Biodegradation rate of diesel range n-alkanes by bacterial cultures Exiguobacterium aurantiacum and Burkholderia cepacia Gita Mohanty, Suparna Mukherji à Centre for Environmental Science and Engineering (CESE), Indian Institute of Technology (IIT) Bombay, Powai, Mumbai 400076, India Abstract The biodegradation rate of hydrocarbons is an important consideration determining the time scale for bioremediation in oil- contaminated environments. Two naturally occurring bacterial cultures, Exiguobacterium aurantiacum and Burkholderia cepacia, were capable of utilizing diesel oil as the sole source of carbon and energy by induction of hydrophobic cell surfaces with water contact angle greater than 701. The cultures demonstrated good degradation characteristics for diesel range n-alkanes (C9–C26) and were also capable of degrading pristane. A significant correlation was observed between maximum decay rate (MTR) of individual n-alkane peak area and initial abundance of n-alkanes in diesel (r 2 ¼ 0.79 and 0.97 for E. aurantiacum and B. cepacia, respectively). Thus, MTR (day 1 ) was essentially constant, in the range of 0.07–0.20, for n-alkanes with a wide range of carbon numbers from C12 to C26. Biodegradation altered the relative abundance of n-alkanes in diesel and resulted in a loss of symmetry in n-alkane distribution. C9, C17–C19, and C26 were completely degraded by both the cultures. In B. cepacia, the residual diesel was enriched in the higher carbon number n-alkanes C20–C25. The MTR of n-C16 present as a component of diesel was comparable to that for n-C16 when present as sole substrate for B. cepacia but not for E. aurantiacum. Scientific relevance: Two cultures E. aurantiacum and B. cepacia were capable of utilizing diesel oil as sole substrate and exhibited uniform decay rates for a wide range of n-alkanes from C12 to C26. In contrast, most researchers have reported decreasing degradation rate with increasing carbon number. Keywords: Oil; NAPL; Biodegradation; Bacteria; Hydrophobicity 1. Introduction Biodegradation of petroleum hydrocarbons in oil and other non-aqueous phase liquids (NAPLs) by bacterial cultures has been reported in both batch and field scale studies (Mills et al., 1999; Olson et al., 1999; Gogoi et al., 2003; De Oteyza et al., 2004). The rate and extent of biodegradation in batch cultures was affected by NAPL composition and concentration, culture characteristics, concentration and their inherent NAPL uptake mechan- ism, nutrient media composition, and other cultivation conditions, such as temperature, pH, and speed of rotary shaking. Important variables affecting the success of bioremediation in contaminated sites were identified as: nature of the NAPL contaminants, duration of con- tamination, available or applied consortia and their predominance, nutrients applied, and other environmental parameters. NAPL biodegradation/bioremediation is usually a slow process due to the hydrophobic nature of the contaminants and consequent bioavailability limita- tions. Longer acclimatization time is required for micro- organisms to degrade complex hydrocarbon substrates where multiple substrate interaction effects and interaction between cultures degrading the various fractions can potentially play a dominant role. Various components in crude oil are metabolized simultaneously, but some are metabolized at a faster rate than others. Based on studies testing the effectiveness of products for bioaugmentation and biostimulation, Aldrett et al. (1997) reported on the significant difference in biodegradation rates of the saturates and aromatics in ARTICLE IN PRESS