Physiological response of Porites lutea to high temperature stress

To explore the synergistic response characteristics of host corals and symbiotic zooxanthellae to temperature stress, the study focus on a typical reef-building coral species-Porites lutea, as the research subject, cycling corals between bleaching and health by precisely controlling the culture temperature, sampling corals before, during and after recovery from bleaching. The coral symbiont calcification rate, maximum quantum yield, symbiotic zooxanthellae density, chlorophyll content, antioxidant enzymes, ammonium assimilase and community composition of zooxanthellae were measured and analyzed. Results showed that the symbiotic zooxanthellae density, chlorophyll content, maximum quantum yield were significantly higher in healthy state than in the other treatment groups (P＜0.05). The symbiotic zooxanthellae density, chlorophyll content, photosynthetic parameters, and calcification rate were significantly reduced compared to the healthy condition when corals begin to bleach at 32 ℃ (P＜0.05). High temperatures also significantly increased antioxidant and ammonium assimilation levels in both the host and symbiotic zooxanthellae. Both CAT and GS enzyme activities in coral tissue and GS enzyme activities in symbiotic zooxanthellae increased and with maximum values observed at the start of bleaching (P＜0.05). After resumption of 26 ℃, the coral parameters gradually improved, but did not return to the initial state. The subclade C15 consistently occupied the dominant position and no significant differences were detected between the healthy, bleaching, and recovered groups (P＞0.05). In summary, high temperature stress can lead to disruptions in the relationship between corals and symbiotic zooxanthellae, thereby eliciting a range of physiological responses in coral symbionts. The subclade C15 is the optimal result of the long-term evolution and natural selection of Porites lutea in response to high temperature stress.