Biotic Control of Surface pH and Evidence of Light-Induced H ⁺ Pumping and Ca ²⁺-H ⁺ Exchange in a Tropical Crustose Coralline Alga

Abstract

Presently, an incomplete mechanistic understanding of tropical reef macroalgae photosynthesis and calcification restricts predictions of how these important autotrophs will respond to global change. Therefore, we investigated the mechanistic link between inorganic carbon uptake pathways, photosynthesis and calcification in a tropical crustose coralline alga (CCA) using microsensors. We measured pH, oxygen (O ₂), and calcium (Ca ²⁺) dynamics and fluxes at the thallus surface under ambient (8.1) and low (7.8) seawater pH (pH _SW) and across a range of irradiances. Acetazolamide (AZ) was used to inhibit extracellular carbonic anhydrase (CA _ext), which mediates hydrolysis of HCO ₃ ^-, and 4,4′ diisothiocyanatostilbene-2,2′-disulphonate (DIDS) that blocks direct HCO ₃ ^- uptake by anion exchange transport. Both inhibited photosynthesis, suggesting both diffusive uptake of CO ₂ via HCO ₃ ^- hydrolysis to CO ₂ and direct HCO ₃ ^- ion transport are important in this CCA. Surface pH was raised approximately 0.3 units at saturating irradiance, but less when CA _ext was inhibited. Surface pH was lower at pH _SW 7.8 than pH _SW 8.1 in the dark, but not in the light. The Ca ²⁺ fluxes were large, complex and temporally variable, but revealed net Ca ²⁺ uptake under all conditions. The temporal variability in Ca ²⁺ dynamics was potentially related to localized dissolution during epithallial cell sloughing, a strategy of CCA to remove epiphytes. Simultaneous Ca ²⁺ and pH dynamics suggest the presence of Ca ²⁺/H ⁺ exchange. Rapid light-induced H ⁺ surface dynamics that continued after inhibition of photosynthesis revealed the presence of a light-mediated, but photosynthesis-independent, proton pump. Thus, the study indicates metabolic control of surface pH can occur in CCA through photosynthesis and light-inducible H ⁺ pumps. Our results suggest that complex light-induced ion pumps play an important role in biological processes related to inorganic carbon uptake and calcification in CCA.