Biodegradation and bioremediation potential of diazinon-degrading Serratia marcescens to remove other organophosphorus pesticides from soils.

The ability of diazinon-degrading Serratia marcescens to remove organophosphorus pesticides (OPPs), i.e. chlorpyrifos (CP), fenitrothion (FT), and parathion (PT) was studied in a mineral salt medium (MSM) and in three soils of different characteristics. This strain was capable of using all insecticides at concentration of 50 mg/l as the only carbon source when grown in MSM, and 58.9%, 70.5%, and 82.5% of the initial dosage of CP, FT, and PT, respectively was degraded within 14 days. The biodegradation experiment showed that autochthonous microflora in all soils was characterized by a degradation potential of all tested OPPs; however, the initial lag phases for degradation of CP and FT, especially in sandy soil, were observed. During the 42-day experiment, 45.3%, 61.4% and 72.5% of the initial dose of CP, FT, and PT, respectively, was removed in sandy soil whereas the degradation of CP, FT, and PT in the same period, in sandy loam and silty soils reached 61.4%, 79.7% and 64.2%, and 68.9%, 81.0% and 63.6%, respectively. S. marcescens introduced into sterile soils showed a higher degradation potential (5-13%) for OPPs removal than those observed in non-sterile soil with naturally occurring attenuation. Inoculation of non-sterile soils with S. marcescens enhanced the disappearance rates of all insecticides, and DT50 for CP, FT, and PT was reduced by 20.7, 11.3 and 13.0 days, and 11.9, 7.0 and 8.1 days, and 9.7, 14.5 and 12.6 days in sandy, sandy loam, and silty soils, respectively, in comparison with non-sterile soils with only indigenous microflora. This ability of S. marcescens makes it a suitable strain for bioremediation of soils contaminated with OPPs.

Changes in bacterial diversity and community structure following pesticides addition to soil estimated by cultivation technique.

An experiment was conducted under laboratory conditions to investigate the effect of increasing concentrations of fenitrothion (2, 10 and 200 mg a.i./kg soil), diuron (1.5, 7.5 and 150 mg a.i./kg soil) and thiram (3.5, 17.5 and 350 mg a.i./kg soil) on soil respiration, bacterial counts and changes in culturable fraction of soil bacteria. To ascertain these changes, the community structure, bacterial biodiversity and process of colony formation, based on the r/K strategy concept, EP- and CD-indices and the FOR model, respectively, were determined. The results showed that the measured parameters were generally unaffected by the lowest dosages of pesticides, corresponding to the recommended field rates. The highest dosages of fenitrothion and thiram suppressed the peak SIR by 15-70% and 20-80%, respectively, while diuron increased respiration rate by 17-25% during the 28-day experiment. Also, the total numbers of bacteria increased in pesticide-treated soils. However, the reverse effect on day 1 and, in addition, in case of the highest dosages of insecticide on days 14 and 28, was observed. Analysis of the community structure revealed that in all soil treatments bacterial communities were generally dominated by K-strategists. Moreover, differences in the distribution of individual bacteria classes and the gradual domination of bacteria populations belonging to r-strategists during the experiment, as compared to control, was observed. However, on day 1, at the highest pesticide dosages, fast growing bacteria constituted only 1-10% of the total colonies number during 48 h of plate incubation, whereas in remaining samples they reached from 20 to 40% of total cfu. This effect, in case of fenitrothion, lasted till the end of the experiment. At the highest dosages of fenitrothion, diuron and at all dosages of thiram the decrease of biodiversity, as indicated by EP- and CD-indices on day 1, was found. At the next sampling time, no significant retarding or stimulating effect was detected. However, in case of CD values the higher differences were observed. The significant impact of pesticides on the physiological state of soil bacteria was not found. They were generally in dormant state (lambda < 0.5), but immediately after pesticides application, the additional reduction of frequency of bacterial cell proliferation (max. decrease of lambda value to 0.15 for thiram on day 14) and prolonged retardation time of colony appearance (max. increase of t(r) value to 1.39 for fenitrothion on day 1) on agar plates were found.