engleski

Impact of the different bioremediation treatments on the structure and activity of functinal community involved in PCB-degradation

Even though production of polychlorinated biphenyls (PCBs) was banned more than 30 years ago, their persistence, toxicity and potential to accumulate in the food web, makes PCBs still ecological and health issues worldwide. Use of microorganisms for cleaning of PCB-contaminated environments has been proposed as a cost-effective and environmentally friendly strategy in comparison with physical-chemical treatments. Even though microbial strains able to co-metabolize PCBs under aerobic or anaerobic conditions have been isolated, in situ removal rates are exceptionally weak, therefore the challenge is to determine why this is so, and to find a way to realize full potential of PCB degraders. In order to better understand biological processes that facilitate PCB-bioremediation in contaminated soils, a small-scale bioremediation experiment was designed. Soil designated for bioremediation had been contaminated with PCBs for more than 10 years as a consequence of transformer station damaged during war event in Croatia. Small-scale field experiment employed three different strategies: (i) bioaugmentation of soil with PCB-degrading mixed culture TSZ7 (BAM treatment) or PCB-degrading strain Rhodococcus sp. Z6 (BAP treatments) and amendments (xylose, carvone and soya lecithin), and (ii) biostimulation of soil only with the amendments (BS treatment). Only natural compounds for both inducing PCB-degradation pathway and enhancing PCB bioavailability were used in the experiment. Study investigated the impact of the treatments (i) on the diversity and abundance of PCB-degrading bacteria (by quantitative PCR, cloning and sequencing of bphA and bphC sequences), (ii) on the PCB-degrading activity of soil microbiota and (by determining PCB residues using gas chromatography analysis) (iii) on the changes in total bacterial community structure (using soil DNA bar coding). Results showed low phylogenetic diversity among PCB-degraders in soils indicating Rhodococcus genus bacteria as the predominant one in all treated soils with the genetic potential to degrade PCBs. Higher levels of bphA and bphC sequences detected in remediated soils further implied that treatments led to the switch in the soil microbial community favoring the development of PCB-degrading Rhodococcus population, shown to represent up to 0.2 % of the total soil bacterial community. It was shown that treatments enhanced removal of PCBs from the soil with approximately 40% of the PCBs being transformed within first year of the experiment. After reaching this maximum further degradation was shown to be limited. Even thought biostimulation was indicated as a sufficient strategy for bioremediation of the targeted soil, the fact that bioaugmentation accelerated PCB-degradation process suggested that soil inoculation with PCB-degraders played a complementary role in the degradation process. Metabolically active soil bacteria were shown to be efficient in degrading structurally versatile PCB congeners present in soil, even ortho-substituted PCBs, known to be highly resistant to microbial degradation. Bar coding analysis of soil DNA extracts revealed that remediation deeply changed the global genetic structure of soil microbial communities. One could conclude that bioremediation treatments affected (i) the abundance of competent PCB-degraders in soil and/or (ii) the induction of microbial community catabolic genes and/or (iii) the bioavailability of PCBs in soil.

polychlorinated biphenyls; soil bioremediation; bphA; bphC; Rhodococcus

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