MEPS 180:81-91 (1999)  -  doi:10.3354/meps180081

ABSTRACT: To address the question of whether free CO₂ can be a growth-limiting nutrient in oceanic waters 3 large marine diatoms, Stephanopyxis palmeriana (Greville) Grunow, Ditylum brightwellii (T. West) Grunow in Van Heurck 1883, and Coscinodiscus sp., were grown in pH-drift experiments under batch culture conditions. The cultures were maintained under quiescent conditions without added buffer in the growth medium, allowing the pH to rise and the free CO₂ concentration to fall as growth proceeded. Growth rates were constant for sustained periods and only decreased as the pH rose to as high as 8.5-8.6 and the free CO₂ concentration fell to ~4 µmol l^-1. Such a low free CO₂ concentration is far below the expected half saturation coefficient for ribulose biphosphate carboxylase-oxygenase (RUBISCO) and suggests that these species were capable of utilizing HCO₃^- in some fashion so as not to allow free CO₂ to become growth-limiting. Additional experiments were conducted with these and other diatoms to demonstrate that turbulent mixing at high pH levels had no effect on inorganic carbon uptake or growth rate and that HCO₃^- uptake probably was occurring. Turbulent mixing should have enhanced inorganic carbon uptake by lowering diffusion gradients had these species been obligate users of free CO₂. These species, being the most susceptible to diffusion-controlled uptake of free CO₂ because of their long diffusion paths, represent the worst case scenario for free CO₂ limitation. Finally, by developing a chemical-biological model it was possible to show that, in order for the pH of marine waters to rise even several tenths above ambient levels, biomass concentrations must increase to levels that are never found, except possibly on occasion in estuarine or coastal waters. Thus it seems unlikely that free CO₂ limits phytoplankton growth in the oceans.

KEY WORDS: Inorganic carbon availability · pH levels · HCO₃^- uptake · Growth · Large marine diatoms