Growth dynamics of a methylotroph (Methylomonas L3) in continuous cultures. II. Growth inhibition and comparison against an unstructured model

Abstract

High methanol concentrations have a negative effect on the growth rate and the biomass yield of growth transients induced by methanol pulses in continuous cultures of Methylomonas L3. The physiological basis of this effect is investigated by measuring the effect of the methanol pulse on the cell energy charge (EC) and ATP, ADP, and AMP concentrations, and by comparing the results of the pulse transients against an unstructured model. The methanol pulse is shown to lead to increased values of the cell EC and ATP concentration, and thus, inhibition and reduced availability of biosynthetic energy are excluded as causes of inhibition. When the biomass and methanol profiles of the transient experiments are compared in phase-plane diagrams against computer simulations based on the model, satisfactory agreement between experimental data and model predictions is found in single-substrate, high-dilution-rate experiments. Conversely, poor agreement between experimental data and simulation results indicates a more severe growth inhibition than the model predicts at low dilution rates and a less severe one in mixed-substrate experiments. Based on these findings and other relevant physiological information, we propose that the large variations in the negative effect of methanol on growth result from the fact that cells accumulate methanol to widely different concentrations depending on their physiological state. In their effort to detoxify from the high intracellular methanol and formaldehyde concentrations, cells oxidize considerably more methanol than they can incorporate into biomass. This leads to a useless ATP surplus, which the cells must hydrolyze without doing any useful biosynthetic work, and this results in lower biomass yields.

Expressed Genome of Methylobacillus flagellatus as Defined through Comprehensive Proteomics and New Insights into Methylotrophy

“…However, while high activities for the enzymes of the H 4 MPT pathway have been measured, mutants in these enzymes revealed only very subtle phenotypes, suggesting a minor role in formaldehyde oxidation. Formate dehydrogenase activity was reported only in cells of M. flagellatus (and other RuMP methylotrophs) when grown in a chemostat at low dilution rates (6,12,13), consistent with the nonessential role of the linear pathway. However, here we detected all the components of the linear pathway, including the two alternative formate dehydrogenases.…”