Heat-tolerant intertidal rock pool coral Porites lutea can potentially adapt to future warming.

The growing threat of global warming on coral reefs underscores the urgency of identifying heat-tolerant corals and discovering their adaptation mechanisms to high temperatures. Corals growing in intertidal rock pools that vary markedly in daily temperature may have improved heat tolerance. In this study, heat stress experiments were performed on scleractinian coral Porites lutea from subtidal habitat and intertidal rock pool of Weizhou Island in the northern South China Sea. Thermotolerance differences in corals from the two habitats and their mechanisms were explored through phenotype, physiological indicators, ITS2, 16S rRNA, and RNA sequencing. At the extremely high temperature of 34°C, rock pool P. lutea had a stronger heat tolerance than those in the subtidal habitat. The strong antioxidant capacity of the coral host and its microbial partners was important in the resistance of rock pool corals to high temperatures. The host of rock pool corals at 34°C had stronger immune and apoptotic regulation, downregulated host metabolism and disease-infection-related pathways compared to the subtidal habitat. P. lutea, in this habitat, upregulated Cladocopium C15 (Symbiodiniaceae) photosynthetic efficiency and photoprotection, and significantly increased bacterial diversity and coral probiotics, including ABY1, Ruegeria, and Alteromonas. These findings indicate that rock pool corals can tolerate high temperatures through the integrated response of coral holobionts. These corals may be 'touchstones' for future warming. Our research provides new insights into the complex mechanisms by which corals resist global warming and the theoretical basis for coral reef ecosystem restoration and selection of stress-resistant coral populations.

Reduced genetic diversity and restricted gene flow of broadcast-spawning coral Galaxea fascicularis in the South China Sea reveals potential degradation under environmental change.

Under the dual effects of climate change and anthropogenic activities, coral reefs in the South China Sea (SCS) are at serious risk of degradation. Galaxea fascicularis is a widely distributed species in the SCS, and the study of its genetics, survival, and adaptability is conducive to further understanding the future characteristics of coral reefs in the SCS. In this study, 146 G. fascicularis samples were selected from 9 survey stations across 12 latitudes in the SCS, and 8 pairs of microsatellite markers were used to characterize their genetic diversity and structure. The results showed moderate genetic diversity index values (Ar = 3.444-4.147, He = 0.634-0.782, Ho = 0.367-0.586). The AMOVA results and pairwise FST values showed a moderate level of genetic differentiation (ΦST = 0.119, P < 0.05) among G. fascicularis populations in the SCS, whereas its genetic structure showed high genetic differentiation (FST = 0.062-0.225) among relatively high-latitude populations (n = 3) and low genetic differentiation (FST = 0.012-0.064) in low-latitude populations (n = 6). The living environment of relatively high-latitude populations is disturbed by high-intensity human activities, leading to the specialization of local populations. Mantel test results showed a significant positive correlation between genetic differentiation among G. fascicularis populations and sea surface temperature (SST) variance (R2 = 0.4885; Mantel test, p = 0.010 < 0.05) in addition to geographical distance (R2 = 0.1134; Mantel, test p = 0.040 < 0.05), indicating that SST and geographical isolation were primary factors affecting the genetic structure of this species in the SCS. The lower genetic diversity and limited gene flow of G. fascicularis indicate limited genetic adaptation, and corresponding vulnerability may be more pronounced under future environmental changes. These findings provide a theoretical basis for the conservation and restoration of coral reefs in the SCS.

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.

High genetic differentiation and moderate genetic diversity of the degenerative branching coral Pocillopora verrucosa in the tropical South China Sea.

Global warming is causing rapid degradation of coral reefs, among which branching corals are degrading the fastest. An assessment of coral genetic diversity and adaptive potential provides a basis for coral reef protection. In this study, we selected the branching coral Pocillopora verrucosa, a widely distributed species in the tropical South China Sea (SCS), to carry out population genetic studies. To analyze the genetic diversity and structure of 319 P. verrucosa samples from 10 populations in 4 SCS regions, twelve pairs of microsatellite primers and two nuclear markers, ITS and ß-tub, were selected. Microsatellite marker results showed moderate genetic diversity for P. verrucosa in the SCS, but relatively low diversity in Dazhou Island and Yongxing Island. The haplotype network showed that P. verrucosa in the SCS was derived from two ancestors, which may be linked to geographical isolation in the Pleistocene glacial period. AMOVA (ΦST = 0.3375) and FST pairwise analysis results based on ß-tub showed that the populations were highly differentiated, with most FST values (21/45) > 0.25. Yongxing and Qilianyu Islands populations were significantly different from those in the Xisha area. Mantel test results showed that genetic differentiation among P. verrucosa populations was significantly and positively correlated with both mean sea surface temperature (SST) and SST variance, and was not correlated with distance, chlorophyll-a, or turbidity. The reproductive mode of brooding planulae was an important factor contributing to high genetic differentiation among populations. The moderate genetic diversity of SCS P. verrucosa indicates that this population has a certain genetic potential in the context of global changes, but the high genetic differentiation between populations increases the risk of local degradation or extinction. This study provides a theoretical basis for the protection and restoration of SCS coral reefs.

Genetic structure of Turbinaria peltata in the northern South China Sea suggest insufficient genetic adaptability of relatively high-latitude scleractinian corals to environment stress.

With the rapid degradation of coral reefs due to global warming and anthropogenic impacts, relatively high-latitude areas, such as the northern South China Sea (SCS), are likely to become refuges for tropical coral species. Here we investigated the genetic features and adaptability of one dominant scleractinian coral species, Turbinaria peltata, in the northern SCS. A total of 81 samples from 5 sites were studied to explore potential mechanisms of adaptability to environmental stress as a result of climate change. Ten microsatellite markers developed in this study, one nuclear gene (internal transcribed spacer, ITS), and one mitochondrial gene (mitochondrial cytochrome oxidase subunit I gene, mtDNA COI) were used. Our results indicated that the genetic diversity of T. peltata in the northern SCS is low (Ar = 1.403-2.011, Ho = 0.105-0.248, He = 0.187-0.421) with the lowest in Dongfang population (DF) (Ar = 1.403, Ho = 0.22, He = 0.187). These results indicate that T. peltata has insufficient genetic adaptability and may unable to handle increasingly complex global changes. A significantly moderate genetic differentiation was observed among T. peltata populations (ΦST = 0.167), in addition to a high genetic differentiation between DF and other populations (FST = 0.272-0.536 > 0.25). The DF population near a fishing port was exposed to severe anthropogenic environmental stress, which may drive the extraordinarily high genetic differentiation between DF and other populations. Furthermore, the Mantel test results showed that the genetic differentiation of the other four populations was strongly correlated with the average sea surface temperature (SST) (R2 = 0.82, Mantel test P < 0.05) and geographical distance (R2 = 0.57, Mantel test P < 0.05). Our results suggest that the genetic structure of T. peltata in the relatively high-latitude of the SCS was significantly affected by average SST, geographical isolation, and anthropogenic activities. These findings provide a theoretical foundation for the protection of relatively high-latitude coral reefs.