## 4 Discussion
[66] reported that the bacterial strain ADL15 tended to degrade shorter _n_-alkanes (_n_-C\({}_{10}\), _n_-C\({}_{11}\), _n_-C\({}_{12}\)) more effectively than the middle-chain _n_-alkanes, while the other bacterial strain, ADL36, could degrade most of the recognizable _n_-alkanes significantly. Therefore, we hypothesized that the difference in diesel degradation between the two strains was partly due to the fact that they had different capacities to utilize diesel constituents. As expected, the Pa strain performed much better than the Kp strain in reducing diesel constituents (especially those with high content), such as C\({}_{14}\), C\({}_{17}\) and C\({}_{21}\) alkanes (with their contents varyingfrom 5.77% to 15.2%), but not C\({}_{12}\) which only accounts for 5.06%. These results suggested that specific bacterial strains or communities are required to utilize given PHs pollutants. We further assessed the growth characteristics of these two strains in mediums where diesel or its constituents were provided as a sole carbon and energy source. This was because the degradation of PHs by bacteria depends mainly on their strong intracellular enzymes [51; 52] and the synthesis and secretion of biosurfactants which are affected by the bacterial growth rate [53]. Therefore, we expected that there would be a positive correlation between PHs degradation efficacy and bacterial growth rate [54; 55; 50; 56]. As expected, compared with the Kp strain, Pa grew faster in diesel medium and was able to utilize more constituents (on both a number-basis and content-basis). This indicated that the Pa strain had a better capacity to utilize diesel as a carbon source. We also confirmed that degradation efficacy was positively correlated with bacterial growth as indicated by either OD\({}_{600}\) or CFU. Although the Pa strain did not grow, Kp grew very well in cyclohexane medium. Pa still performed better in degrading diesel which was because cyclohexane accounted for only 0.6% of diesel on a content-basis. Our result suggested that high growth rate, especially in constituents with high content, should be attended to when selecting strains for PHs biodegradation. The addition of surfactants could significantly promote PHs degradation of oily sludge [72], which might be due to the reduced surface tension [73], and/or enhanced hydrophobicity [46], and/or emulsifiability [74]. Importantly, some microorganisms have been reported to produce biosurfactants. For example, [PERSON] and [PERSON] [75] identified that a novel biosurfactant (proline lipid) was produced by a marine oil degrading bacterium, _Alcanivorax dieselolei_ B-5. In our study, Pa had a lower surface tension value and higher hydrophobicity and emulsifiability than Kp. These results motivated us to investigate whether biosurfactant production contributed to the better performance of Pa in degrading PHs. As expected, we found Pa could excrete glycolipid and we further identified this glycolipid to be rhamnoid [65]. This compound was found to serve as a surfactant which enhances the solubility of PHs and renders them more accessible for biodegradation [76; 77]  
We also found that degradation efficacy was positively correlated with hydrophobicity and emulsifiability. In fact, rhamnoids produced by Pa are well known to interact with toxic xenobiotics--closing them in micelles and making them less toxic to bacteria. The reduction in the toxicity of phenols can be explained by a combination of toxin accumulation in biosurfactant micelles and hydrophobic interactions of phenols with rhamnoid-based dissolved organic carbon. These results provide evidence that, next to the effect of the micelle formation, hydrophobic interactions with rhamnoid-based dissolved organic carbon affect the bioavailability of phenols [78]. Besides which, the surfactant-mediated treatment increases hydrocarbon solubilization and potentially facilitates biodegradation.