Phosphate Uptake by Microorganisms in Lake Water: Deviations from Simple Michaelis–Menten Kinetics

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Abstract
Orthophosphate (31Pi) uptake rates by natural Lake Michigan microbial assemblages were measured to test a hypothesis that the instantaneous velocity of31Piuptake at low added substrate concentrations is higher than predicted by the simple Michaelis–Menten equation. Analysis of data from most experiments verified this prediction:31Piturnover times (Tcalc) obtained by back-extrapolating from "low" substrate regions in Woolf plots ranged from 25% to nearly 3000% of those calculated from "high" substrate regions. Simulation analysis demonstrated that deviations in Tcalccould be at least an order of magnitude higher than previously predicted. Large (>1000%) discrepancies from the simple Michaelis–Menten equation could be caused by "skewed" or "clumped" distributions, where the range in both species half-saturation constants (Kt) and relative abundances is very wide and species with the lowest Ktvalues are most abundant. A comparison of Ktvalues for mixed microbial assemblages in Lake Michigan (0.16–19.4 μg P∙L−1) with those from laboratory culture studies (11–364 μg P∙L−1) demonstrates that natural microbial populations have adapted to P-limited environments by synthesizing uptake systems that have Ktvalues at least an order of magnitude below those detected in culture studies.