Seawater pH, and not inorganic nitrogen source, affects pH at the blade surface of Macrocystis pyrifera : implications for responses of the giant kelp to future oceanic conditions

• Published in: Physiologia Plantarum
• Main Authors: Fernandez, PA, Roleda, MY, Leal, PP, Hurd, CL
• Format: Article in Journal/Newspaper
• Language: English
• Published: Blackwell Munksgaard 2017
• Subjects: Biological Sciences; Plant Biology; Phycology (incl. Marine Grasses); Ocean acidification
• Online Access: http://dx.doi.org/oi:10.1111/ppl.12478; https://doi.org/10.1111/ppl.12478; http://www.ncbi.nlm.nih.gov/pubmed/27293117; http://ecite.utas.edu.au/112297

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.