Date Published: April 4, 2015
Publisher: Springer Berlin Heidelberg
Author(s): Veena S. More, Preeti N. Tallur, Francois N. Niyonzima, Sunil S. More.
A bacterium capable of degrading pendimethalin was isolated from the contaminated soil samples and identified as Bacillus lehensis XJU based on 16S rRNA gene sequence analysis. 6-Aminopendimethalin and 3,4-dimethyl 2,6-dinitroaniline were identified as the metabolites of pendimethalin degradation by the bacterium. The biodegradation of pendimethalin by freely suspended and the immobilized cells of B. lehensis on various matrices namely agar, alginate, polyacrylamide, and polyurethane foam was also investigated. The batch degradation rate was nearly the same for both free and immobilized cells in agar and alginate, whereas polyacrylamide- and PUF-immobilized cells degraded 93 and 100 of 0.1 % pendimethalin after 96 and 72 h, respectively. At higher concentration, the degradation rate of freely suspended cells decreased; whereas the same immobilized cells on polyurethane foam completely degraded 0.2 % pendimethalin within 96 h. The repeated batch degradation with the polyurethane foam-immobilized cells was reused for 35 cycles without losing the 0.1 % pendimethalin degrading ability. In contrast, agar-, alginate- and polyacrylamide-immobilized cells could be reused for 15, 18, and 25 cycles, respectively. When the pendimethalin concentration was increased to 0.2 %, the immobilized cells could be reused but the pendimethalin degradation rate was decreased. Polyurethane foam-immobilized cells exhibited better tolerance to pH and temperature alterations than freely suspended cells and could be stored for more than 3 months without losing pendimethalin degrading ability. The immobilization of cells capable of degrading pendimethalin may serve as an ideal technique for the complete degradation of the herbicide in the environment.
Microorganisms are one of the tools used to detoxify toxic compounds present in the environment. Free suspended or immobilized microbial cells can be used for this purpose. However, the immobilized microbial cells have many advantages over free suspended cells under different conditions. For instance, the immobilization of whole cells increases degradation rate owing to increased cell population density, cell wall permeability, and extracellular microbial enzymes stability are improved, cells can be easily removed from the reaction mixture, higher operational stability and storage stability, reuse of immobilized cells in continuous reactors, and allows the bioreactors to operate at flow rates different from the growth rate of the microorganisms (Bettmann and Rehm 1984; Hall and Rao 1989; Cassidy et al. 1996; Ha et al. 2009; Zheng et al. 2009). In addition, the immobilized cell systems act as a protective cover in the presence of toxic compounds and are more resistant to pH or temperature changes. However, free suspended cells have better mass transfer aspects compared to immobilized bacterial or fungal cells (Trevors et al. 1992; Zheng et al. 2009).
The degradation of pendimethalin by B. lehensis XJU based on TLC, reverse phase HPLC, UV visible, and mass spectral studies has resulted in the formation of 3,4-dimethyl 2,6-dinitroaniline and 6-aminopendimethalin. Similarly, the biodegradation of the herbicide pendimethalin by B. circulans, resulted in the formation of two metabolites, viz., 6-aminopendimethalin by pendimethalin reduction, and 3,4-dimethyl 2,6-dinitroaniline by pendimethalin oxidative dealkylation (Megadi et al. 2010). Biodegradation of pendimethalin by freely suspended cells of B. lehensis XJU and by cells immobilized in agar, SA, PUF, and polyacrylamide was compared with respect to their degradation rate and tolerance against increasing concentrations of pendimethalin. In batch cultures, the freely suspended cells degraded pendimethalin comparatively well at lower concentrations (0.1 %) and degradation rate decreased at higher concentrations. The cells immobilized in SA, PUF, agar, and polyacrylamide matrices were able to survive and degrade pendimethalin at higher concentrations. The cells immobilized in PUF degraded pendimethalin up to the concentration of 0.7 % (w/v) in batch cultures. The present results revealed that the toxicity of pendimethalin at high concentration level could inhibit the metabolism resulting in lower removal efficiency by the free cells. In addition, it indicates that in an immobilized cell culture, the carrier material act as a protective cover against the toxicity of pendimethalin. The enhanced pendimethalin degradation rate can also be ascribed to higher cell population density and higher activity of the cells immobilized in or on these carrier inert materials. Similarly, the inert materials served as a protective cover towards 2-nitrotoluene (Mulla et al. 2013) and 3-nitrobenzoate (Mulla et al. 2012). Likewise, the higher degradation rates of various toxic nitroaromatics were attributed to higher cell density in the matrices (Cassidy et al. 1996; Qi et al. 2012). Increased concentration of pendimethalin was better tolerated and degraded by PUF-immobilized cells than B. lehensis XJU cells immobilized in agar, SA, and polyacrylamide, as well as free cells. The cells of Micrococcus luteus Z3 immobilized in PUF had the higher nitrobenzene degradation capacity compared to free suspended cells (Qi et al. 2012). The moderate degradation rate observed with bacterial cells immobilized in agar, alginate, and polyacrylamide can be attributed to the leakage that may result from the mechanical instability and monomer and/or radical toxicity (Hall and Rao 1989; Trevors et al. 1992).
The present investigation has revealed the biodegradation of pendimethalin by the bacterial isolate B. lehensis XJU. It also showed the pendimethalin biodegradation by freely suspended and immobilized B. lehensis XJU on different matrices. The microorganisms capable of degrading toxic compounds can therefore be immobilized by entrapment in an inexpensive supports and the immobilized cells retain their ability over a considerable period of time, especially for PUF. Thus, the immobilized microbial cell systems may find applications in the treatment of various contaminated environment sites. However, prior to large-scale application of such systems, further studies are needed for determining the optimal operating conditions.