SeawaterpH, and not inorganic nitrogen source, affectspHat the blade surface of Macrocystis pyrifera: implications for responses of the giant kelp to future oceanic conditions

• Published in: Physiologia Plantarum
• Main Authors: Fernández, Pamela A., Roleda, Michael Y., Leal, Pablo P., Hurd, Catriona L.
• Other Authors: Pamela A. Fernández, The Royal Society of New Zealand Marsden
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 2016
• Subjects: Ocean acidification
• Online Access: http://dx.doi.org/10.1111/ppl.12478; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fppl.12478; https://onlinelibrary.wiley.com/doi/pdf/10.1111/ppl.12478

Ocean acidification ( OA ), the ongoing decline in seawater pH , is predicted to have wide‐ranging effects on marine organisms and ecosystems. For seaweeds, the pH at the thallus surface, within the diffusion boundary layer ( DBL ), is one of the factors controlling their response to OA . Surface pH is controlled by both the pH of the bulk seawater and by the seaweeds' metabolism: photosynthesis and respiration increase and decrease pH within the DBL ( pH DBL ), respectively. However, other metabolic processes, especially the uptake of inorganic nitrogen (N i NO 3 − and NH 4 + ) may also affect the pH DBL . Using Macrocystis pyrifera , we hypothesized that (1) NO 3 − uptake will increase the pH DBL , whereas NH 4 + uptake will decrease it, (2) if NO 3 − is cotransported with H + , increases in pH DBL would be greater under an OA treatment ( pH = 7.65) than under an ambient treatment ( pH = 8.00), and (3) decreases in pH DBL will be smaller at pH 7.65 than at pH 8.00, as higher external [H + ] might affect the strength of the diffusion gradient. Overall, N i source did not affect the pH DBL . However, increases in pH DBL were greater at pH 7.65 than at pH 8.00. CO 2 uptake was higher at pH 7.65 than at pH 8.00, whereas HCO 3 − uptake was unaffected by pH . Photosynthesis and respiration control pH DBL rather than N i uptake. We suggest that under future OA , Macrocystis pyrifera will metabolically modify its surface microenvironment such that the physiological processes of photosynthesis and N i uptake will not be affected by a reduced pH .