International Journal of Marine Science, 2025, Vol.15, No.4, 199-208 http://www.aquapublisher.com/index.php/ijms 200 mechanisms of non-calcified groups to cope with climate change. This study will review the domestic and foreign research progress in recent years from the aspects of marine acidification, the physicochemical mechanism, the metabolic characteristics of sea cucumber, the impact of acidification and ecological effects, and look forward to future research directions. 2 Physical and Chemical Mechanism of Marine Acidification 2.1 The relationship between carbon dioxide emissions and the decline in seawater pH The fundamental reason for ocean acidification is the increase in atmospheric CO₂ concentration. After excessive CO₂ is absorbed by the ocean, it reacts with water molecules to form carbonic acid, further ionization produces hydrogen ions, resulting in a drop in the pH of seawater. The average seawater pH has decreased by about 0.1 since industrialization, meaning that the hydrogen ion concentration has increased by about 30%. At first, the carbonate buffer system of the ocean can neutralize part of the acidity, but as CO₂ continues to dissolve, the buffer capacity gradually weakens. The IPCC report predicts that if high emissions are maintained, the drop in ocean surface pH will significantly exceed the natural fluctuation range of the past few million years by 2100. 2.2 Spatial differences and dynamic changes in acidification in tropical seas The degree of ocean acidification varies in different regions. The acidification of the open ocean surface is relatively uniform, while tropical nearshore and coral reef areas show more complex pH dynamics due to local biogeochemical processes. For example, photosynthesis in the reef area during the daytime reef increases the pH of water, and the release of CO₂ at night biorespiration reduces the pH, and the fluctuation amplitude of day and night can reach more than 0.2 (Cryer et al., 2021). Some semi-enclosed sea areas may acidify faster than the global average due to seawater stagnation and high organic matter decomposition. The study found that frequent low pH extreme events occurred in the coral triangle in the Western Pacific from 2015 to 2022, with Ωₐᵣ and pH drops higher than those in other sea areas. On the contrary, in tropical oceans far away from the mainland, the carbonate saturation of seawater is still relatively high, providing a certain "buffer" space for coral reefs. In addition, factors such as eutrophication and upflow will also aggravate local acidification: organic matter decomposition and freshwater input in the nearshore area of the estuary often cause a rapid decline in pH, forming an acidification "hot spot" (Isah et al., 2022). 2.3 Effect of marine acidification on carbonate system and marine buffering capacity As the CO₂ content in seawater increases, the carbonate system changes significantly: the carbonate ion concentration decreases and the bicarbonate ion concentration increases. This leads to a decrease in saturation of calcium carbonate in seawater, weakening the biocalcification effect. Reef-making corals and shellfish are more difficult to form skeletons and shells at low pH, with significantly lower growth rates. Increased seawater acidity is also gradually weakening the ocean's chemical buffering capacity. When the pH drops, the bicarbonate/carbonate balance system moves in the direction of releasing more H⁺, reducing the ability of seawater to resist foreign acids (Figure 1) (Sulpis et al., 2022). In summary, on the one hand, ocean acidification directly affects the carbonate chemical balance of seawater, and on the other hand, it also reduces the ocean's self-regulation ability of acidification, which will further amplify the potential impact on marine ecology. 3 Metabolic Characteristics of Tropical Sea Cucumbers 3.1 Basic characteristics of respiration and energy metabolism pathways Sea cucumbers are benthic slowly moving debris animals, and their basal metabolic rate is relatively low. Sea cucumbers have a unique respiratory structure - a breathing tree at the end of the intestine, which absorbs water and expels water through the anus for gas exchange. This breathing method is not efficient, but it is enough to meet its slow lifestyle oxygen needs. In adverse environments, sea cucumbers can also actively reduce their metabolic rate to save energy. For example, when the temperature is high in summer, the prickly ginseng will enter a summer hibernation state, and the metabolic level will drop significantly, thus passing through the period of high temperature and food scarcity.
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