Between shells and seas: Effects of ocean acidification on calcification and osmoregulation in yellow clam (Amarilladesma mactroides)

The decline in ocean pH due to rising CO2 levels is a critical factor impacting marine ecosystems. Ocean acidification (OA) is expected to negatively affect various organisms, particularly those with mineralized structures. While the effects of OA on the calcification of shells and exoskeletons are documented, the impact on homeostatic processes, such as osmoregulation, is less understood. Osmoregulation is vital for maintaining water and salt balance within marine organisms, crucial for their survival and physiological functions. Acidification may alter ion exchange mechanisms, affecting the regulation of ions. In this study, we evaluated the effects of intermediate OA (pH 7.6) with or without hypersaline stress (35 parts per thousand) on calcification and osmotic balance biomarkers in the bivalve Amarilladesma mactroides after 96h of acute exposure. We found that pH did not affect hemolymph osmolality or extracellular Ca2+ concentration. However, OA impaired the bivalve's ability to maintain its mineralized structures by decreasing Ca2+-ATPase enzyme activity in the mantle. The increase in carbonic anhydrase activity indicated a specific response to maintain acid-base balance in the tissue, i.e., compensating for the effects of acidification by neutralizing CO2 accumulation and stabilizing internal pH. In the gills, both enzymes showed increased performance under higher salinity and reduced pH. Exposure to less alkaline pH inhibited carbonic anhydrase and Na+/K+-ATPase activity, potentially affecting the regulation of essential inorganic osmolytes.