Abstract Acinetobacter sp. CR was grown on a model oil, which consisted of an inert oil matrix of pristane with n-heneicosane dissolved in it as the sole carbon source, in a stirred-tank bioreactor. This bacterium takes up substrates from the oil phase by direct contact with the oil phase. A previously established mathematical model was applied to reveal the effect of agitation condi- tions on the growth and n-alkane degradation kinetics of the bacterium. Higher impeller speed resulted in both lower microbial growth and lower n-alkane degradation rate of the bacterium, although it increased the specific surface area of the oil, which was measured by a previ- ously developed device. This result was due to the decreased number of cells adhering to the oil surface, i.e., intense agitation inhibited the adhesion of cells to the oil surface. The addition of a surfactant below a critical micelle concentration (CMC) inhibited the degra- dation of n-heneicosane dissolved in pristane, although the biodegradability of the substrate recovered gradually with the increase in the dose of surfactant over CMC. The results suggest that efforts to increase the specific surface area of the oil phase have the undesirable result of inhibiting oil degradation when the dominant microbi- al degraders take up substrates in oil by direct contact with the oil. Introduction Crude oil hydrocarbons are highly hydrophobic materials that are hard to biodegrade, and do not decompose, due to their poor availability to microorganisms (Bury and Miller 1993). Increasing the specific surface area of the oil phase in order to enhance the biodegradation of hydrocarbons has been considered a promising strategy. In particular, surfactants are frequently used to disperse oil phase in the field of bioremediation. However, con- flicting results have been reported about the effective- ness of surfactants on the biodegradation rate of hydro- carbons. Some research groups have found that the pres- ence of surfactant enhances biodegradation, while others have found the opposite (Stelmack et al. 1999). This discrepancy is considered to arise from differences in the mechanism of hydrocarbon uptake by dominant de- graders in different environments. Mechanisms for hydrocarbon uptake by microorgan- isms have been intensively studied for applications in hydrocarbon fermentation and oil bioremediation. Three mechanisms have been generally proposed depending on the type of microorganism involved (Goswami and Singh 1991): (1) uptake of hydrocarbons dissolved in the aqueous phase (Bouchez et al. 1995; Grimberg et al. 1996; Mulder et al. 1998), (2) direct cell contact with, and uptake of, a hydrocarbon drop (or particle) consider- ably larger than the cell (Bouchez et al. 1999; Choi et al. 1999; Efroymson and Alexander 1991; Erickson et al. 1969; Goswami and Singh 1991; Nakahara et al. 1977; Ortega-Calvo and Alexander 1994; Stelmack et al. 1999; Verkooyen and Rietema 1980; Wang and Ochoa 1972; Zilber et al. 1980), and (3) uptake of “solubilized/pseu- dosolubilized” or “accommodated” hydrocarbon droplets much smaller than cells by the action of a biosurfactant (Aiba et al. 1969a; Bouchez et al. 1995; Goma 1978; Goswami and Singh 1991; Moo-Young and Shimizu 1971; Roy et al. 1979; Shreve et al. 1995; Yoshida and Yamane 1971). Pseudomonas stutzeri (Grimberg et al. 1996) is an example of a microorganism that takes up hydrocarbons by mechanism 1, while Arthrobacter sp. R1 (Efroymson and Alexander 1991), Candida albicans (Erickson et al. 1969) and Candida intermedia (Wang and Ochoa 1972) are examples of 2, and Candida guilliermondii (Aiba et al. 1969a), Candida lipolytica (Goma 1978; Moo-Young and Shimizu 1971), En- domycopsis lipolytica (Roy et al. 1979), Pseudomonas aeruginosa (Shreve et al. 1995) and Candida tropicalis (Yoshida and Yamane 1971) use mechanism 3. K. Hori · Y. Matsuzaki · Y. Tanji · H. Unno ( ✉ ) Graduate School of Bioscience and Biotechnology, Tokyo Institute of Technology, 4259 Nagatsuta-cho, Midori-ku, Yokohama 226–8501, Japan e-mail: hunno@bio.titech.ac.jp Tel.: +81-45-9245762, Fax: +81-45-9245818 Appl Microbiol Biotechnol (2002) 59:574–579 DOI 10.1007/s00253-002-1021-9 ORIGINAL PAPER K. Hori · Y. Matsuzaki · Y. Tanji · H. Unno Effect of dispersing oil phase on the biodegradability of a solid alkane dissolved in non-biodegradable oil Received: 12 February 2002 / Revised: 21 March 2002 / Accepted: 2 April 2002 / Published online: 10 May 2002 © Springer-Verlag 2002