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

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; NO3- and NH4+) may also affect the pH(DBL). Using Macrocystis pyrifera, we hypothesized that (1) NO3- uptake will increase the pH(DBL), whereas NH4+ uptake will decrease it, (2) if NO3- 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. CO2 uptake was higher at pH 7.65 than at pH 8.00, whereas HCO3- 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.