Role of inoculum preparation and density on the bioremediation of 2,4-D-contaminated soil by bioaugmentation

Abstract

The effect of inoculum preparation and density on the efficiency of remediation of 2,4-dichlorophenoxyacetic acid (2,4-D) by bioaugmentation was studied in non-sterile soil. A 2,4-D-degrading Pseudomonas cepacia strain (designated BRI6001) was used initially in liquid culture to determine the effects of pre-growth induction and of inoculum density. The time for complete 2,4-D degradation was reduced by 0.5 day for each log increase of inoculum density. In mixed (BRI6001 and soil bacteria) liquid cultures, a competition effect for 2,4-D became apparent at low inoculum levels (less than 10 10⁵ cfu/ml BRI6001 for 10⁸ cfu/ml soil bacteria) but only when the soil bacteria included indigenous 2,4-D degraders. In static non-sterile soil, the effect of inoculum density on 2,4-D degradation was comparable to that in liquid culture but only at high inoculation levels. At lower levels, a biological effect for 2,4-D degradation became apparent, as was observed in mixed liquid cultures, whereas at intermediate levels, a combination of biological, physical and chemical factors decreased the efficiency of bioaugmentation. The acclimation period for 2,4-D degradation in soil bioaugmented with BRI6001 reflected mainly the time required for cell induction and, presumably, for overcoming the physical limitation of diffusion of both 2,4-D and added bacteria in the soil matrix.