1. Introduction
Marine heatwaves (MHWs) are extreme climate events of anomalously high surface temperature, which might last for days to months from local to regional scales (Hughes et al., 2019). With the ongoing climate change, MHWs have become more frequent and severe, resulting in escalating damage to coral reefs (Hughes et al., 2018; Oliver et al., 2018; Dietzel et al., 2020). These MHWs lead to abnormally high water temperatures, causing corals to bleach by expelling the symbiotic algae within a short perioding of time, which would affect the survival and reproduction of corals, even destroying the entire coral ecosystem (Hoegh-Guldberg et al., 2007; Hughes et al., 2018).
It is worth noting that more and more studies have found that corals have some tolerance to high temperatures, and based on this tolerance, corals may survive after a short time of heat stress (Hughes et al., 2003). Studies have found that Corals might adjust their zooxanthellae density or ratio (Barker, 2018; Yu et al., 2020), modify the composition of their membrane lipids, particularly by increasing the presence of unsaturated fatty acids with double bonds, and enhance the production of antioxidant substances, enzymatic antioxidants like superoxide dismutase (SOD), catalase (CAT), and glutathione-S-transferase (GST), as well as non-enzymatic antioxidants such as vitamins, carotenoids, and tocopherols to acclimate to the high temperature(Krueger et al., 2017; Kultz, 2020). These adaptive mechanisms bolster the coral’s ability to withstand stress and protect the integrity of their cell membrane structures.
In addition to corals and zooxanthellae changes, there are also research findings that have highlighted the critical role of coral symbiotic bacteria in helping corals cope with heat stress (Claar et al., 2020; Sun et al., 2023). Previous studies have found that the composition and function of coral’s symbiotic bacteria might change to facilitate the coral’s adaptation and ecological plasticity in adapting to rapid environmental changes (Roder et al., 2014; Frade et al., 2016; Lee et al., 2016; Neave et al., 2016).
Due to global warming, there is an increasing trend of MHWs (Frolicher et al., 2018; Oliver, 2019a; Oliver et al., 2019b). Although there have been numerous studies have reported on the mechanisms of coral response to MHWs, research on the impacts of repeated exposure to MHWs remains limited (Claar et al., 2020; Marzonie et al., 2023).
Turbinaria peltata is widely distributed in the Indo-Pacific region, and it is an essential ecological and dominant species in the South China Sea. Due to its resistance to environmental changes and stress, it can be used as an important model organism to study the response mechanism of coral holobiont to the environment. So in this study, we investigate the physiological and microbiological effects of repeated MHWs on reef-building coral Turbinaria peltata to assess its responses to extreme climatic conditions. These findings will provide valuable data and reference for understanding the response and adaptation mechanisms of corals to repeated MHWs.