The Effect Of Non-ionic Surfactant On Biodegradation Of Pyrene By PAH-degrading Bacteria

Polycyclic aromatic hydrocarbons (PAHs), composed of two or more fused aromatic rings, are ubiquitous chemicals produced via natural and anthropogenic processes. The presence of high molecular weight (HMW) PAHs in contaminated soils continues to pose significant environmental problems due to the increased persistence and genotoxicity of PAHs with the increasing of their molecular size. A very limited number of bacteria have been isolated from soil that can grow in pure cultures on HMW-PAHs. Pyrene, a four-ring PAH with a wide distribution in the aquatic environment, represents a major portion of the total PAHs found in contaminated sites. It has been used as a model compound in metabolism studies of HMW-PAHs biodegradation due to its relatively simple oxidation and structural similarity to several carcinogenic PAHs.Degradation of pyrene in the environment occurs predominantly by microbial processes and in recent years the study of the utilization of bacterial processes for PAHs bioremediation has increased significantly. However, biodegradation of pyrene is more difficult than low molecular weight (LMW) PAHs due to its low bioavailability to the degrading microbes. To overcome this problem, surfactant is introduced into the HMW-PAHs bioremediation. It has been reported that surfactants can enhance the solubilization of pollutants in the medium, thus improving their bioavailability. Although some surfactants exhibit negative effects on biodegradation process of PAHs, some less toxic or harmless non-ionic surfactants have been chosen to enhance the biodegradation of PAHs. Surfactant seems to be an effective means to improve the bioavailability of HMW-PAHs in biodegradation but the relationship between biodegradation of PAHs and surfactant in pure culture still remains unclear; as a result, this limits its application in bioremediation. As such, the study of the effect of non-ionic surfactant on biodegradation of PAHs is particularly important to understand the biodegradation processes and is also useful for the choice of suitable surfactant for bioremediation of PAHs. The main objective of this paper is to determine the toxic effect of pyrene on Bacillus subtilis (B. subtilis) and the PAH-degrading bacteria Burkholderia cepacia (B. cepacia) in order to understand the effect of non-ionic surfactant Tween80on biodegradation of pyrene.In this study, microcalorimetry has been successfully applied to investigate the toxicity of pyrene and the effect of Tween80on biodegradation. Power-time curves were acquired and calorimetric parameters including the growth rate constant (k), half inhibitory concentration (IC50), and total thermal effect (QT) were determined. These thermo kinetic parameters are good indicators to observe the changes of microbial activity and they are in good agreement with the OD of microbe and enzyme assay data. B. subtilis, B. cepacia and B. cepacia with500mg L-1Tween80were completely inhibited when the concentration of pyrene reached200,400and1600μg ml-1, respectively. B. cepacia shows better tolerance to pyrene than B. subtilis. It is concluded that the bacteria B. cepacia has significant potential for biodegrading pyrene.Tween80significantly improves the biodegradation process and contributes to the survival of B. cepacia by increasing the bioavailability of pyrene. However, with the same initial pyrene concentration, its biodegradability is reduced with the increasing Tween80concentration. For example, nearly50%of the pyrene is degraded when Tween80concentration initially presents500mg L-1. In contrast, less than35%of pyrene could be degraded, if initial Tween80concentration is increased to1000mg L-1. Although biodegradability of pyrene is decreased with the increase of Tween80, the biodegradation amount of pyrene is indeed improved than control because of the enhanced aqueous solubility and bioavailability of pyrene by Tween80.Enzyme assay data show that Tween80increases the expression and activity of catechol2,3-dioxygenase (C23O) in B. cepacia, however, the stimulation decreases with the increase of Tween80, which is consistent with the results from the pyrene biodegradation. Since Tween80increase the solubility and biodegradation of pyrene in the culture medium, relatively higher accumulation of intermediate metabolites and their utilization may be a factor causing the relatively higher expression and activity of C23O in the presence of Tween80.The enhanced bioavailability and biodegradation of pyrene by Tween80shows potential use of Tween80in the PAHs bioremediation.