Ocean acidification shifts carbonate chemistry heterogeneity in molluscan respiratory microenvironments

Ocean acidification (OA) poses a threat to marine calcifiers by modifying carbonate chemistry in ambient seawater, where localized fluctuations exert more immediate physiological impacts than bulk seawater changes. We investigated the heterogeneity of carbonate chemistry parameters (pH, DIC, pCO₂, TA-DIC) within the respiratory microenvironments (RE) of four molluscan species (Mytilus galloprovincialis, Haliotis discus hannai, Chlamys farreri, Crassostrea gigas) under ambient (pH 8.1) and OA conditions (pH 7.8). Our results demonstrated that regardless of bulk seawater pH, RE consistently exhibited significantly elevated dissolved inorganic carbon (DIC) and pCO₂ levels, coupled with reduced pH and lower TA-DIC ratios compared to bulk seawater (P < 0.05). Notably, H. discus hannai displayed the most pronounced RE modifications, with the highest DIC and pCO₂, alongside the lowest pH and TA-DIC values among studied species, revealing distinct interspecific variability. Acute OA exposure significantly increased pCO₂ heterogeneity (P < 0.05) while decreasing pH, DIC, and TA-DIC heterogeneity within the RE of three species (C. farreri, C. gigas, and H. discus hannai), suggesting suppressed CO₂ excretion capacity under rapid acidification stress. Prolonged OA exposure progressively enhanced spatial heterogeneity in DIC and pCO₂ levels, indicating physiological acclimation to facilitate metabolic CO₂ excretion. A critical finding was the accelerated pCO₂ increasing rate in RE compared to bulk seawater during OA stress, this steep elevation in pCO₂ level in RE may require extra-efforts to facilitate CO₂ excretion. These findings provide insights into the mechanistic links between OA-driven carbonate chemistry modifications and molluscan respiratory physiology, highlighting species-specific vulnerability patterns and adaptive responses.