AbstractsBiology & Animal Science

Degradation of 2,6-dichlorobenzonitrile and 2,6-dichlorobenzamide in groundwater sedimentary deposits and topsoil

by Veera Pukkila




Institution: University of Helsinki
Department: Department of Environmental Sciences, Environmental Ecology
Year: 2015
Keywords: environmental Ecology / ympäristöekologia
Record ID: 1135481
Full text PDF: http://hdl.handle.net/10138/154671


Abstract

The worldwide use of pesticides (herbicides, insecticides, and fungisides) currently amounts to 2.4 billion kilos. Only a small proportion of pesticides actually reach the target organism, whereas the majority becomes a potent contaminant that threatens the environment and humans. Microbes, present everywhere in the environment, have the ability to degrade many kinds of man-made chemical compounds, xenobiotics. By studying the degrading microbes and the optimal conditions for microbial degradation, bioremediation techniques may be developed to clean contaminated sites. A metabolite of the herbicide 2,6-dichlorobenzonitrile (dichlobenil), is 2,6-dichlorobenzamide (BAM). BAM is frequently detected in groundwater worldwide, and thus the use of dichlobenil is nowadays banned in the EU. Dichlobenil is degraded in soil relatively quickly, but BAM is much more persistent. Due to its high water solubility and low sorption affinity, BAM easily leaches down to deeper soil layers and even to groundwater where it is considered stable. This study focused on the degradation of dichlobenil and BAM in Finnish groundwater sedimentary deposits and topsoil. The biotic and abiotic factors associated with effective dichlobenil or BAM degradation were studied. The aim was to examine how the presence of microbes and oxygen, and the chemical characteristics of soil and groundwater deposits, affect the degradation rates of dichlobenil and BAM. In addition, the indigenous microbes degrading these compounds were enumerated, and some were isolated and identified. Dichlobenil was degraded in all studied groundwater sedimentary deposits and topsoil. The presence of microbes and oxygen, and high carbon and nitrogen contents enhanced dichlobenil degradation. As expected, BAM was more resilient to microbial degradation than dichlobenil. Significant aerobic microbial degradation of BAM was detected only in one out of five deposits, and in another weak biodegradation was observed. In these two deposits the concentrations of the elements manganese, zinc, cobalt, lead, and nickel were high. Aerobic bacterial strains growing in the presence of dichlobenil or BAM were isolated from all studied groundwater sedimentary deposits and topsoil. The isolates belonged to the phyla Proteobacteria, Actinobacteria, and Bacteroidetes, Gammaproteobacteria being the largest group of isolates. The dichlobenil or BAM degradation capacity of the isolates was rather low (5-46%) and not demonstrated for all isolates. In conclusion, the diversity of dichlobenil and BAM degrading aerobic microbes in Finnish groundwater sedimentary deposits and topsoil was relatively high. Especially the high number and diversity of isolated BAM degrading strains was unexpected, as only few BAM degrading strains have been reported earlier. Due to their low degradation capacity the potential of these isolates in bioremediation is not considered high, but they could be used e.g. to identify the genes and enzymes involved in the degradation of dichlobenil and BAM. Maailmassa…