KINETICS OF NUTRIENT UPTAKE AND GROWTH IN PHYTOPLANKTON

Microscopic algae ran grow rapidly in natural waters that are extremely low in essential macro and micro nutrients. Yet, their nutrient uptake systems exhibit only mediocre nutrient affinities, the saturation constants being often 10–1000 times the (estimated) ambient concentrations. The large difference which exists between the saturation constants for growth (K_μ) and short term uptake (K_ρ) are due to the acclimation capabilities of the organisms. Over the acclimation range, K_μ to K_ρ, the algae can maintain maximum growth rate by modulating both their internal nutrient quotas (Q) and their maximum short term nutrient uptake rates (ρ_max) in response to variations in external nutrient concentrations. The commonly assumed hyperbolic relationships for steady growth and uptake (viz “chemostat theory”) are coherent with a hyperbolic expression for short term uptake including a variable maximum (ρ_max). The ratio of the saturation constants for growth and uptake is then directly related to the extreme in quotas and maximum uptake rates: K_μ/K_ρ= Q_min/Q_max·ρ^lo_max/ρ^hi_max. This result is applicable even when the exact hyperbolic laws are not. Published data on Fe, Mn, P and N limitation in algae are generally in accord with the theory and demonstrate a wider acclimation range for trace than for major nutrients.