EXOGENOUS SOURCES OF INORGANIC CARBON FOR PHOTOSYNTHESIS BY MARINE MACROALGAE1

Thirty-five species of marine macroalgae were tested for their ability to remove inorganic carbon from seawater using the pH-drift technique. Six of these species, all Rhodophyta, were unable to use HCO₃^–. The remaining species exhibited a range in ability to use HCO₃^– and deplete inorganic carbon (C_τ); the three most effective species, all Chlorophyta, raised the pH to over 10.50, depleted the concentration of CO₂ effectively to zero, and depleted the concentration of C_τ to less than 50% of that at air-equilibrium. In contrast, the six species restricted to CO₂ did not raise the pH above 9.0 at a CO₂ concentration of about 1.5 μmol · L^-1 and depleted the concentration of C_τ to about 80% of that at air-equilibrium. Ability to raise pH and deplete C_τ was linked to the habitat in which the species grew. Five of the six species which lack the ability to use HCO₃^– grow subtidally in relatively low light beneath a canopy of larger Phaeophyta. None of these species grow in rockpools where carbon-depletion may occur. Species from rockpools were all effective at removing inorganic carbon. Competition for C_τ may be one of the factors that determines species composition in rockpools. There was a species-specific difference between the calculated concentration of C_τ at the end of a pH-drift experiment and that measured directly. Most, but not all, species with the ability to generate high pH-values showed a lower than calculated final concentration of C_τ consistent with precipitation of CaCO₃. A number of Rhodophyta with no, or a limited, ability to use HCO₃^– showed the opposite response, with final concentrations of C_τ exceeding that calculated from the pH. Calculations based on the maximum gross rate of production of CO₂ from HCO₃^– in the absence of external carbonic anhydrase confirmed the results of the pH-drift experiments by demonstrating HCO₃^– -use in Monostroma fuscum (Post et Rupr.) Wittr. And Ulva lactuca L. and the lack of this ability in Lomentaria articulate (Huds.) Lyngb. Rates of net photosynthesis at air-equilibrium were greater than 95% of those at 2.3 mmol C_τ· L^-1 for many of the species which were able to use HCO₃^–, but on average only 72% for the six species restricted to CO₂.