International Journal of Marine Science, 2024, Vol.14, No.3, 231-244 http://www.aquapublisher.com/index.php/ijms 233 vulnerable to ocean acidification due to their reliance on calcium carbonate (CaCO3) to form shells and skeletons. The decrease in pH reduces the availability of carbonate ions, essential for calcium carbonate formation. This reduction leads to weaker and thinner shells, making these organisms more susceptible to predation and environmental stress. Studies have shown that ocean acidification can decrease the calcification rates of corals, leading to slower growth and weaker reef structures (Henry et al., 2020). Additionally, mollusks, such as oysters and mussels, show impaired shell formation, which can reduce their survival and reproductive success (Mostofa et al., 2015). The long-term impact of these changes includes potential declines in populations of calcifying species, which could disrupt marine food webs and ecosystem services, such as coastal protection and biodiversity (Scherer et al., 2022). 3.2 Impacts on fish and invertebrates Ocean acidification affects fish and invertebrates in multiple ways, including physiological stress, altered behavior, and reproductive challenges. Fish rely on chemoreception for vital functions such as finding food, avoiding predators, and locating mates. Acidified conditions can impair these sensory abilities, leading to disorientation and decreased survival rates (Tembo, 2017). For instance, studies on juvenile fish, like the European sea bass, have shown that acidification alters their stress response, leading to prolonged recovery times and changes in neurotransmitter levels, which affect behavior and motor activity (Servili et al., 2022). Invertebrates, such as copepods and polychaetes, exhibit varied responses to acidification, often experiencing reduced growth, delayed development, and increased oxidative stress (Lee et al., 2019). These physiological and behavioral changes can impact individual fitness and, over time, lead to shifts in population dynamics and community structure (Wang et al., 2018). 3.3 Consequences for marine microbes Marine microbes play crucial roles in nutrient cycling and carbon flow within marine ecosystems. Ocean acidification can significantly alter microbial community composition and function, affecting overall ecosystem health. Microbial processes such as photosynthesis and nitrogen fixation are sensitive to changes in pH. For example, studies indicate that acidification can enhance the photosynthetic activity of some phytoplankton species while inhibiting others, leading to shifts in community structure and potential disruptions in primary production (O'Brien et al., 2016) (Figure 1). The microbial communities associated with marine organisms, such as corals, may shift under acidified conditions, potentially increasing host susceptibility to diseases (Zunino et al., 2021). These changes at the microbial level can have cascading effects throughout the marine food web, influencing the health and stability of larger marine ecosystems. The image depicts photos taken along a pCO2/pH gradient in Papua New Guinea (year 2014) and illustrates three potential scenarios for coral reefs under present-day and future (years 2050 and 2100) ocean acidification (OA) conditions if we continue on current predicted CO2 emissions trajectories. In the present-day scenario, the coral reef is shown to be healthy, with high structural complexity and diversity, likely colonized by beneficial microbial associates, and experiencing a low incidence of disease. As pCO2 levels increase, the scenarios for the years 2050 and 2100 illustrate the successive degradation of coral reef heterogeneity (structural diversity), destabilization of microbial associations, and an increase in disease. By 2050, the coral reef starts to show signs of degradation, with beneficial microbial associations beginning to destabilize and disease prevalence increasing, leading to a reduction in structural diversity. By 2100, the degradation has progressed further, and the reef transitions to an alternative stable state dominated by competitive species such as sponges, macroalgae, and seagrass, along with sediment/rubble. The arrows from green to red indicate the positive to negative changes a coral reef might experience over time. This transition highlights the severe ecological degradation that coral reefs may face if CO2 emissions continue on the current trajectory.
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