Biomedical Engineering Reference
In-Depth Information
generation of proton-motive force by the respiratory chain and its utilization for ATP
synthesis, as well as the mechanism of uncoupler action [57,58].
In recent years, more and more attentions have been paid on this method in
environmental engineering for reduction of excess sludge production in CAS process. Certain
conditions and reagents can uncouple oxidation from phosphorylation, such as the presence of
excess carbon source and limited nutrients; the presence of anti-metabolites; high
temperature; alternative aerobic-anaerobic cycle; etc [59].
1. The Presence of Excess Carbon Source and Limited Nutrients
There is evidence [60] that the observed growth yield decreases significantly with the
increase of carbon source. It has been thought that the observed growth yield is a net result of
interaction among growth, maintenance metabolism and uncoupling degree between
anabolism and catabolism. The observed growth yield decreased with the increase of S
0
/X
0
ratio due to energy spilling under substrate sufficient conditions. Bacteria have various
metabolic flexibilities with respect not only to available catabolic and anabolic substrates, but
also with respect to the continuously changing nutrients [61] which is an indispensable factor
for living, such as
Klebsiella pneumonia
,
Escherichia coli
,
Pseudomonas fluorescens
,
Pseudomonas putida
,
Paracoccis denitrificans
,
Bacillus subtilis
and
Bacillus
stearothermophilus
[62]. The phenotypic and genotypic responses to low-nutrient growth
conditions basically include the regulation of gene expression, variation in the cellular
structure and composition, changes in enzyme-specific activity and capacity involving in
uptake and/or assimilation of the limiting nutrients, as well as changes in the affinity of these
enzymes to different nutrients [61,63]. Under nutrients limited conditions, the uncoupling of
oxidative phosphorylation and ATP synthesis occur due to the overproduction of metabolites.
One possible mechanism may be the leakage of ions, such as protons or K
+
that in turn
weakens the electrical potential across the plasma membrane, resulting in PMF dissipation
and finally uncouples oxidative phosphorylation [34]. Such a situation would be encountered
during the biological treatment of high strength industrial wastewater in which nutrients are
often insufficient.
2. At Unfavorable Temperatures
Senez [49] has reported that there exists a “critical” range of temperature between
optimal growth and total inhibition of bacteria. The maximal efficiency of growth may be
realized only over a restricted range of temperature, and simultaneous decrease of both the
rate and yield of growth would take place outside this range, even with adequate nutrition in
the system. Under such conditions, the biosynthesis rate is reduced, while no corresponding
fall in the rate of catabolism occurs. Zakharov and Kuzmina [64] found that oxidative
phosphorylation to be thermolabile in
Thermus thermophilus
and suggested that elevated
temperatures dramatically increased the membrane permeability of the protons. It has also
been demonstrated that elevated temperature will cause species shifts toward a thermophilic
population while unacclimatized species may achieve reduced biomass production with
thermally induced uncoupled oxidative phosphorylation [34].
