Thermal Stress and Resilience of Corals in a Climate-Changing World

Coral reef ecosystems are under the direct threat of increasing atmospheric greenhouse gases, which increase seawater temperatures in the oceans and lead to bleaching events. Global bleaching events are becoming more frequent and stronger, and understanding how corals can tolerate and survive high-temperature stress should be accorded paramount priority. Here, we review evidence of the different mechanisms that corals employ to mitigate thermal stress, which include association with thermally tolerant endosymbionts, acclimatisation, and adaptation processes. These differences highlight the physiological diversity and complexity of symbiotic organisms, such as scleractinian corals, where each species (coral host and microbial endosymbionts) responds differently to thermal stress. We conclude by offering some insights into the future of coral reefs and examining the strategies scientists are leveraging to ensure the survival of this valuable ecosystem. Without a reduction in greenhouse gas emissions and a divergence from our societal dependence on fossil fuels, natural mechanisms possessed by corals might be insufficient towards ensuring the ecological functioning of coral reef ecosystems.

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Mangrove forests mitigate coral bleaching under thermal stress from climate change

10.1101/2021.06.04.447049 ◽

2021 ◽

Author(s):

Jack Johnson ◽

Jaimie Dick ◽

Daniel Pincheira-Donoso

Keyword(s):

Climate Change ◽

Thermal Stress ◽

Coral Reef ◽

Coral Bleaching ◽

Critical Role ◽

Global Scale ◽

Mangrove Forests ◽

Reef Corals ◽

Conservation Actions ◽

Coral Reef Ecosystems

Anthropogenic marine heatwaves are progressively degrading coral reef ecosystems worldwide via the process of coral bleaching (the expulsion of photosynthetic endosymbionts which reveals the coral skeleton). Corals from mangrove lagoons are hypothesised to increase resistance and resilience to coral bleaching, highlighting these areas as potential natural refuges for corals. Our study, the first conducted at a global-scale, reveals that coral reefs associated with mangrove forests are less likely to bleach under thermal stress, and thus, under scenarios of climate warming. The onset of severe bleaching occurred after 3.6 Degree Heating Weeks (DHW) in mangrove-associated reefs, compared to 2.23 DHW for non-mangrove associated reefs. These findings highlight the critical role of mangrove forests for coral reef persistence under climate change. Accordingly, conservation actions targeting the protection of mangroves are expected to contribute to the resilience and resistance of reef corals from bleaching as marine heatwaves continue to become more common.

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Nutrient and sediment loading affect multiple facets of functionality in a tropical branching coral

Journal of Experimental Biology ◽

10.1242/jeb.225045 ◽

2020 ◽

Vol 223 (21) ◽

pp. jeb225045

Author(s):

Danielle M. Becker ◽

Nyssa J. Silbiger

Keyword(s):

Coral Reef ◽

Nitrogen Content ◽

Thermal Performance ◽

Nutrient Loading ◽

Metabolic Responses ◽

Coral Host ◽

Percent Cover ◽

Sediment Loading ◽

Anthropogenic Stressors ◽

Coral Reef Ecosystems

ABSTRACTCoral reefs, one of the most diverse ecosystems in the world, face increasing pressures from global and local anthropogenic stressors. Therefore, a better understanding of the ecological ramifications of warming and land-based inputs (e.g. sedimentation and nutrient loading) on coral reef ecosystems is necessary. In this study, we measured how a natural nutrient and sedimentation gradient affected multiple facets of coral functionality, including endosymbiont and coral host response variables, holobiont metabolic responses and percent cover of Pocillopora acuta colonies in Mo′orea, French Polynesia. We used thermal performance curves to quantify the relationship between metabolic rates and temperature along the environmental gradient. We found that algal endosymbiont percent nitrogen content, endosymbiont densities and total chlorophyll a content increased with nutrient input, while endosymbiont nitrogen content per cell decreased, likely representing competition among the algal endosymbionts. Nutrient and sediment loading decreased coral metabolic responses to thermal stress in terms of their thermal performance and metabolic rate processes. The acute thermal optimum for dark respiration decreased, along with the maximal performance for gross photosynthetic and calcification rates. Gross photosynthetic and calcification rates normalized to a reference temperature (26.8°C) decreased along the gradient. Lastly, percent cover of P. acuta colonies decreased by nearly two orders of magnitude along the nutrient gradient. These findings illustrate that nutrient and sediment loading affect multiple levels of coral functionality. Understanding how local-scale anthropogenic stressors influence the responses of corals to temperature can inform coral reef management, particularly in relation to the mediation of land-based inputs into coastal coral reef ecosystems.

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Light-harvesting in mesophotic corals is powered by a spatially efficient photosymbiotic system between coral host and microalgae

10.1101/2020.12.04.411496 ◽

2020 ◽

Author(s):

Netanel Kramer ◽

Raz Tamir ◽

Or Ben-Zvi ◽

Steven L. Jacques ◽

Yossi Loya ◽

...

Keyword(s):

Coral Reef ◽

Light Harvesting ◽

Optical Scattering ◽

Light Transport ◽

Coral Host ◽

Light Conditions ◽

Plastic Response ◽

Coral Reef Ecosystems ◽

Fundamental Insight ◽

Insight Into

SummaryThe coral-algal photosymbiosis fuels global coral-reef primary productivity, extending from sea level to as deep as 150 m (i.e., mesophotic). Currently, it is largely unknown how such mesophotic reefs thrive despite extremely limited light conditions. Here, we show that corals exhibit a plastic response to mesophotic conditions that involves a spatially optimized regulation of the bio-optical properties by coral host and symbiont. In contrast to shallow corals, mesophotic corals absorbed up to three-fold more light, resulting in excellent photosynthetic response under light conditions of only ~3% of the incident surface irradiance. The enhanced light harvesting capacity of mesophotic corals is regulated by average refractive index fluctuations in the coral skeleton that give rise to optical scattering and facilitate light transport and absorption by densely pigmented host tissue. The results of this study provide fundamental insight into the energy efficiency and light-harvesting mechanisms underlying the productivity of mesophotic coral reef ecosystems, yet also raise concerns regarding their ability to withstand prolonged environmental disturbances.

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