EXOGENOUS SOURCES OF INORGANIC CARBON FOR PHOTOSYNTHESIS BY MARINE MACROALGAE

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(T)); 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(T) 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-mu-mol.L-1 and depleted the concentration of C(T) to about 80% of that at air-equilibrium. Ability to raise pH and deplete C(T) 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(T) may be one of the factors that determines species composition in rockpools. There was a species-specific difference between the calculated concentration of C(T) 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(T) 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(T) 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 articulata (Huds.) Lyngb. Rates of net photosynthesis at air-equilibrium were greater than 95% of those at 2.3 mmol C(T).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.