Bicarbonate uptake via an anion exchange protein is the main mechanism of inorganic carbon acquisition by the giant kelp Macrocystis pyrifera (Laminariales, Phaeophyceae) under variable pH

<em>Macrocystis pyrifera</em> is a widely distributed, highly productive, seaweed. It is known to use bicarbonate (HCO₃⁻) from seawater in photosynthesis and the main mechanism of utilization is attributed to the external catalyzed dehydration of HCO₃⁻ by the surface-bound enzyme carbonic anhydrase (CA_ext). Here, we examined other putative HCO₃⁻ uptake mechanisms in <em>M. pyrifera</em> under pH_T 9.00 (HCO₃⁻: CO₂ = 940:1) and pH_T 7.65 (HCO₃⁻: CO₂ = 51:1). Rates of photosynthesis, and internal CA (CA_int) and CA_ext activity were measured following the application of AZ which inhibits CA_ext, and DIDS which inhibits a different HCO₃⁻ uptake system, via an anion exchange (AE) protein. We found that the main mechanism of HCO₃⁻ uptake by <em>M. pyrifera</em> is via an AE protein, regardless of the HCO₃⁻: CO₂ ratio, with CA_ext making little contribution. Inhibiting the AE protein led to a 55%–65% decrease in photosynthetic rates. Inhibiting both the AE protein and CA_ext at pH_T 9.00 led to 80%–100% inhibition of photosynthesis, whereas at pH_T 7.65, passive CO₂ diffusion supported 33% of photosynthesis. CA_int was active at pH_T 7.65 and 9.00, and activity was always higher than CA_ext, because of its role in dehydrating HCO₃⁻ to supply CO₂ to RuBisCO. Interestingly, the main mechanism of HCO₃⁻ uptake in <em>M. pyrifera</em> was different than that in other Laminariales studied (CA_ext-catalyzed reaction) and we suggest that species-specific knowledge of carbon uptake mechanisms is required in order to elucidate how seaweeds might respond to future changes in HCO₃⁻:CO₂ due to ocean acidification.