Coral reefs are vital marine ecosystems that harbor a significant proportion of the ocean’s biodiversity. However, these ecosystems are increasingly threatened by anthropogenic activities, particularly the emission of greenhouse gases leading to climate change and ocean acidification. Ocean acidification refers to the reduction in pH of marine waters due to the absorption of CO₂ from the atmosphere, forming carbonic acid (H₂CO₃), which dissociates into bicarbonate (HCO₃−) and hydrogen ions (H+), thus lowering pH. This sequence of reactions leads to an increase in hydrogen ion concentration, causing a decrease in pH. The reduction in carbonate ions (CO₃2−) is particularly detrimental to marine calcifiers, including corals, which rely on carbonate for the formation of their calcium carbonate (CaCO₃) skeletons. Coral reefs are constructed by the deposition of CaCO₃ by coral polyps. Zooxanthellae, symbiotic algae living within coral tissues, provide essential nutrients through photosynthesis, facilitating calcification. Acidification disrupts this symbiotic relationship by impairing photosynthetic efficiency and reducing the availability of carbonate ions necessary for skeletal growth. As ocean acidification progresses, the concentration of carbonate ions diminishes, making it energetically more challenging for corals to secrete their skeletons, thereby slowing growth rates and compromising structural integrity. Coral bleaching occurs when corals, under stress, expel their zooxanthellae, leading to a loss of pigmentation and a decline in energy reserves. Stressors include elevated sea temperatures, pollution, and acidification. The loss of zooxanthellae not only deprives corals of their primary food source but also disrupts calcification processes. Thermal stress is a predominant factor in coral bleaching. Elevated sea temperatures can destabilize the photosynthetic machinery of zooxanthellae, producing reactive oxygen species (ROS) that damage both the algae and coral tissues. Prolonged exposure to high temperatures exacerbates acidification effects, intensifying bleaching events. The decline in coral health due to bleaching and acidification has profound ecological impacts, including the loss of habitat for numerous marine species, reduced biodiversity, and compromised fisheries. Socioeconomically, coral reef degradation affects tourism, coastal protection, and the livelihoods of communities dependent on reef resources. Reduction of CO₂ emissions through global policy agreements and renewable energy adoption. Local conservation efforts, such as marine protected areas (MPAs) have the potentials to enhance reef resilience. Conservation efforts may be complemented by research into coral species and strains with higher tolerance to acidification and thermal stress, potentially involving selective breeding and genetic modification. Marine water acidification and coral bleaching are intricately linked phenomena driven by anthropogenic climate change. The decline of coral reefs signals a broader environmental crisis that necessitates urgent scientific, policy, and community responses to mitigate adverse effects and foster adaptive resilience in marine ecosystems.
Adeniran-Obey S. O., Isibor P. O. & Imoobe T. O., 2024. Marine water acidification and coral bleaching. In: Isibor P.O. (Ed.), Arctic marine ecotoxicology: climate change, pollutants, and their far-reaching effects pp 403-420. Springer Cham. Chapter (restricted access).
