## 1 Introduction  
Petroleum hydrocarbons (PHs) are among the most abundant pollutants in soil and water environments [1]. It is reported that about eight million tons of petroleum are released into the water environment every year [2]. Leakage of PHs commonly occurs during exploration, production, refining, storage and transportation [3]. For example, a rig explosion resulted in the escape of four million barrels of petroleum into the Gulf of Mexico in 2010 [4]. The European Environment Agency reported that about 34% of contaminated sites in Europe were related to mineral oil (including common petroleum substances such as polycyclic aromatic hydrocarbons (PAHs) and volatile aromatic hydrocarbons) in 2006 and this value has since increased to 53% [5]. Besides surface water, PHs leakage also contaminates groundwater. According to the United States Environmental Protection Agency, in 2000, 25% of underground storage tanks for petroleum had leakage which directly resulted in serious groundwater pollution in the United States [6,7]. Toxic and persistent constituents of PHs such as olefinic and paraffinic hydrocarbons, monoaromatic hydrocarbons (e.g., benzene, toluene, and xylene) pose a great threat to human beings [1,8,9,10]. Hence, developing an economical and effective way to remove PHs from water ecosystems is a major goal to maintain the health of ecosystems and human beings. There are several methods to mitigate the risks of PHs contamination of the health of ecosystems and effects on human health, including bioremediation [11,12,13,14] and a variety of physico-chemical treatments, e.g., separation tank treatment [15], coagulant addition [16,17], and membrane separation [18]. Although physico-chemical treatments are effective in degrading these pollutants, they are limited by the high operational costs and generation of secondary toxic compounds [19]. Compared to these methods, bioremediation has received immense attention because it costs less in chemicals and energy and is environment-friendly. In this process, these PHs, serving as the energy and carbon source for microorganisms, are transformed or degraded into non-hazardous (such as microbial biomass and/or CO\({}_{2}\)) or less-hazardous chemicals (such as glycerinum) [20,21]. As implementation of bioremediation is heavily dependent on microorganisms' performance, an in-depth understanding of factors that affect their performance is required. Many microorganisms are able to degrade PHs, such as protozoa, fungi and bacteria. Specifically, grazing protozoa were reported to be able to carry out naphthalene mineralization up to four times faster than the control in flasks with sediment slurry and seawater [22]. Fungi were also reported to degrade PHs, which is mediated by the cytochrome P-450 system and/or soluble extracellular enzymes, including lignin peroxidase, manganese peroxidase and lacases [23]. Additionally, filamentous fungi, basidiomycetes, white rot fungi and deuteromycetes have also shown the ability to reduce the abundance of PAHs by effectively degrading a broad range of the PHs that mainly compose diesel fuels [24,25]. Although many microorganisms can degrade PHs pollutants, bacteria play a dominant role [26,27]. Of various strains obtained from enrichment cultivation of oil-contaminated soils from an oil field in China, 22 strains could utilize diesel as a sole source of carbon and energy and 11 strains could degrade the total PHs of diesel by more than 70% in seven days [28]. Another study found that when two bacterial strains (_Mycobacterium hyalinum_ and _Cidasporium carrianni_) were applied at the same time, a significant synergistic effect was observed, and almost complete (99%) degradation of diesel in was achieved within five days [29]. Compared to bacteria, there are fewer kinds of fungi and protozoa that can degrade PHs, and their average degradation efficacy is also lower [30,31]. Therefore, a comprehensive understanding of how bacteria perform in degrading PHs is of great benefit in the construction of effective bioremediation systems.