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

Macrocystis pyrifera 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 M. pyrifera 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 M. pyrifera 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 M. pyrifera 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.