Bioaugmentation of biomixtures with actinobacteria for atrazine removal: optimization of inoculum concentration

Abstract

Biopurification systems (BPS) or biobeds are bioprophylaxis systems to prevent pesticide point-source contamination, whose efficiency relies mainly on the pesticide removal capacity of the biomixture, the majority component of a BPS. The microbial metabolic abilities of the biomixture could be improved through bioaugmentation with microorganisms with specific degrading capacities, like actinobacteria. In this sense, Streptomyces sp. M7 is a previously selected actinobacterium with well-known pesticide-degrading abilities.The aim of this work was to optimize the concentration of Streptomyces sp. M7 (M7) inoculated in organic biomixtures for atrazine (ATZ) removal. For this purpose, the biomixtures B1 and B2 were formulated with soil, peat, and sugarcane bagasse and filter cake, respectively, inoculated with three concentrations of M7 (2, 4, and 8 g kg-1), and contaminated with atrazine (50 mg kg-1). The residual concentration of ATZ and different microbial groups were determined along a 28 d-assay.In general terms, at the end of the assay, an increasing trend was shown in the microbial developments of the different groups studied in both contaminated and bioaugmented biomixtures for the three concentrations of inoculum used. In B1, the microbial counts were significantly higher with 8 g kg-1 inoculum, with respect to lower inoculum concentrations: total heterotrophic microorganisms, total bacteria, fungi, and actinobacteria reached counts of 3.2 x 107; 2.7 x 107; 1.7 x 105, and 1.5 x 107 CFU g-1, respectively. In B2, total heterotrophs, total bacteria, and actinobacteria were significantly higher for the 4 g kg-1 inoculum concentration, reaching 7.9 x 107; 1.1 x 108, and 5.9 x 107 CFU g-1, respectively; fungal counts did not show significant differences. ATZ removal showed no significant differences in B1 and B2 between the three concentrations of M7 inoculum evaluated. The concentration of 2 g kg-1 of M7 was selected for further studies considering lower costs and optimum removal efficiency.