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 (pHDBL), respectively. However, other metabolic processes, especially the uptake of inorganic nitrogen (Ni; NO3-and NH4+) may also affect the pHDBL. Using Macrocystis pyrifera, we hypothesized that (1) NO3-uptake will increase the pHDBL, whereas NH4+uptake will decrease it, (2) if NO3-is cotransported with H+, increases in pHDBLwould be greater under an OA treatment (pH = 7.65) than under an ambient treatment (pH = 8.00), and (3) decreases in pHDBLwill be smaller at pH 7.65 than at pH 8.00, as higher external [H+] might affect the strength of the diffusion gradient. Overall, Nisource did not affect the pHDBL. However, increases in pHDBLwere greater at pH 7.65 than at pH 8.00. CO2uptake was higher at pH 7.65 than at pH 8.00, whereas HCO3-uptake was unaffected by pH. Photosynthesis and respiration control pHDBLrather than Niuptake. We suggest that under future OA, Macrocystis pyrifera will metabolically modify its surface microenvironment such that the physiological processes of photosynthesis and Niuptake will not be affected by a reduced pH.
Department of Botany, University of Otago, Dunedin, 9054, New Zealand.
Pamela A
PA
Fernández
Department of Botany, University of Otago, Dunedin, 9054, New Zealand.
Michael Y
MY
Roleda
Department of Botany, University of Otago, Dunedin, 9054, New Zealand.
Pablo P
PP
Leal
Department of Botany, University of Otago, Dunedin, 9054, New Zealand.
Catriona L
CL
Hurd
D001639:Bicarbonates / Q000378:metabolism*; D002245:Carbon Dioxide / Q000378:metabolism*; D060833:Cellular Microenvironment; D006863:Hydrogen-Ion Concentration; D044662:Macrocystis / Q000378:metabolism*; D009584:Nitrogen / Q000378:metabolism*; D009792:Oceans and Seas; D010100:Oxygen / Q000378:metabolism; D010788:Photosynthesis; D012623:Seawater / Q000737:chemistry*
D001639:Bicarbonates; D002245:Carbon Dioxide; D009584:Nitrogen; D010100:Oxygen