International Journal of Aquaculture, 2024, Vol.14, No.1, 1-8 http://www.aquapublisher.com/index.php/ija 5 Climate change has severe impacts on the ecological relationship between tropical hard corals and symbiotic algae. Factors such as elevated sea temperatures, ocean acidification, storms, and sea level rise pose threats to the survival and reproduction of hard corals, rendering these coral reef ecosystems more fragile. 3 Adaptive Biochemical Mechanisms to Climate Change In the face of environmental changes induced by climate change, the delicate relationship between tropical hard corals and their symbiotic algae becomes even more fragile. However, these organisms are not helpless; they exhibit impressive biochemical adaptive mechanisms to cope with new environmental challenges. 3.1 Photosynthetic adaptation Symbiotic algae are crucial for the survival of hard corals as they provide the necessary energy through photosynthesis. However, the increased temperature and light intensity resulting from climate change can stress symbiotic algae, leading to coral bleaching. To address this challenge, some species of hard corals have demonstrated photosynthetic adaptability. An important adaptive strategy involves certain hard coral species adjusting the type of symbiotic algae to cope with different environmental conditions. Under high-temperature conditions, some hard corals can establish symbiotic relationships with heat-tolerant algae varieties, thus alleviating the pressure on symbiotic algae caused by elevated temperatures (Kawamura et al., 2021). This phenomenon provides additional survival opportunities for hard corals, enabling them to thrive in a broader range of environmental conditions. Hard corals and symbiotic algae (Figure 4) can also regulate the rate of photosynthesis to minimize the damage of photosynthetic products to hard corals. Hard corals can adjust the density and pigment content of symbiotic algae to adapt to different light conditions (López-Londoño et al., 2022). This photosynthetic adaptation mechanism helps hard corals mitigate the risk of symbiotic algae being exposed to excessive sunlight while maintaining a sufficient energy supply. Figure 4 Coral and symbiotic algae 3.2 Changes in symbiotic algae species With the rise in sea temperatures, some hard corals have begun to establish new symbiotic relationships with different types of photosynthetic algae to adapt to higher temperatures. This change in symbiotic algae species is an adaptive mechanism known as "symbiotic algae switching." This process typically involves hard corals forming new symbiotic relationships with heat-tolerant varieties of symbiotic algae, thereby increasing the chances of survival in high-temperature conditions. A research team from California State University developed a global ecological and evolutionary model simulating the response of 1,925 coral reefs to warming and ocean acidification under four climate scenarios (Li
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