Molecular ecology of chlorobenzoate degraders in soil

Abstract

A series of three experiments were conducted to determine the diversity of indigenous chlorobenzoate (CB) degraders in soil and to investigate the use of different methods of bioaugmentation for remediation of contaminated soil. In the first study, soil was amended with either 500 or 1000 μg of 3-CB g⁻¹ and was either uninoculated or inoculated with the 3-CB degrader Comamonas testosteroni BR60. Bioaugmentation with C. testosteroni BR60 increased 3-CB degradation at both contaminant levels, and the increase was more pronounced at the higher level due to contaminant inhibition of indigenous 3-CB degraders. Bioaugmentation also appeared to reduce the deleterious effects that 3-CB contamination had on indigenous soil microbial populations as evidenced by changes in culturable heterotrophic bacterial populations. In the second study, two similar pristine soils were contaminated with 500 μg of 2-, 3-, or 4-CB g⁻¹ . The two soils differed in their ability to degrade the compounds with one degrading 2- and 4-CB and the other degrading 3- and 4-CB. Several hundred degraders were isolated, grouped according to DNA fingerprints, and selected degraders were identified by 16S rDNA sequences. The identity of the CB degraders differed between the two soils. The results indicated that the development of 2-, 3-, and 4-CB degrader populations was site-specific even for the soils that developed under similar soil-forming conditions. The third study also used the two soils from the second study. This project investigated the potential for use of activated soil, which contained an indigenous degrader population, as a bioaugmentation inoculant. An aliquot of a given soil that contained an indigenous 2-, 3-, or 4-CB degrader population was added to a soil that did not have an indigenous degrader population for the same contaminant. The study found that bioaugmentation with activated soil increased degradation of each 2-, 3-, and 4-CB but only if the activated soil was pre-exposed to the contaminant prior to use for bioaugmentation. The results from these three studies indicate that CB degrader populations are diverse and variable in pristine soils and, if not present in contaminated soils, appropriate degrader populations may be established via different bioaugmentation strategies.