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

Abstract

Orthophosphate ( 31 P i ) uptake rates by natural Lake Michigan microbial assemblages were measured to test a hypothesis that the instantaneous velocity of 31 P i uptake at low added substrate concentrations is higher than predicted by the simple Michaelis–Menten equation. Analysis of data from most experiments verified this prediction: 31 P i turnover times (T calc ) 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 T calc could 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 (K t ) and relative abundances is very wide and species with the lowest K t values are most abundant. A comparison of K t values 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 K t values at least an order of magnitude below those detected in culture studies.