Kinetics of nutrient-limited transport and microbial growth.

INTRODUCTION Nutrient-limited growth is the usual state for most microorganisms of the environment. In aquatic systems, nutrients are usually absorbed until concentrations decrease to levels that are sufficient for the growth of only those organisms with good transport systems, or ones that can grow very slowly. Many terrestrial systems are also restricted by nutrient concentrations, because much organic carbon exists in recalcitrant macromolecules which are hydrolyzed slowly with extracellular enzymes. In nutrient-limited systems the rate of microbial growth is set either by ability to transport the nutrient of greatest need or by the rate of use of a previously stored reserve. Kinetics is the science that relates microbial growth rates to the nutrient concentations on which they depend. It can be divided into two processes, the kinetics of concentration-limited accumulation of nutrients and the efficiency that these nutrients provide for growth. Nutrient uptake is often observed by the rate of nutrient transfer into organisms over time. Growth is observed in closed systems by the rate of proliferation of a population, or, in open systems, by the rate at which a population can be removed without disturbing the steady state or by injected tracer movement through it. Theoretical treatments provide the ties between nutrient transport concentrations, which limit the rate of transporter activity in given systems, and the rate of nutrient accumulation expressed as growth. The purpose of this review is to bring theory together with experimental data that relate nutrient concentrations, nutrient fluxes, and microbial growth rates. Formulations are presented with common nomenclature throughout, and kinetic observations are organized in such a way that all reported observations regarding the accumulation ability of various organisms for a particular nutrient can be compared. In addition to comparing the kinetics of transport and growth of different microorganisms, the compilations are a test of the ability of the experimental systems to resolve the kinet-ics involved. GROWTH RATE The rate of growth expresses the change in population with time. This change depends on the population as well as time, so that dX dt-jA (1) where ,. is the instantaneous growth rate constant. (Definitions and units are given in Table 1.) Integrating between initial and final populations 270