Dynamic changes in phycosphere carbonate chemistry reveal rapid modulation of carbon uptake in single diatom cells

Diatoms use multiple mechanisms of dissolved inorganic carbon (DIC) uptake to maintain CO₂ supply to RuBisCO (ribulose-1,5-bisphosphate carboxylase/oxygenase), but how these different pathways are coordinated in response to rapid changes in their environment remains unclear. By measuring the dynamic changes in phycosphere carbonate chemistry around single diatom cells, we show that DIC uptake is rapidly modulated. We find that external carbonic anhydrase (eCA) plays a major role in DIC uptake in large centric diatoms at ambient seawater CO₂, but there is a strong switch to direct HCO₃^- uptake under low CO₂. Inhibition of eCA leads to rapid activation of HCO₃^- uptake, which can compensate for the loss of eCA activity under lower irradiances. These different modes of DIC uptake have distinct influences on phycosphere carbonate chemistry that may directly affect nutrient acquisition and biotic interactions. Our findings reveal that dynamic modulation of DIC uptake allows diatoms to rapidly respond to changes in CO₂ supply and demand.