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The increasing threat of ocean warming has led to the more frequent endangerment of coral reefs, including the heat-tolerant Pavona decussata. To shed light on the molecular mechanisms involved in the response of coral to ocean warming, we investigated the gene expression profiles of P. decussata after natural thermal stress. Using PacBio Sequel II sequencing technology, we obtained relatively complete transcriptome data for P. decussata and then analyzed its gene expression quantitatively with Illumina RNA-seq technology. We acquired information on gene function, structure, and expression profile from coral host and zooxanthellae. Analysis of Illumina sequencing data revealed that unbleached coral host might rely on the active utilization of amino acids to maintain a stable living condition based on the tricarboxylic acid cycle under high temperature stress, and that zooxanthellae might benefit from ammonium produced by coral host. Moreover, the downregulation of unbleached coral host gene expression in innate immune pathways centered on the transcription factors that heat shock factor and nuclear factor (NF)-κB, as well as the tyrosine kinase pathway, might be crucial for maintaining the equilibrium of the zooxanthellae under thermal stress. Thus, the differences in these molecular mechanisms could determine, to some extent, whether coral host can maintain a symbiotic relationship with algae under heat stress. This study elucidated the molecular mechanisms underlying differences in thermal tolerance within P. decussata species and supported further theoretical basis in coral molecular biology and ecological conservation, which enhance our comprehension of coral responses to future climate change. [ABSTRACT FROM AUTHOR]

Coral reef bleaching is usually characterized by expulsion of symbiotic zooxanthellae, loss of zooxanthellae pigmentation, or both. We collected 128 samples comprising 39 species of 21 genera of reef-building corals from Luhuitou and Xiaodonghai in Sanya of Hainan Island and Daya Bay of Guangdong Province, respectively, and analyzed the symbiotic zooxanthellae population density. The results show that: (1) the symbiotic zooxanthella density varies from 0.67×106 to 8.48×106 cell/cm2, displaying significant interspecies variability, with branch corals usually having relatively less zooxanthellae (ranging from 0.67×106 to 2.47×106 cell/cm2) than massive species (from 1.0×106 to 8.48×106 cell/cm2); (2) corals inhabiting within 4 m water depth have higher levels of symbiotic zooxanthellae than those living at the bottom (-.7 m depth) of the reef area; (3) there is no discernable difference in the zooxanthellae density between corals from relatively high latitude Daya Bay (-22°N) and those from relatively low latitude Sanya (-18°N) at comparable sea surface temperatures (SST); (4) in partially-bleached corals, the density of zooxanthellae shows the following order: healthy-looking part> semi-bleached part > bleached part. Based on the above results, we suggest that (1) the zooxanthellae density difference between branching and massive coral species is the main cause that branching corals are more vulnerable to bleaching than massive corals. For example, symbiotic zooxanthellae levels are low in branching Acropora and Pocillopora corals and thus these corals are more susceptible to bleaching and mortality; (2) symbiotic zooxanthellae density can also be affected by environmental conditions, such as sediment loads, diving-related turbidity, and aquaculture-related nitrate and phosphate input, and their increase may reduce symbiotic zooxanthellae density in corals. [ABSTRACT FROM AUTHOR]

Relatively high-latitude waters are supposed as a refuge for corals under ocean warming. A systematic assessment of the Weizhou Island reef in the northern South China Sea, a relatively high-latitude region, shows that the ecosystem restoration index decreased from 0.96 to 0.62 during the period between 1990 and 2015. Although the biotic community, supporting services, and regulating services remained at good or very good states, the provisioning services, cultural services, and especially habitat structure deteriorated to very poor or moderate states. Gray relational analysis showed that these ecological declines exhibited a strong relationship with human pressures from tourism activities and the petrochemical industry. The recoveries of the biotic community and supporting services that benefited from wintertime warming appeared to be partly offset by intensive human pressures. The long-term effects on ecosystem structure and functions suggest that anthropogenic disturbances have impaired the possibility of this area serving as a potential thermal refuge for reef-building corals in the South China Sea. This study thus provides an integrated approach for assessing the adaptive responses of coral reef ecosystems to climate change and local human activities. [ABSTRACT FROM AUTHOR]