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

• Published in: Journal of Phycology
• Main Authors: Fernandez, PA, Hurd, CL, Roleda, MY
• Format: Article in Journal/Newspaper
• Language: English
• Published: Blackwell Publishing Inc 2014
• Subjects: Environmental Sciences; Ecological Applications; Ecological Impacts of Climate Change; Ocean acidification
• Online Access: https://doi.org/10.1111/jpy.12247; http://ecite.utas.edu.au/98376

Macrocystis pyrifera is a widely distributed, highly productive, seaweed. It is known to use bicarbonate ( HCO 3 − ) from seawater in photosynthesis and the main mechanism of utilization is attributed to theexternal catalyzed dehydration of HCO 3 − by the surface-bound enzyme carbonic anhydrase (CA ext ). Here, we examined other putative HCO 3 − uptake mechanisms in M.pyrifera under pH T 9.00 ( HCO 3 − : CO 2 =940:1) and pH T 7.65 ( HCO 3 − : CO 2 =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 HCO 3 − uptake system, via an anion exchange (AE) protein. We found that the main mechanism of HCO 3 − uptake by M.pyrifera is via an AE protein, regardless of the HCO 3 − : CO 2 ratio, with CA ext making little contribution. Inhibiting the AE protein led to a 55% decrease in photosynthetic rates. Inhibiting both the AE protein and CA ext at pH T 9.00 led to 800% inhibition of photosynthesis, whereas at pH T 7.65, passive CO 2 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 HCO 3 − to supply CO 2 to RuBisCO. Interestingly, the main mechanism of HCO 3 − 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 HCO 3 − : CO 2 due to ocean acidification.