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.