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: Fernández, Pamela A., Hurd, Catriona L., Roleda, Michael Y.
• Other Authors: Wernberg, T., The Royal Society of New Zealand Marsden fund
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2014
• Subjects: Ocean acidification
• Online Access: http://dx.doi.org/10.1111/jpy.12247; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fjpy.12247; https://onlinelibrary.wiley.com/doi/pdf/10.1111/jpy.12247

M acrocystis 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 the external 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%–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 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 L aminariales 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.