COMPARATIVE EFFECTS OF pH AND ALUMINUM ON SILICA-LIMITED GROWTH AND NUTRIENT UPTAKE IN ASTERIONELLA RALFSII VAR. AMERICANA (BACILLARIOPHYCEAE)1

The acidophilic diatom Asterionella ralfsii cf. var. americana Korn. was grown in continuous culture to examine the influences of both pH and Al on Si-limited growth and uptake kinetics. In contrast to nutrient-replete cultures of A. ralfsii, lowering pH from approximately 6 to 5 reduced algal cell density, chlorophyll a concentration, and intensity of in vivo fluorescence (IVF) at steady state. The lower pH treatments were also characterized by lower Si cell quotas and higher residual dissolved Si concentrations in chemostats with similar nutrient supply rates. Physiological responses to Al stress differed from those to pH reduction when cultures were Si-limited. Nominal Al additions of 20 μmol·L−1 reduced chlorophyll a concentration and IVF values at higher pH, but all other biomass and chemical parameters remained constant at steady state. The combined efects of Al and reduced pH were more severe than either stress alone, inducing culture washout at pH 4.8. Short-term Si uptake experiments performed at pH 6 showed that Al influenced Michaelis-Menten parameter estimates. Half-saturation (Ks and maximum uptake rate (Vm) constants increased approximately 8- and 2-fold in the presence of Al, respectively, but this difference was only significant for Vm. Similar to previously observed effects of Al on cell morphology in A. ralfsii, Si uptake kinetics were more sensitive to Al additions than to Silimited growth per se.