Research On The Bioaccumulation, Toxic Effect And Biodegradation Of Hexachlorobenzene By The Microbial Communities

Hexachlorobenzene (HCB) is considered as a model persistent organic pollutants (POPs). Bioaccumulation and toxic effects of hexachlorobenzene in the microbial communities and biodegradation of hexachlorobenzene are two research focuses. Supported by the Key Project of Ministry of Education, some researches are done on bioaccumulation and toxic effects of HCB in the microbial communities, isolation of anaerobic microbial community and aerobic microbial community which be able to degradate of HCB.Methods:In order to study the adsorption and transfer behaviors of hexachlorobenzene within the aquatic microbial community, the samples were separated into four fractions and hexachlorobenzene of each fraction were determined. Using the polyurethane foam units (PFU) method to monitor bioaccumulation and toxic effects of hexachlorobenzene in the microbial communities. Microbial communityies which was able to biodegrade hexachlorobenzene were enriched from sediment from contaminated site. Application of restriction fragment length polymorphism screening approach analyzing the HCB degradation microbial communities.Results: 1) Experiments were conducted to study the influence of hexachlorobenzene on an aquatic microbial community and the adsorption and transfer behaviors of hexachlorobenzene within the aquatic microbial community. The results showed that the accumulating consumption of oxygen and biomass were inhibited significantly with 6μg/L hexachlorobenzene. The concentrations of hexachlorobenzene in the dissolved decreased whilst the concentration increased in the higher trophic level (flagellate and >8μm fractions). Approximately 75% hexachlorobenzene was associated to the microbial community fraction and about 25% to the dissolved fraction. The acute toxicity of fipronil on the protozoan community colleted by PFU from fresh water as target organism was studied. Results showed that the HCB was low toxicity to the protozoan community.2) The bioaccumulation and toxic effect of HCB in the microbial (protozoan in particular) communities were investigated by using the standardized polyurethane foam units (PFU) method. In January and August 2006, field samples were taken from eight stations established along the Fu-He River Wuhan, an water body receiving HCB industrial wastewaters containing HCB and other pollutants. The concentration ratios of HCB in microbial communities and in water were 9.66-18.64, and the microbial communities accumulated 13.29-56.88μg/L of HCB in January and 0.82-10.25μg/L HCB in August. Correlation analysis showed a negative correlation between the HCB contents in the microbial assemblage and the number of species and the diversity index of the protozoan communities. This study demonstrated the applicability of the PFU method in monitoring the effects of HCB on the level of microbial communities.3) An anaerobic microbial community which was able to grow at the presence of hexachlorobenzene was enriched from sediment from contaminated site. The result showed that the mixed microorganisms were able to utilize hexachlorobenzene as sole carbon and energy source. It was shown that up to 95% of HCB could be degraded during 45 days incubation at 30℃in mineral salts medium (pH7.0) with 1mg/L HCB. Application of restriction fragment length polymorphism screening approach analyzing the microbial community revealed 5 clusters, and 3 major clusters were sequenced. Nearly complete 16S rDNA sequence analysis show that the microbial community was dominated by Dysgonomonas and Ideonella groups.4) An aerobic microbial community which was able to grow at the presence of hexachlorobenzene was enriched from sediment from contaminated site after incubating about 2 months. The result showed that the mixed microorganisms were able to utilize hexachlorobenzene as sole carbon and energy source. It was shown that up to 55% of HCB could be degraded during 18 days incubation at 30℃in mineral salts medium (pH7.0) with 4.5mg/L HCB. The calculated rate of hexachlorobenzene biodegradation was 137.5μg/ (L·d). Application of restriction fragment length polymorphism screening approach revealed 9 clusters, and 3 major clusters were sequenced. Nearly complete 16S rDNA sequence analysis show that the microbial community was dominated by Diaphorobacter, Acinetobacter, Alcaligenes and Azospirillum groups. To our knowledge, this is the first report describing aerobic dechlorination of hexachlorobenzene via dehalorespiration by a microbial community which was enriched from contaminated site.