Degradation of propanil by bacterial isolates and mixed populations from a pristine lake

The microbial transformation rates of propanil, a commonly used herbicide, were investigated using water from a pristine lake in northeast Georgia. Microbial degradation rates were measured using natural water microflora, the natural water microflora amended with five bacterial species (Aerobacter aerogenes, Aeromonas hydrophila, Acinetobacter calcoaceticus, Proteus mirabilis, and Aeromonas salmonicida) isolated from the same lake, and the five isolates individually. Transformation rate constants for propanil were compared for the mixed microbial assemblages and isolates at similar initial bacterial concentrations (approximately 5.0 x 10(-3) bacteria/mL). Degradation started within 60 hours and was completed by 160 hours in all experiments. The mean first-order rate constant for natural microflora was -(4.80 +/- 0.620) x 10(-3) h-1. Natural waters amended with the bacterial isolates yielded rate constants ranging from -(0.39 +/- 0.186) x 10(-3) h-1 to -(2.13 +/- 0.029) x 10(-3) h-1 with an overall mean of -(1.63 +/- 0.242) x 10(-3) h-1. After 660 hours following the first amendment of propanil, (i.e., 500 hours after propanil degradation was complete), each sample was again amended with propanil. Subsequent degradation rates ranged from -(21.3 +/- 0.186) x 10(-3) h-1 to -(64.2 +/- 0.786) x 10(-3) h-1 and the mean rate constant was -(37.5 +/- 0.922) x 10(-3) h-1. No significant differences were observed between first-order rate constants among isolates following the first or the second addition of propanil. After the second spike, however, the average of rate constants was approximately 20 times greater than that following the first spike. Rates for the individual isolates varied greatly from one isolate to another, ranging from virtually no degradation with A. calcoaceticus to -(21.6 +/- 0.332) x 10(-3) h-1 for the composite treatment of all isolates.