Species-specific proton and oxygen flux in Hawaiian corals under ocean acidification—a microsensor analysis of the concentration boundary layer

Coral reefs are essential for the foundation of marine ecosystems. However, ocean acidification (OA) driven by rising atmospheric carbon dioxide (CO₂) threatens coral growth and biological homeostasis. In this study, we examined the microenvironmental fluxes of two Hawaiian coral species— Montipora capitata and Pocillopora acuta to elevated pCO₂, focusing on proton (H⁺) and oxygen (O₂) flux within the concentration boundary layer (CBL) at the zone of primary calcification (ZPC). Utilizing pH and O ₂ microsensors under controlled light and dark conditions, we characterized species-specific CBL traits and quantified material fluxes. Our results revealed that while both species maintained a positive net proton flux, P. acuta showed a pronounced reduction in dark proton efflux (-188%) and a significant increase in light O₂ flux (+ 175%), suggesting impaired metabolic and calcification dynamics. In contrast, M. capitata showed minimal changes in both flux parameters under similar OA conditions. Statistical analyses using linear models showed several significant interactions between species, treatment, and light conditions, identifying physical, chemical, and biological drivers for species responses to OA. We also present a conceptual model correlating external measures with internal physiologies to explain our findings. We indicate that OA exacerbates microchemical gradients in the CBL and potentially acts to reduce calcification in vulnerable species like P. acuta while highlighting the resistance of M. capitata . This study advances our understanding of how species-specific microenvironmental processes could influence coral responses to changing ocean chemistry.