Physiological Responses of Marine Invertebrates to Ocean Acidification

Abstract

Background and Objectives: Ocean acidification, driven by the dissolution of increasing atmospheric carbon dioxide into the ocean, has led to a decrease in pH and alterations in seawater’s crucial carbonate chemistry. This phenomenon significantly affects marine organisms, in particular the diverse and abundant marine invertebrates. These creatures not only have important functions in ecosystem, but are also commercially important. Due to their importance, the present review article compiles information aiming at providing detailed insights into ocean acidification and physiological responses of marine invertebrates.

Content: Ocean is a crucial ecosystem that plays a vital role in the absorption and sequestration of atmospheric carbon dioxide. When carbon dioxide from the atmosphere is absorbed by the ocean, it undergoes a reaction with seawater, resulting in the breakdown into hydrogen ions (H+ ), leading to a decrease in seawater pH value. Due to the reversible nature of the reaction, the hydrogen ion concentration is well regulated by the buffer capacity of the ocean, resulting in a reasonably stable pH. However, since the current levels of anthropogenic carbon dioxide emissions are elevated, an increase in hydrogen ion concentration in the ocean is observed, leading to the pH reduction. Such a condition leads to a decreased abundance of carbonate ions and a reduction in the saturation of calcium carbonate minerals. Ocean acidification has a potential also to disrupt several physiological processes in marine invertebrates: (i) acid-base imbalance: ocean acidification can cause acid-base imbalances within organisms, resulting in acidosis in both intra- and extracellular fluids, potentially causing cellular dysfunction. Invertebrates strive to maintain homeostasis either by the elimination of surplus hydrogen ions or by enhancing buffering capacity, both of which necessitate energy expenditure; (ii) aerobic metabolic depression, which results from a lower pH in hemolymph, in turn impairing the oxygen-binding capacity of hemoglobin and hemocyanin, as well as diminishing hemolymph circulation. In addition, regular metabolism undergoes a transition from aerobic to anaerobic pathways, resulting in a significant decrease in energy release and altering the catabolism from carbohydrates and fats to proteins; (iii) energy budget constraints: marine invertebrates experiencing ocean acidification have reduced energy budgets as a result of decreased ingestion rates or deficits in digestion and absorption. Available evidences indicate that there are constraints on the scope for growth or energy reserves for reproduction. In addition, the energy acquired reallocates in various strategies. Several molluscs such as Crepidula fornicata and Mytilus edulis reallocate their energy toward calcification to counteract shell dissolution caused by ocean acidification. On the other hand, Hexaplex trunculus increases its body mass as its shell thickness decreases. Musculista senhousia allocates more energy to reproduction rather than growth; (iv) digestive impairments: reduced pH level in the digestive system poses negative impact on the activities of digestive enzymes and can damage the structure of such digestive organs as the digestive gland and intestine. This suggests that the organisms may acquire a reduced amount of energy from their feed; (v) calcification and calcareous structure dissolution: carbonate structures, such as mollusc shells and coral skeletons, which play crucial roles in protecting soft tissues from environmental changes and predators, providing structural support and offering attachment substrates, are compromised. The reduction in carbonate ions, essential for calcification, along with decreased calcium carbonate saturation, diminishes calcification and causes dissolution of these structures. Ocean acidification clearly has detrimental impacts on the physiology of marine invertebrates, which is likely to impact the survival of these animals. Therefore, to mitigate these impacts, it is imperative for individuals to be aware of their contributions to carbon dioxide emissions and to works towards reducing them to slow the impacts on marine resources.