed samples were compared to a pooled reference aRNA sample composed of aRNA from the four t0C fragments. Since all RNA samples were compared to the reference sample, direct comparisons of gene expression across all time points and conditions can be performed.

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Nitrogen pollution interacts with heat stress to increase coral bleaching across the seascape.

Publications

Nitrogen pollution interacts with heat stress to increase coral bleaching across the seascape.

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