Bacterial community dynamics during the bioremediation of diesel contaminated soil using brewery spent grain

Brewery spent grain (BSG) has been shown to be nutrient rich and support the growth of microbes. It has thus been exploited in studies for the bioremediation of hydrocarbon contaminated soils. However, previous studies using BSG, have tended to investigate contaminant reduction and microbial growth. The associated bacterial community dynamics, metabolites, catabolic genes, and any changes over time have not been investigated. This knowledge could proffer a more informed rate enhancing strategy to optimise the remediation process. The study reported here has undertaken research to evaluate the bioremediation potential of BSG in diesel contaminated soil in terms of bacterial population dynamics, bacterial metabolic potential in key hydrocarbon degradation pathways, presence and abundance of degradation genes and shifts in bacterial populations towards known hydrocarbon degraders. It has also evaluated the breakdown of the diesel and attempted to correlate this with changes in bacterial populations. The results show that BSG accelerates the biodegradation of diesel contamination in soils. Bacterial counts increased significantly in soil samples with BSG added, from log 3.9 up to log 5.6. The alkB gene for alkane degradation and catA and xylE genes for aromatic hydrocarbon degradation, were significantly higher in the microbial community of soils treated with BSG; implying that BSG supported the proliferation of both aromatic and aliphatic hydrocarbon degrading microbes possessing these genes. Also, the metagenomic evaluation of bioremediation over time, adopted in this study, revealed shifts in bacterial populations in favour of Flavobacterium, Pseudomonas and Acinetobacter species and the latter two were shown to be intrinsic populations of the BSG control treatment. Of these species, Acinetobacter spp. were associated with the xylE gene while Pseudomonas putida was associated with both the catA and alkB genes; thus enhancing both aliphatic and aromatic hydrocarbon biodegradation. The BSG control treatment was also shown to harbour five other known hydrocarbon degrading bacterial species.