International Journal of Marine Science, 2024, Vol.14, No.1, 14-20 http://www.aquapublisher.com/index.php/ijms 17 In addition, coral reefs are of great significance for the protection of coastal areas. The structure of coral reefs can slow down the impact of waves and storms on coastal areas, forming a natural protective barrier. During storm surges and tsunamis, coral reefs can absorb and slow down energy, protecting coastal areas from damage. Especially for island countries and coastal communities that are frequently hit by storms and waves, the presence of coral reefs is crucial. They are in the ocean carbon cycle, where symbiotic algae absorb carbon dioxide through photosynthesis and convert it into organic matter. This helps to reduce carbon dioxide in the atmosphere and alleviate the effects of global warming. In addition, the bones of coral animals are rich in calcium. When they die, calcium can deposit on the seabed to form coral stones, which absorb a large amount of carbon dioxide during the formation process, helping to slow down global warming. 2 Background and Mechanism of Ocean Acidification 2.1 Greenhouse gas emissions and carbon cycling During the process of ocean acidification, greenhouse gas emissions and carbon cycling have had a significant negative impact. Greenhouse gas emissions mainly refer to the emission of carbon dioxide (CO2). Carbon dioxide is a greenhouse gas generated by human activities, such as burning fossil fuels, deforestation, and land use change. These activities result in a large amount of carbon dioxide being released into the atmosphere. A portion of carbon dioxide is absorbed by the atmosphere, forming carbonate ions that dissolve in seawater. As carbon dioxide increases, the concentration of carbonate ions in seawater also increases, leading to the process of ocean acidification. Carbon cycle refers to the cycling process of carbon elements between different environments on Earth. Carbon can exist in various forms, including carbon dioxide in the atmosphere, carbonate ions dissolved in seawater, organic matter on land, and organisms. The carbon cycle involves the absorption, release, and transformation of carbon elements. The ocean is one of the largest carbon reservoirs on Earth, with carbonate ions in seawater playing an important role. The ocean absorbs approximately 25% of anthropogenic carbon dioxide emissions and stores them through carbon cycling processes. There is a close relationship between greenhouse gas emissions and carbon cycling during ocean acidification. Greenhouse gas emissions lead to an increase in the concentration of carbon dioxide in the atmosphere, and a portion of carbon dioxide is dissolved in seawater to form carbonate ions. As carbon dioxide increases, the concentration of carbonate ions in seawater also increases, leading to an increase in seawater acidity, known as ocean acidification. This acidic environment has had a negative impact on marine life and ecosystems, especially posing a threat to the growth and development of calcareous organisms such as coral reefs. The carbon cycle also involves other important processes, such as biological absorption and release of carbon, deposition and dissolution of carbon. Plants and plankton in the ocean absorb carbon dioxide through photosynthesis, convert it into organic matter, and release oxygen. These organic compounds can become the foundation of the food chain, absorbed and utilized by other organisms. On the other hand, the decomposition of organic matter and biological death will release carbon dioxide into seawater. 2.2 Chemical processes of acid-base balance and ocean acidification Ocean acidification refers to the increase in acidity in seawater, mainly caused by an increase in carbon dioxide in the atmosphere. The carbon dioxide (CO2) generated by human activities is released into the atmosphere. Carbon dioxide has the ability to dissolve in water, including seawater, through an equilibrium process of material exchange. Between the atmosphere and the ocean, carbon dioxide can be absorbed through the CO2+H2O⇌ H2CO3 reaction, which converts carbon dioxide into carbonic acid (H2CO3) and begins the process of ocean acidification (Figure 4). Carbonic acid can further dissociate into carbonate ions (HCO3-) and hydrogen ions (H+), which are important components of ocean acid-base equilibrium. In seawater, the concentration of carbonate ions and hydrogen ions increases with the increase of carbon dioxide, which leads to an increase in acidity.
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