## 1 Introduction
The biodegradation process for PHs is influenced by many factors, such as the constituent content of diesel fuels [32]. Diesel is a complex mixture of PHs with carbon numbers of C\({}_{10}\)-C\({}_{22}\). For 0\({}^{\#}\) diesel, among the 70 compounds detected, 8% of mass fraction was aromatic hydrocarbons, 90% was saturated alkanes, and 2% was alcohol, acid and olefins [33]. For aromatic hydrocarbons, half were monocyclic aromatic hydrocarbons (e.g., substituted benzene) while the other half were PAHs (e.g., naphthalene, anthracene, and phenanthrene). For saturated alkanes, 69% were C\({}_{13}\)-C\({}_{22}\) straight alkanes, 15% were isomers and 6% were naphthenic hydrocarbons [34]. Importantly, the extent of their biodegradation varied [30], depending on the carbon number [35], branch number [36] as well as the biotoxicity of aromatic hydrocarbons [37]. The order of biodegradation difficulty was: olefins < alkanes < PAHs < branched alkanes < naphthalene hydrocarbons [27; 38]. This biodegradation process is also manipulated by the production of biosurfactant [39; 40], a type of metabolite of PHs produced by microorganisms [41; 42]. For example, the strain DHT-GL (_Pseudomonas aeruginosa_) was able to produce biosurfactant identified as glycolipid on yeast peptone glucose medium and reduce its surface tension from 55 to 30 dN/cm after 96 h growth. In addition, this biosurfactant not only increased emulsification of the oil but also increased the adhesion of PHs to the cell surface of other bacteria [43]. These surfactants were reported to reduce the surface tension of liquid [44], emulsify solubilization and increase the surface hydrophobicity of the bacterial cells [45; 46]. The specific biosurfactant function seems to be connected with environmental conditions and corresponding regulatory systems. The range of rhamnotrophs covers solubilization, modification of surface properties, stimulation of bacterial motility, formation and disruption of biofilms, virulence and anti-microbial activity [42]. These effects increased the probability of contact between the cell and PHs and the adhesion of hydrocarbon, leading to improvement of the hydrophobic substrate [47]. Many microorganisms produce biosurfactants to increase the bioavailability of poorly available substrates [24]. Therefore, bacteria producing biosurfactants are hypothesized to be more effective in degrading PHs [48; 49]. In addition to the constituent content of PHs and biosurfactants, growth characteristics of microorganisms might also contribute to bacterial performance in degradation of PHs [50]. One of the possible reasons is that PHs degradation by bacteria is mainly dependent on their strong intracellular enzymes, such as monooxygenase and hydroxylase [51; 52]. Synthesis and secretion of these enzymes are impacted by the growth rate and metabolism of the cells [53]. It has been reported that fast-growing bacteria have a higher degradation efficacy than their slow-growing counterparts, and there is a positive correlation between them [50; 54; 55; 56]. Two PH-degrading bacterial strains were screened in our laboratory, and we found that they have a high diesel degradability both as a single strain and as a community. Meanwhile, one of them was able to produce biosurfactant while the other was not (Supplementary Materials; Figures S1-S3). Using them to biodegrade diesel, we aimed to explore both biotic and abiotic factors that contributed to the performance of the two strains to degrade PHs. We hypothesized that the strain with higher efficacy for diesel degradation would be able to produce biosurfactant and utilize higher levels of diesel constituents, with a higher growth rate.