Js. Liu et al., The coupling between catabolism and anabolism of Methanobacterium thermoautotrophicum in H-2- and iron-limited continuous cultures, ENZYME MICR, 25(10), 1999, pp. 784-794
The aim of the present work was to investigate whether uncoupling of catabo
lism from anabolism, which was often observed in heterotrophic microorganis
ms under energy-sufficient growth conditions, also occurs in the autotrophi
c bacterium Methanobacterium thermoautotrophicum. For this purpose, M. ther
moautotrophicum was cultivated in continuous cultures that were limited by
the trace element iron. The influences of both dilution rate and iron suppl
y rate on the coupling between anabolism and catabolism were investigated.
As compared to continuous cultures of M. thermoautotrophicum limited by the
energy substrate H-2,H- a 5-fold decrease in the biomass concentration and
a 3-fold decrease in H-2, CO2, and CH4 conversion rates were observed in i
ron-limited cultures. However, the specific substrate and product conversio
n rates increased as compared to the values determined in energy-limited cu
ltures. Thus, iron limitation provoked an uncoupling of catabolism from ana
bolism. At a dilution rate of 0.096 h(-1) and at an iron concentration of 1
7 mu M in the feed. the specific H-2 consumption rare was 100% higher than
the rate determined under H-2-limiting conditions, whereas at a dilution ra
te of 0.168 h(-l), the values differed only by 5%. Uncoupling of catabolism
from anabolism also increased dramatically when the iron supply rate was l
owered but the dilution rate was kept constant. Thus, the extent of uncoupl
ing is a function of both the dilution rate and the iron supply rate. It wa
s found that the specific consumption rate of H-2 increased in parallel wit
h the partial pressure of H-2 in the culture medium. This suggested that th
e catabolic activity of M. thermoautotrophicum was not stringently controll
ed at the enzymatic level and can be considerably stimulated by the excess
of H-2 in the medium. Hypotheses as to the fate of the excess energy derive
d from uncoupled catabolism are discussed, but the physiological reason for
the partial uncoupling between catabolism and anabolism remains yet to be
clarified. (C) 1999 Elsevier Science Inc. All rights reserved.