The impact of environmental conditions on the heat tolerance of Acropora hyacinthus

Abstract

While coral bleaching events are becoming increasingly frequent due to climate change, some nearshore zones exposed to high turbidity and cold-water upwelling may serve as thermal refuges to effectively mitigate the effects of heat stress. Understanding how corals in these zones adapt to environmental stressors can aid in coral management. In this study, we collected Acropora hyacinthus from two zones near Hainan Island: Changjiang, a nearshore coastal zone with high turbidity, and Wuzhizhou, a zone affected by the Qiongdong upwelling in the summer. The corals were subjected to heat stress in the laboratory up to 32 °C, and their responses were compared and analyzed. A. hyacinthus from Changjiang exhibited superior thermal tolerance, potentially due to its higher total energy reserves, consistent composition of symbiotic microbial communities (especially the photosynthetically active Symbiodiniaceae C1 and abundance of probiotic bacteria), and stability of metabolic activity. Upon heat stress, it up-regulated various metabolic pathways, including arachidonic acid metabolism, pantothenate and CoA biosynthesis, and ABC transporters, which indicated good adaptive capacity to high-temperature settings. Conversely, A. hyacinthus from Wuzhizhou was more vulnerable. Upon heat stress, it had a notable rise in bacterial diversity, a reduction in the abundance of probiotic bacteria, and down-regulated metabolic pathways. Thus, compared to corals in nearshore turbid zones, corals sheltered by the summer upwelling in the South China Sea east of Hainan Island are more sensitive to rising temperatures and are at a higher risk if the Qiongdong upwelling weakens due to further climate change.