EFFECTS OF PH ON THE GROWTH AND CARBON UPTAKE OF MARINE-PHYTOPLANKTON

This study examines the growth and photosynthetic response of marine p hytoplankton to a naturally occurring range of pH's. Growth rates were determined for Thalassiosira pseudonana and Thalassiosira oceanica vi a in vivo fluorescence measurements; photosynthetic rates were measure d via carbon-14 uptake using mesocosm tank assemblages of phytoplankto n. A pH range of 7.0 to 9.4 was used for both sets of experiments and consistent declines of growth rate and photosynthesis were observed at high pH levels (> pH 8.8). The pH response of the 2 phytoplankton spe cies and the tank assemblages appeared to correlate with calculated co ncentrations of free carbon dioxide indicating a possible carbon subst rate limitation at high pH. Half-aturation values for calculated [CO2] were determined for cell growth rates and for photosynthesis using th e Monod equation (K(mu) = 0.5 muM) and Michaelis-Menten equation (K(s) = 1.3 muM), respectively. These values were within the range of value s measured for phytoplankton in previous studies. Based on this eviden ce, it is suggested that at high pH levels the availability of CO2 may become limiting to marine phytoplankton growth and photosynthesis.