title: 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.
abstract: 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 (CAext ). Here, we examined other putative HCO3 (-) uptake mechanisms in M. pyrifera under pHT 9.00 (HCO3 (-) : CO2  = 940:1) and pHT 7.65 (HCO3 (-) : CO2  = 51:1). Rates of photosynthesis, and internal CA (CAint ) and CAext activity were measured following the application of AZ which inhibits CAext , 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 CAext making little contribution. Inhibiting the AE protein led to a 55%-65% decrease in photosynthetic rates. Inhibiting both the AE protein and CAext at pHT 9.00 led to 80%-100% inhibition of photosynthesis, whereas at pHT 7.65, passive CO2 diffusion supported 33% of photosynthesis. CAint was active at pHT 7.65 and 9.00, and activity was always higher than CAext , 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 (CAext -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.
authors.affiliation: Department of Botany, University of Otago, PO Box 56, Dunedin, 9054, New Zealand.
authors.forename: Pamela A
authors.initials: PA
authors.lastname: Fernández
authors.affiliation: Department of Botany, University of Otago, PO Box 56, Dunedin, 9054, New Zealand.|Institute for Marine and Antarctic Studies (IMAS), University of Tasmania, Private Bag 129, Sandy Bay, Hobart, Tasmania, 7001, Australia.
authors.forename: Catriona L
authors.initials: CL
authors.lastname: Hurd
authors.affiliation: Department of Botany, University of Otago, PO Box 56, Dunedin, 9054, New Zealand.|Bioforsk Norwegian Institute for Agricultural and Environmental Research, Kudalsveien 6, Bodø, 8049, Norway.
authors.forename: Michael Y
authors.initials: MY
authors.lastname: Roleda
keywords: Macrocystis pyrifera; bicarbonate; carbon acquisition; carbon dioxide; carbonic anhydrase; inorganic carbon; macroalgae; ocean acidification; photosynthesis