Ocean acidification (OA) projections predict ocean pH to decline between 0.2 and 0.4 by 2100 with potential negative consequences for marine calcifiers without acclimation or adaption strategies to accomodate greater [H + ] in seawater. Biotic control of calcified reef macroalgae thalli surface diffusive boundary layer (DBL) chemistry may overcome low pH in seawater as one strategy to accommodate OA conditions. To investigate this strategy, we examined surface DBL O 2 and pH dynamics in five calcifying macroalgae (Halimeda, Udotea, Jania, Neogoniolithon, crustose coralline algae [CCA]) from the Florida Reef Tract under ambient (8.1) and low (7.65) pH using microsensors (100 μm) at the thalli surface in a flow-through flume. The role of photosynthesis and photosystem II (PSII)-independent proton pumps in controlling DBL pH were examined. Four of the five macroalgae exhibited a strong positive linear relationship between O 2 production and increasing pH in the first 15-30 s of irradiance. Once a quasi-steady-state O 2 concentration was reached (300 s), all species had DBL pH that were higher (0.02-0.32) than bulk seawater. The DBL pH increase was greatest at low pH and dependent on PSII. Some evidence was found for a light-dependent, but PSII-independent, proton pump. High DBL Δ pH upon illumination was likely in response to carbon concentrating mechanisms (CCMs) for photosynthesis. CCMs may be a HCO 3 −-H + symport, OHantiport or other DIC transport system, accompanied by proton efflux. HCO 3 dehydration by external carbonic anhydrase (CA ext) also produces OHthat can neutralize H + in the DBL. CO 2 or HCO 3 uptake for photosynthesis may also engage H + /OHfluxes as part of intracellular acid-base regulation changing DBL pH. A higher Δ pH within the DBL at low pH could be accounted for by greater CO 2 diffusion and/ or lower efficiencies in exporting cellular H + across a lower concentration gradient, and/or a more efficient removal of H + by CA ext-driven dehydration of HCO 3 −. In the dark, Δ pH was less than in the light as these dynamics were primarily due to photosynthesis. We present a conceptual model of inorganic carbon uptake and ion transport pathways, as well as other processes associated with photosynthesis that drive DBL Δ pH and sustain tropical macroalgal calcification in the light under OA. In the dark, unless PSII-independent proton pumps are present, which do not appear to be ubiquitous amongst species, acidification processes likely dominate, resulting in CaCO 3 net dissolution, particularly under OA conditions. nate (CO 3 2−) by~50-60% (Fabry et al., 2008; Koch et al., 2013). The decline in CO 3 2lowers the saturation state (Ω) of carbonate minerals of calcite and aragonite (~60%). The high concentration of Ca 2+ in