Adaptive changes of coral Galaxea fascicularis holobiont in response to nearshore stress

Evolutionary radiation and microbial community dynamics shape the thermal tolerance of Fungiidae in the southern South China Sea

Fungiidae have shown increased thermal adaptability in coral reef ecosystems under global warming. This study analyzes the evolutionary divergence and microbial communities of Fungiidae in the Sanjiao Reef of the southern South China Sea and explores the impact of coral evolution radiation and microbial dynamics on the heat tolerance of Fungiidae. The results found that Cycloseris was an ancient branch of Fungiidae, dating back approximately 147.8953 Mya, and Fungiidae differentiated into two ancestral clades (clades I and II) before 107.0312 Ma. Fungiidae exhibited specific symbioses with the Cladocopium C27 sub-clade. Notably, the Cladocopium C1 sub-clade has a high relative abundance in clade I, whereas the heat-tolerant Cladocopium C40 and C3u sub-clades subdominante in clade II. Regarding bacterial communities, Cycloseris costulata, the earliest divergent species, had higher bacterial β-diversity, while the latest divergent species, Lithophyllon scabra, displayed lower bacterial α-diversity and higher community stability. Beneficial bacteria dominante Fungiidae's bacterial community (54%). The co-occurrence network revealed that microbial networks in clade II exhibited lower complexity and greater resilience than those in clade I. Our study highlights that host evolutionary radiation and microbial communities shaped Fungiidae's thermal tolerance. The variability in subdominant Symbiodiniaceae populations may contribute to interspecific differences in thermal tolerance along the evolutionary branches of Fungiidae. The presence of abundant beneficial bacteria may further enhance the thermal ability of the Fungiidae. Furthermore, the later divergent species of Fungiidae have stronger heat tolerance, possibly driven by the increased regulation ability of the host on the bacterial community, greater microbial community stability, and interaction network resistance.IMPORTANCECoral reefs are facing significant threats due to global warming. The heat tolerance of coral holobionts depends on both the coral host and its microbiome. However, the association between coral evolutionary radiation and interspecific differences in microbial communities remains unclear. In this study, we investigated the role of evolutionary radiation and microbial community dynamics in shaping the thermal acclimation potential of Fungiidae in the Sanjiao Reef of the southern South China Sea. The study's results suggest that evolutionary radiation enhances the thermal tolerance of Fungiidae. Fungiidae species that have diverged more recently have exhibited a higher presence of heat-tolerant Symbiodiniaceae taxa, more stable bacterial communities, and a robust and resilient microbial interaction network, improving the thermal adaptability of Fungiidae. In summary, this study provides new insights into the thermal adaptation patterns of corals under global warming conditions.

Physiological and microbiome adaptation of coral Turbinaria peltata in response to marine heatwaves

Against the backdrop of global warming, marine heatwaves are projected to become increasingly intense and frequent. This trend poses a potential threat to the survival of corals and the maintenance of entire coral reef ecosystems. Despite extensive evidence for the resilience of corals to heat stress, their ability to withstand repeated heatwave events has not been determined. In this study, we examined the responses and resilience of Turbinaria peltata to repeated exposure to marine heatwaves, with a focus on physiological parameters and symbiotic microorganisms. In the first heatwave, from a physiological perspective, T. peltata showed decreases in the Chl a content and endosymbiont density and significant increases in GST, caspase-3, CAT, and SOD levels (p < .05), while the effects of repeated exposure on heatwaves were weaker than those of the initial exposure. In terms of bacteria, the abundance of Leptospira, with the potential for pathogenicity and intracellular parasitism, increased significantly during the initial exposure. Beneficial bacteria, such as Achromobacter arsenitoxydans and Halomonas desiderata increased significantly during re-exposure to the heatwave. Overall, these results indicate that T. peltata might adapt to marine heatwaves through physiological regulation and microbial community alterations.

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.

Consistent responses of coral microbiome to acute and chronic heat stress exposures

Frequent and intense heat waves lead to bleaching and even death of reef-building corals, and the thermal tolerance ultimately depends on the genetic composition of the holobiont. Here, we compared the effects of acute and chronic heat stress exposures on coral Porites cylindrica holobiont. Regardless of the temperature treatment, corals at 33 °C showed signs of bleaching and a significant decrease in photochemical efficiency (Fv/Fm). However, Symbiodiniaceae communities were relatively stable and all dominated by the same genus Cladocopium (C15). The relative abundanbce of core microbiome varied significantly, and they may provide several functions important to holobiont fitness. Both heat stress exposures induced the significant structural reorganization of coral-associated bacteria, with bacterial diversity and community heterogeneity significantly increasing with the temperature treatment. The modified stochasticity ratio (MST) revealed that stochastic processes dominated bacterial community assembly in thermally stressed corals. Certain core bacterial members that were hypothesized to fulfil functional niche decreased significantly, with the enrichment of potentially pathogenic and opportunistic bacteria in heat stress exposures. Thermally stressed corals had more positive correlation, higher network complexity and tighter associations among microbial taxa, relative to healthy corals. Overall, the coral microbiome exhibits similar responses to acute and chronic heat stress, and our study provides new insights about the deleterious impacts of complex warming oceans on coral holobiont.