The direct total count of bacteria and the concentration of isoprenoid quinones were measured in 82 samples from different aquatic and soil environments. The geometric mean of the total count (TC)/total quinine (TQ) ratios obtained with different environments ranged from 1.1 to 2.2´10 3 cells fmol -1 . When the concentration of the total respiratory quinone (TRQ), i.e., ubiquinones and menaquinones, were taken into account, the geometric mean of the TC/TRQ ratios fell into a range of 2.2 to 4.1´10 3 cells fmol -1 . A high positive correlation was noted between TC and TQ (r 2 =0.9864) or TRQ (r 2 =0.9990). Based on the relationship between TC and TRQ, 1 nmol of the total quinone was estimated to be equivalent to 2.5´10 9 cells of bacteria on average. These results indicate that the concentration of the respiratory quinones can be used as a good measure of bacterial counts and biomass in the environment.

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Recent reports have suggested the usefulness of a metabolic uncoupler to reduce excess sludge production in the activated sludge process. This study was conducted to more thoroughly study the effects of different chemical uncouplers on microbial biomass production, metabolic activity, and community structure in a laboratoryscale activated sludge system fed with synthetic sewage. Results showed that the amount of biomass produced decreased sharply with increasing concentrations of uncouplers in all cases. Among the five congeners of chlorophenols and nitrophenols as uncouplers studied, 4-nitrophenol was the most effective in reducing biomass production. The addition of 200 mM 4-nitrophenol resulted in a reduction in the biomass yield to less than 10% compared the control culture without the additive, while the BOD removal efficiency with the uncoupler remained approximately 80% of the control level. Differences in biomass reduction efficiency among the uncouplers tested suggested that uncouplers with a lower pKa value have a higher potential to reduce biomass production. A short-term sequencing batch cultivation of sludge with any uncoupler exerted no or little effect on respiratory enzyme activities and the microbial community structure of the sludge. When the inhibitory effect of 4-nitrophenol on biomass production was studied in prolonged sequencing batch cultures, it was found to gradually diminish with time and had almost vanished after 4 weeks of operation. At this stage, a significant change in microbial community structure was demonstrated by respiratory quinone profiling. These results indicate that dosing an uncoupler in a short period is useful for minimizing excess sludge production in the activated sludge process without a significant decrease in BOD removal efficiency, but this positive effect is questionable over a long period of operation.