Physiological plasticity of corals to temperature stress in marginal coral communities

Local adaptation of trophic strategy determined the tolerance of coral Galaxea fascicularis to environmental fluctuations

The escalation of global change has resulted in heightened frequencies and intensities of environmental fluctuations within coral reef ecosystems. Corals originating from marginal reefs have potentially enhanced their adaptive capabilities in response to these environmental variations through processes of local adaptation. However, the intricate mechanisms driving this phenomenon remain a subject of limited investigation. This study aimed to investigate how corals in Luhuitou reef, a representative relatively high-latitude reef in China, adapt to seasonal fluctuations in seawater temperature and light availability. We conducted a 190-day plantation experiment with the widespread species, Galaxea fascicularis, in Luhuitou local, and from Meiji reef, a typical offshore tropical reef, to Luhuitou as comparison. Drawing upon insights from physiological adaptations, we focused on fatty acid (FA) profiles to unravel the trophic strategies of G. fascicularis to cope with environmental fluctuations from two origins. Our main findings are threefold: 1) Native corals exhibited a stronger physiological resilience compared to those transplanted from Meiji. 2) Corals from both origins consumed large quantities of energy reserves in winter, during which FA profiles of local corals altered, while the change of FA profiles of corals from Meiji was probably due to the excessive consumption of saturated fatty acid (SFA). 3) The better resilience of native corals is related to high levels of functional polyunsaturated fatty acid (PUFA), while insufficient nutrient reserves, possibly due to weak heterotrophic ability, result in the obstruction of the synthesis pathway of PUFA for corals from Meiji, leading to their intolerance to environmental changes. Consequently, we suggest that the tolerance of G. fascicularis to environmental fluctuations is determined by their local adapted trophic strategies. Furthermore, our findings underscore the notion that the rapid adaptation of relatively high-latitude corals to seasonal environmental fluctuations might not be readily attainable for their tropical counterparts within a brief timeframe.

Response of coral bacterial composition and function to water quality variations under anthropogenic influence

Microbial communities play key roles in the adaptation of corals living in adverse environments, as the microbiome flexibility can enhance environmental plasticity of coral holobiont. However, the ecological association of coral microbiome and related function to locally deteriorating water quality remains underexplored. In this work, we used 16S rRNA gene sequencing and quantitative microbial element cycling (QMEC) to investigate the seasonal changes of bacterial communities, particularly their functional genes related to carbon (C), nitrogen (N), phosphorus (P) and sulfur (S) cycle, of the scleractinian coral Galaxea fascicularis from nearshore reefs exposed anthropogenic influence. We used nutrient concentrations as the indicator of anthropogenic activities in coastal reefs, and found a higher nutrient pressure in spring than summer. The bacterial diversity, community structure and dominant bacteria of coral shifted significantly due to seasonal variations dominated by nutrient concentrations. Additionally, the network structure and nutrient cycling gene profiles in summer under low nutrient stress was distinct from that under poor environmental conditions in spring, with lower network complexity and abundance of CNPS cycling genes in summer compared with spring. We further identified significant correlations between microbial community (taxonomic composition and co-occurrence network) and geochemical functions (abundance of multiple functional genes and functional community). Nutrient enrichment was proved to be the most important environmental fluctuation in controlling the diversity, community structure, interactional network and functional genes of the coral microbiome. These results highlight that seasonal shifts in coral-associated bacteria due to anthropogenic activities alter the functional potentials, and provide novel insight about the mechanisms of coral adaptation to locally deteriorating environments.

Learning from the past is not enough to survive present and future bleaching threshold temperatures

In the past decade, the frequency of mass coral bleaching events has increased due to seawater temperature anomalies persisting for longer periods. Coral survival from temperature anomalies has been based on how each species in each location responds to stress, which is unique to individual species and may be due to the way stressful experiences accumulate through time in the form of ecological and physiological memory. A deeper understanding of ecological and physiological memory in corals is necessary to understand their survival strategies into the future. Laboratory experiments can help us simulate seawater temperatures experienced by corals in the past and compare their responses to those of the present and future. In this study, we sampled corals with different life history traits from one location perturbed by seawater temperature incursions (variable site) and from a second, relatively undisturbed location (stable site). We sampled across two seasons to observe the responses to bleaching threshold temperatures in the past (1998-29 °C), present (2018-31 °C), and future (2050-33 °C). Corals were healthy at 29 °C and 31 °C, but a fast-growing, temperature-susceptible coral species experienced high mortality at 33 °C compared to a slow-growing, temperature-resistant coral species. Moreover, corals from the variable site and during the spring season fared better under temperature stress. The results of this study provide insight into the possible role of life-history traits on coral's response to seasons and locations in terms of memory to long-term and short-term thermal anomalies and climate change.

Selective deep water coral bleaching occurs through depth isolation

Climate change is degrading coral reefs around the world. Mass coral bleaching events have become more frequent in recent decades, leading to dramatic declines in coral cover. Mesophotic coral ecosystems (30-150 m depth) comprise an estimated 50-80 % of global coral reef area. The potential for these to act as refuges from climate change is unresolved. Here, we report three mesophotic-specific coral bleaching events in the northern Red Sea over the course of eight years. Over the last decade, faster temperature increases at mesophotic depths resulted in ~50 % decline in coral populations, while the adjacent shallow coral reefs remained intact. Further, community structure shifted from hard coral dominated to turf algae dominated throughout these recurrent bleaching events. Our results do not falsify the notion of the northern Red Sea as a thermal refuge for shallow coral reefs, but question the capacity of mesophotic ecosystems to act as a universal tropical refuge.

Lower cold tolerance of tropical Porites lutea is possibly detrimental to its migration to relatively high latitude refuges in the South China Sea

As high temperature stress due to climate change threatens tropical corals, cooler areas at relatively high latitudes may be potential refuges. Tolerance to low temperatures is critical in determining whether corals can successfully migrate to higher latitudes. However, the physiological and molecular adaptations that protect corals from low temperature stress are unclear. In this study, scleractinian Porites lutea samples from the tropical Xisha Islands (XS) and subtropical Daya Bay (DY) in the South China Sea were subjected to a reduction in ambient temperature from 26 to 12°C. Differences in physiological changes and gene expression were analysed. P. lutea from both XS and DY exhibited physiological bleaching under low temperature stress, and the Symbiodiniaceae density, Fv/Fm, and chlorophyll-α content were significantly reduced. Symbiosome antioxidative stress and metabolic enzyme activity first increased and then decreased. RNA-seq analysis showed that the host responded to low temperature stress by activating immune, apoptotic, and autophagic pathways and reducing metabolic levels. Nevertheless, Symbiodiniaceae lacked the physiological regulatory capacity to adapt to low temperatures. The lower cold tolerance of XS tropical P. lutea may attribute to lower oxidative stress resistance, lower photosynthetic capacity, worse energy supply, and higher susceptibility to bacterial and viral infections and diseases in XS corals. The difference in cold tolerance may result from genetic differences between the geographic populations and is possibly detrimental to the migration of tropical coral to relatively high latitude refuges. This study provides a theoretical basis for anthropogenically assisted coral migration as a response to global change.

Microcosm study on cold adaptation and recovery of an exotic mangrove plant, Laguncularia racemosa in China

Laguncularia racemosa (a white mangrove) is an exotic mangrove species commonly distributed in southern intertidal zones in China since it was introduced for reforestation purposes in 1999. However, the invasiveness of this exotic species and its cold adaptability have rarely been reported. The present work determined the cold resistance level of L. racemosa and its recovery from cold stress, aiming to speculate its potential invasive capability in China. Results showed that the germination of L. racemosa seeds in sand or in simulated sea field models was significantly inhibited by a series of cold treatments, with no germination at 5 °C and decreased in germination at low temperatures (15-25 °C). Low temperature also reduced net photosynthetic rate (A), water use efficiency (WUE), transpiration rate (E), and stomatal conductance (Gs) of the seedlings of L. racemosa. On the other hand, cold stress up-regulated in leaves of malondialdehyde (MDA) and antioxidant activities, including superoxide dismutase (SOD), glutathione reductase (GR), and ascorbate peroxidase (APX). Additionally, these physiological and biochemical indexes of cold-stressed L. racemosa could recover to the original levels if the plants were returned to room temperature with a few exceptions. For instance, the cold exposure duration altered seedlings' physiology, but the photosynthetic related activities could not recover if cold treatment lasted for 120 h. This study suggests that L. racemosa can tolerate low temperatures to some extent, thus settle and even invade the coast of China at high latitudes having cold winter, which poses a challenge to the conservation and management of local mangrove ecosystems.

Does Predation Exacerbate the Risk of Endosymbiont Loss in Heat Stressed Hermatypic Corals? Molecular Cues Provide Insights Into Species-Specific Health Outcomes in a Multi-Stressor Ocean

Ocean warming has been a major driver of coral reef bleaching and mass mortality. Coupled to other biotic pressures, corals’ ability for acclimatization and adaptation may become compromised. Here, we tested the combined effects of warming scenarios (26, 30, and 32°C) and predation (wound vs. no wound) in coral health condition (paleness, bleaching, and mortality), cellular stress responses (heat shock protein 70 kDa Hsp70, total ubiquitin Ub, and total antioxidant capacity TAC), and physiological state (integrated biomarker response index, IBR) of seven Scleractinian coral species, after being exposed for 60 days. Results show that although temperature was the main factor driving coral health condition, thermotolerant species (Galaxea fascicularis, Psammocora contigua, and Turbinaria reniformis) displayed increased paleness, bleaching, and mortality in predation treatments at high temperature, whereas thermosensitive species (Acropora tenuis, Echinopora lamellosa, and Montipora capricornis brown and green morphotypes) all died at 32°C, regardless of predation condition. At the molecular level, results show that there were significant main and interactive effects of species, temperature, and predation in the biomarkers assessed. Temperature affected Hsp70, Ub, and TAC, evidencing the role of protein folding and turnover, as well as reactive oxygen species scavenging in heat stress management. Predation increased Hsp70 and Ub, suggesting the activation of the pro-phenoloxidase system and cytokine activity, whereas the combination of both stressors mainly affected TAC during moderate stress and Ub under severe stress, suggesting that redox balance and defense of homeostasis are crucial in tissue repair at high temperature. IBR levels showed an increasing trend at 32°C in predated coral fragments (although non-significant). We conclude that coral responses to the combination of high temperature and predation pressure display high inter-species variability, but these stressors may pose a higher risk of endosymbiont loss, depending on species physiology and stress intensity.