Biodegradation of petroleum hydrocarbons in soils co-contaminated with petroleum hydrocarbons and heavy metals derived from petroleum

Abstract

The biodegradation of sites co-contaminated by organic pollutants and Heavy Metals is often a challenge due to the inhibition of microbial activities. Microbes play important role in the mineralization of petroleum hydrocarbons to CO2 by utilizing petroleum hydrocarbons as a carbon/ energy source. Heavy metals are often constituents of petroleum. Petroleum spills may result to the release associated metals (e.g. Ni, Cd, Pb, As) into the environment. Subsequent spills may cause an increase in metal concentrations in soils that may build to concentrations above intervention values. This may result to the inhibition of important biological activities such as the biodegradation of organic contaminants. This research investigates the effects of Ni, Cd and Pb contamination on biodegradation of petroleum hydrocarbons in complex soil system using a microbiological approach combined with geochemical approach. Such an approach will provide a more detailed understanding of the patterns of oil degradation under different and increasing metal stresses and how microbial communities change in such environments. Results indicated that Ni has stimulatory or no effects on biodegradation of petroleum hydrocarbons in soils depending on the chemical form of added Ni. The stimulatory effect was observed in Ni-Porphyrin contaminated soils and declined with increasing Ni concentration. In NiO soils, no effects occurred at low concentrations and increased concentration of Ni resulted to increased inhibition of biodegradation. This is unlike NiCl2 amended soils where Ni effects on biodegradation were neutral irrespective of Ni concentration. The microbial diversity study of the microbial soil community indicated that there was a selective enrichment of species in the soil communities. Phylogenetic study indicated that the dominant microorganism in the community is a strain of Rhodococcus (100%), which was closely related to most Rhodococcus strains isolated from hydrocarbon-contaminated environments, metal contaminated environments and extreme environments. Results indicated that Cd inhibited biodegradation of crude oil in soils, irrespective of Cd form or concentrations. The inhibitory effect increased with increasing concentrations. Also, the microbial diversity study of the microbial soil community indicated that there was a selective enrichment of species in the soil communities. Similar to Ni, Phylogenetic study indicated that the dominant microorganism in the community is a strain of Rhodococcus. Also biodegradation of petroleum was significantly reduced in crude oil degrading short-term Pb contaminated soils, irrespective of Pb form or concentration. However, in long-term Pb contaminated soils, while maximal rate of petroleum degradation reduced at high- Pb concentration, no effect was observed at low lead concentration. Also, the microbial diversity study of the microbial soil community indicated that there was a selective enrichment of species in the soil communities. Two dominating specie were identified in Pb-soils depending on soil. Both are closely related to a strain belonging to Bacillales that were originally isolated Rock, Scopulibacillus darangshiensis strain (98%) and oil contaminated soil Bacillus circulans (99%). While the former dominated in Pb -short-term contaminated soils as well as Pb-long term contaminated soil at high concentration, the later dominated long-term-Pb contaminated soil at low concentration