Abstract:
Agrochemicals have been used extensively all over the world for improved food security, industrial development, and poverty reduction. In Western Kenya, herbicides are used to clear weeds in sugarcane growing plantations. Uncontrolled and unregulated use of these herbicides results in contamination of both soils and the associated drainage systems. Their use may have adverse effects such as disruption of microbial, animal and plant diversity in addition to serious effects to human health. Repeated application of the herbicides results in biochemical adaptation of native microbes especially the bacteria, which in turn lead to the enhanced mineralization of the herbicides. Hexazinone and 2,4-dichlorophenoxyacetic acid (2,4-D), are among the most commonly used herbicides in Nzoia sugar cane farms in Western Kenya. The main objective of this study was to isolate and characterize bacterial degraders of hexazinone and 2,4-D from soils collected from Nzoia Sugar Company sugarcane farms in Western Kenya and also to determine their effects on colony forming units (CFUs) and total dehydrogenase (DHA) activity. Isolation was achieved through incubation experiments in mineral salt medium amended with the herbicides. Growth of isolates indicated by turbidity of broths was monitored by optical density measurements, whereas degradation by isolates was determined by quantification of residual herbicides using high performance liquid chromatography. DHA activity analysis was achieved through triphenyl tetrazolium chloride (TTC) based method. The DHA activity showed that 2,4-D had inhibitory effects on total DHA activity while hexazinone had boosting ability for the bacterial growth. Degradation experiments yielded four bacteria strains encoded as N13010H1, N15030H2, N15030H3 and N212H4 which were able to degrade hexazinone and three bacteria degraders encoded as N139D1, N13010D3 and N13010D4 with potential to degrade 2,4-D. Biochemical and molecular characterization showed that, N13010H1 was Bacterium NLAE zl-H322. N15030H2 was identified as Enterobacter sp, N15030H3 as Bacillus cereus while N212H4 was identified as Staphylococcus aureus. All the four isolates had high potential for hexazinone degradation ranging from 57.6 to 82% with N15030H3 showing the highest potential. For 2,4-D degraders, N13010D3 was identified as Serratia marcescens, while N139D1 and N13010D4 were identified as Bacillus sp and Uncultured bacterium clone, respectively. The biodegradation capacity of 2,4-D by the three isolates ranged from 65 to 82% with N13010D3 showing the highest potential. The isolates can be modified and utilized for bioremediation of hexazinone and 2,4-D contaminated soils.
