Consideration on the Proton Translocation Dehydration Theory

The proton translocation-induced dehydration of bacterial surface is considered a key element of the proton translocation-dehydration theory. In accordance with the chemiosmotic mechanism on most of the aerobic bacteria, ATP is generated by oxidative phosphorylation, in which process electrons are transported through the electron transport system (ETS) from an electron donor (substrate) to a final electron acceptor (O2). The molecules directly using the H+ gradient built up by electron transport can be considered H+-ATP as pumps. In anaerobic methanogens, ATP synthesis is linked with methanogenesis by electron transport, proton pumping, and a chemiosmotic mechanism (Prescott et al., 1999). Similar to aerobic respiration, anaerobic respiration is effective because it is more efficient than fermentation and allows ATP synthesis by electron transport and oxidative phosphorylation in the absence of oxygen. Thus, it appears that proton translocation-driven phosphorylation is a common mechanism for energy generation in both aerobic and anaerobic respirations. It should be pointed out that some bacteria, for example, Streptococcus, have no respiration chain and can produce ATP only via substrate-level phosphorylation. In this case, the proton gradients across those bacterial surfaces are often generated by proton extrusion catalyzed by membrane ATPases (H+/ATPases) at the expense of ATP. It follows that the metabolic end-product efflux is an additional mechanism for proton extrusion from Streptococci and other bacterial cells that result in the generation of proton gradients. Protons are disposed off as acid to regulate their cytoplasmic pH conditions. This in turn can cause protonation and dehydration on the bacterial surfaces.

The fundamentals of energy metabolism show that proton translocation across cellular membrane exists in both aerobic and anaerobic respirations. It has been well established that anaerobic respiration is not as efficient as aerobic respiration in ATP synthesis, because the alternate electron acceptors, such as nitrate, sulfate, or carbon dioxide have less positive reduction potentials than oxygen (Prescott et al., 1999). This implies that less energy is available to generate ATP in anaerobic respiration. In other word, the proton translocation activity across cellular membrane in anaerobic respiration is much lower than that in aerobic process.

The proton translocation-induced dehydration theory suggests that microbial granulation could be observed in any aerobic or anaerobic system, and is independent of the types of substrate, bioreactors, and operation conditions. However, microbial granulation has never been reported in conventional activated sludge systems in the last 100 years of operation, and that anaerobic granules are formed mostly in UASB process. Feasibility and efficiency of other types of anaerobic bioreac-tors with development of anaerobic granules have not been sufficiently demonstrated yet. The proton translocation-dehydration theory provides useful information in understanding how anaerobic granules are developed in a molecular level. However, this theory does not account for those conditions-associated metabolic changes/requirements of microorganisms, which are considered significant contributors to the formation of UASB granules.

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