Dynamics of pH, O2 and Ca2+ at the seawater-surface interface of a tropical coralline alga and evidence of a proton pump

To examine the coupling of photosynthesis and calcification in a tropical crustose coralline alga (CCA; Porolithon sp.), pH, oxygen (O₂), and calcium (Ca²⁺) were simultaneously measured with microsensors at the thallus surface. We exposed Porolithon sp. to pH 8.1 and 7.8 at a range of irradiances (0-800 mmol photons cm^-2 s^-1). Inhibitors (acetazolamide, AZ, and 4,4’ diisothiocyanatostilbene-2,2’-disulphonate, DIDS) were used to block specific inorganic carbon uptake pathways for photosynthesis and calcification, including extracellular carbonic anhydrase mediated hydrolysis of HCO₃^- (AZ) and direct HCO₃^- uptake by anion exchange transport (DIDS). The diffusive boundary layer (DBL) ranged from 100-300 µm, causing a surface micro-environment distinct from bulk seawater. For example, thalli surface pH was always ~0.3 units above seawater at saturating irradiance, regardless of seawater pH. However, in the presence of AZ, surface pH was only elevated 0.2 units above seawater. The combination of AZ and DIDS inhibited gross photosynthesis 77-92% at saturating irradiance. Light-dark dynamic studies showed rapid light-induced H⁺ flux with and without photosynthesis inhibition (DCMU), revealing a proton pump that is light-mediated, but photosynthesis-independent. The Ca²⁺ dynamics were strong, and temporally variable, but were closely coupled to pH dynamics, suggesting Ca²⁺/H⁺ exchange and the presence of Ca²⁺-ATPase. Remarkably, Ca²⁺ always exhibited a net positive uptake into the thallus, even in the dark. Thus, Porolithon sp. has the potential to elevate surface pH through a light-induced H⁺ pump, which may be linked to Ca²⁺-H⁺ exchange or Ca²⁺-ATPase.