Integrated Comparative Genomic Analysis and Phenotypic Profiling of Pseudomonas aeruginosa Isolates From Crude Oil

Abstract

Pseudomonas aeruginosa is an environmental microorganism that can thrive in diverse ecological niches including plants, animals, water, soil, and crude oil. It also one of the microorganism widely used in tertiary recovery of crude oil and bioremediation. However, the genomic information regarding the mechanisms of survival and adapation of this bacterium in crude oil is still limited. In this study, three Pseudomonads strains (named as IMP66, IMP67, and IMP68) isolated from crude oil were taken for whole-genome sequencing by using a hybridized PacBio and Illumina approach. The phylogeny analysis showed that the three strains were all P. aeruginosa species and clustered in clade 1, the group with PAO1 as a representitive. Subsequent comparative genomic analysis revealed a high degree of individual genomic plasticity, with a probable alkane degradation genomic island, one type I-F CRISPR-Cas system and several prophages integrated into their genomes. Nine genes encoding alkane hydroxylases (AHs) homologs were found in each strain, which might enable these strains to degrade alkane in crude oil. P. aeruginosa can produce rhamnolipids (RLs) biosurfactant to emulsify oil, which enables their survival in crude oil enviroments. Our previous report showed that IMP67 and IMP68 were high RLs producers, while IMP66 produced little RLs. Genomic analysis suggested that their RLs yield was not likely due to differences at genetic level. We then further analyzed the quorum sensing (QS) signal molecules that regulate RLs synthesis. IMP67 and IMP68 produced more N-acyl-homoserine lactones (AHLs) signal molecules than that of PAO1 and IMP66, which could explain their high RLs yield. This study provides evidence for adaptation of P. aeruginosa in crude oil and proposes the potential application of IMP67 and IMP68 in microbial-enhanced oil recovery and bioremediation.

Introduction

Microorganisms can survive in many harsh environments, such as glaciers, hot springs, and crude oil. In order to survive in these harsh environments, bacteria usually evolve a series of corresponding functional proteins or metabolic systems through adaptive evolution including gene mutation and horizontal gene transfer (HGT) (Ollivier and Magot, 2005). Crude oil is a complex mixture of thousands of compounds that is difficult to be utilized and even toxic for most of bacteria (Xu et al., 2018). Interestingly, bacteria such as Pseudomonads can survive in such harsh environment. Alkanes are saturated hydrocarbons of different carbon-chain length and structure, which are the major components of crude oil. Although they are chemically inert, most of them can be efficiently degraded by several microorganisms (Rojo, 2009; Sun et al., 2018). It has been reported that more than 60 genera of aerobic bacteria and 5 genera of anaerobic bacteria are able to degrade n-alkanes (Prince, 2005).

Alkane hydroxylases (AHs), which catalyze the first step of alkane biodegradation, are the key enzymes in aerobic degradation of alkanes by bacteria. These enzymes hydroxylate alkanes to alcohols, which are further oxidized to fatty acids and catabolized via the bacterial β-oxidation pathway (Rojo, 2009). The constituent proteins of the AH system include alkane hydroxylases, one or two rubredoxin (rubA, rubA1, and rubA2) and rubredoxin reductase (rubB) (Geissdorfer et al., 1999). Two common AHs in bacteria are the alkane hydroxylase (AlkB)-related AHs and the cytochrome P450 family proteins (Nie et al., 2014a). Furthermore, flavin-binding LC-alkane hydroxylase (almA) and thermophilic soluble LC-alkane hydroxylase (ladA) have also been found to be involved in the hydroxylation of alkanes (Wentzel et al., 2007; Wang and Shao, 2012). A number of bacteria have been found to contain multiple AHs (van Beilen et al., 2006). For example, the co-existence of AlkB and CYP153 was found in Dietzia sp. DQ12-45-1b (Wang et al., 2011; Nie et al., 2014b), and multiple AHs were found in Amycolicicoccus subflavus DQS3-9A1^T (Nie et al., 2013). Several Pseudomonas species have also been proved to have the ability to utilize n-alkane (Kok et al., 1989; Yuste et al., 2000; Zhang et al., 2011). Among them, P. putida GPo1 has been extensively studied, which can grow on intermediate chain length n-alkanes by virtue of the OCT plasmid-encoded alkane hydroxylase (Chakrabarty et al., 1973). The OCT plasmid carries the alk regulon and consists of two gene clusters: alkBFGHJKL and alkST (van Beilen et al., 1994, 2001). In recent years, many alkane-degrading P. aeruginosa strains have been isolated from crude oil, including P. aeruginosa N002, P. aeruginosa SJTD-1, and P. aeruginosa DQ8 (Zhang et al., 2011; Liu et al., 2012; Das et al., 2015). Genome sequencing has shown that many of these strains contain homologs of the GPo1 alkane hydroxylases genes, which are located on chromosome in contrast to the plasmid-encoded GPo1 (van Beilen et al., 2001).

Under specific environmental conditions, P. aeruginosa could produce rhamnolipids (RLs), which are a class of glycolipids biosurfactants and mainly composed of 3-(3-hydroxyalkanoyloxy) alkanoic acids (HAA), mono-rhamnolipids (MRLs) and di-rhamnolipids (DRLs) (Deziel et al., 2003; Soberon-Chavez et al., 2005). In crude oil environment, RLs emulsify crude oil; which is always considered to have prominent ability to solubilize various hydrocarbons for the biodegradation (Beal and Betts, 2000; Noordman and Janssen, 2002). In P. aeruginosa, the biosynthesis of RLs is positively regulated by RhlI/RhlR quorum sensing (QS) system that directly activates the transcription of rhlAB, which encodes the enzymes catalyzing the first step of RLs biosynthesis. QS is a cell density-dependent communication mechanism which enables bacteria to coordinate cooperative behaviors in response to the accumulation of self-produced autoinducer signals (N-acyl-homoserine lactones, AHLs) in their local environment (Passador et al., 1993; Davies et al., 1998; Daniels et al., 2004). In P. aeruginosa, QS depends on two AHLs regulatory circuits, Las and Rhl. In the Las system, LasI directs the synthesis of N-(3-oxododecanoyl)-homoserine lactone (3-oxo-C12-HSL), which interacts with the transcriptional regulator LasR to control target genes, such as rhlI/rhlR. RhlI is also a signal synthase which generates another signal molecular, N-butanoyl-homoserine lactone (C4-HSL). Subsequently, C4-HSL combines the signal receptor RhlR to form a RhlR/C4-HSL complex that activates the transcription of several genes, including the rhlAB operon (Pearson et al., 1997).

In our previous work, we reported the identification of three rhamnolipid-producing Pseudomonas isolates from crude oil (named as IMP66, IMP67, and IMP68), which exhibited sigificant different phenotypes in RLs and pyocyanin production (Das et al., 2014). The 16s rRNA analysis has suggested that two of them (IMP67 and IMP68) are P. aeruginosa and IMP66 is identified as Pseudomonas sp. Pyr41 (Das et al., 2014). In this study, we performed the comparative genomic analysis for these three Pseudomonas strains to further understand their genetic background. Our analysis indicated that all three strains belong to P. aeruginosa species. Subsequent analysis showed that these three isolates exhibited a high degree of individual genome plasticity along with abundant numbers of alkane hydroxylase encoding genes, conferring survival advantages for P. aeruginosa strains living in the crude oil environment. Moreover, the strains with high yield of RLs produce large amounts of QS signal molecules. This study illustrates the environmental adaptability of P. aeruginosa, and proposes potential applications of these strains in bioremediation as well as in microbial-enhanced oil recovery.

Materials and Methods

Bacterial Strains, Plasmids, and Culture Conditions

IMP66, IMP67, and IMP68 were isolated from the crude oil of Karamay W#8805, Xin-Jiang province, China. Unless otherwise indicated, P. aeruginosa strains were grown in LB without sodium chloride (LBNS) or basal salt medium (BSM, K₂HPO₄ 3.815 g, KH₂PO₄ 0.5 g, (NH4)₂HPO₄ 0.825g, KNO₃ 1.2625 g, Na₂SO₄ 0.2 g, CaCl₂ 0.02 g, FeCl₃ 0.002 g, MgCl₂ 0.02 g/l) at 37°C. Culture was supplemented with 300 μg/ml carbenicillin to select and maintain plasmid-carrying P. aeruginosa. A plasmid-borne fusion of the QS-controlled rhlAB promoter sequence (423 bp) and GFP coding sequence was used to assess the promoter activity of rhlAB in this study. Primers P_rhlAB_F (5′-cgccagagcgtttcgac-3′) and P_rhlAB_R (5′-ttcacacctcccaaaaattttcgaac-3′) were used to generate an amplicon of the rhlAB promoter by I-5^TM High-Fidelity DNA Polymerase (Tsingke Biological Technology). The PCR product was purified and ligated into pPROBE-AT vector (Miller et al., 2000) through Gibson assembly (Gibson et al., 2010). Recombinant plasmid pTH22 (rhlAB:gfp) was then transformed into Escherichia coli DH5α and subsequently into P. aeruginosa for determination of the transcription of rhlAB.

Genome Assembly and Annotation

Genomic DNA was isolated from an overnight broth culture using the Wizard genomic DNA purification kit (Promega, Madison, WI, United States). A spectrophotometer (NanoDrop 2000 UV-Vis; Thermo Scientific, MA, United States) was used to determine the DNA quantity and ensure the DNA quality was suitable for sequencing. Whole-genome sequencing was performed using the long-read PacBio SMRT sequencing (Pacific Bioscience) platform and the Illumina HiSeq sequencer. Genome sequences were de novo assembled using HGAP assembly protocol, which is available with the SMRT Analysis packages and accessed through the SMRT Analysis Portal version 2.1. The pilon v1.5 was then used to correct the assembly (Walker et al., 2014). Annotation was performed using the NCBI Prokaryotic Genome Annotation Pipeline (PGAAP)¹ (Tatusova et al., 2016). The circular genome map of the three strains including all predicted ORFs with COG functional assignments, rRNA, tRNA, G + C content, and GC skew information were generated using CGview (Stothard and Wishart, 2004).

Phylogenetic Analysis

The evolutionary position of the three strains relative to other pathovars and species of Pseudomonas was determined by multi-locus sequence analysis (MLSA) as described previously (Loper et al., 2012). For phylogenetic tree construction, ten conserved housekeeping genes (acsA-aroE-dnaE-guaA-gyrB-mutL-ppsA-pyrC-recA-rpoB) from 51 closely related Pseudomonas spp. were retrieved from Pseudomonas Genome Database (Winsor et al., 2016). Maximum Likelihood analysis was performed for 100 bootstrap replications in RAxML. The final phylogenetic tree was constructed using the iTOLs software (Letunic and Bork, 2016).

In order to detect the strain-level genome content of the three strains, a rapid large-scale prokaryote pan genome analysis by Roary was performed to identify the conserved (core) and non-conserved (accessory) genes (Page et al., 2015), and then created a concatenated core genome for subsequent phylogenetic analysis. 90 closely related Pseudomonas spp. retrieved from Pseudomonas Genome Database (Winsor et al., 2016) were used for the core-genome estimating by Roary bacterial genome analysis pipeline. A cut-off with 90% identity was used and the core genes were defined as those presented in 99% of isolates to generate a multi-FASTA alignment file. These data were then used as the input to the next iteration. RAxML was then run over the final multi-FASTA alignment to provide a high-quality phylogenetic tree in newick format. We used a generalized time reversible (GTR) model with gamma (Y) heterogeneity across nucleotide sites and 100 bootstrap replicates. The final phylogenetic tree was constructed using the iTOLs software (Letunic and Bork, 2016).

Comparative Genome Analysis

The MicroScope platform (Vallenet et al., 2016) provided some of the tools used for the comparative genomic analyses, such as the RGP Finder for determination of regions of genomic plasticity (RGPs). RGPs were defined as DNA segments over 5 kbp possibly related to events of horizontal exchanges, and they were identified using a series of constraints such as G + C% deviation, compositional biases as measured by the tools Alien-Hunter (Vernikos and Parkhill, 2006) and SIGI-HMM (Waack et al., 2006), synteny breaks and proximity to tRNAs. Pan-genome analysis including the genes shared by all these four strains was performed by an “all-against-all” BLAST of the protein sequences of the above genomes using the EDGAR 2.3 software framework (Blom et al., 2016). The genes aligned based on amino acid sequences were used for identifying orthologs. For orthology estimation, EDGAR used a generic orthology threshold calculated from the similarity statistics of the compared genomes, and then generated the veen diagram.

AHLs Extraction and Quantification

Extraction and quantification of AHLs were based on a previously described method (Ortori et al., 2011). Briefly, strains were grown in LBNS media for 24 h, 1 ml of bacterial cells were spun down and supernatants were harvested for AHLs extraction. Acidified ethyl acetate was used for AHLs extraction and the extraction was repeated three times. The organic layers were combined and evaporated under vacuumrotary evaporation. Residues were dissolved in 50 μl methanol and subjected to LC-MS/MS analysis using mass spectrometer (QTRAP 6500 System, AB SCIEX). The calibration curve was prepared from a linear dynamic range (from 0.33 to 33.3 μM) of standard 3-oxo-C12-HSL (Sigma-Aldrich, Germany).

Results

Phylogenetic Analysis of Newly Sequenced Strains With the Other Pseudomonas spp.

IMP66, IMP67, and IMP68 were isolated from the crude oil of Karamay W#8805, Xin-Jiang province, China. The complete genomes of these three strains were sequenced and assembled. A previous report suggested that only IMP67, and IMP68 were P. aeruginosa based on 16S rRNA analysis. It has been showed that the phylogenetic tree based on 16S rRNA lacked resolution at intrageneric level (Anzai et al., 2000). We then used the ten house-keeping genes to further analysis the phylogenetic position of these three strains. A phylogenetic tree was plotted using a maximum likelihood (ML) approach based on multi-locus sequence analysis of 54 representative Pseudomonas spp. strains, including 51 stains with complete genome sequence and these three newly sequenced strains (IMP66, IMP67, and IMP68). E. coli K12 was used as an out-group. Four tightly monophyletic groups could be identified from the phylogenetic tree based on the 10 concatenated housekeeping genes (Figure 1, SC1 to SC3, and SC5). Strains IMP66, IMP67, and IMP68 were all clustered together within the group of P. aeruginosa and were all identified as P. aeruginosa strains (Figure 1).

FIGURE 1

In order to investigate the evolutionary history of these three newly sequenced strains, we reconstructed the phylogeny of the genus using concatenated core genomes from 90 P. aeruginosa isolates, including representative P. aeruginosa strains with completely genomes (such as PAO1, PA14, and PA7), IMP66, IMAP67, and IMP68, as well as several crude oil-isolated P. aeruginosa strains (Pb18, M8A1, M8A4, M28A1, SJTD-1, DQ8). There were 51,389 SNPs identified in the 820 core genes of the 90 isolates. P. aeruginosa strains were divided into three major clades (Figure 2A), which was consistent with the previous reports (Freschi et al., 2015; Castañeda-Montes et al., 2018). Interestingly, all crude oil isolates were consistently distributed in clade 1, which was represented by strain PAO1 and contained the strains from a wide range of sources, including natural environment and clinical settings (Figure 2A). Although most of the strains from crude oil were randomly distributed within clade 1, strain IMP66, IMP67, and IMP68 were clustered together (Figure 2B), which was consistent with the results of housekeeping gene analysis (Figure 1). Taken together, our data suggested that these three P. aeruginosa strains were closely related in the evolutionary process.

FIGURE 2

Comparative Genomic Analysis of IMP66, IMP67, IMP68, and PAO1

IMP66, IMP67, and IMP68 were similar in genome size, with a single chromosome of about 6.5 Mb (Table 1 and Figure 3A). The three strains had the same G + C% content of 66.4% (Table 1), which was consistent with other P. aeruginosa strains. The total numbers of coding sequences (CDSs) in the three strains were close to 6100, which were much higher than that of PAO1. We also performed a general comparison of the genomic features including genome size, GC content, and predicted number of CDSs among several crude oil sourced strains and well-studied strain PAO1 (Table 1). The results showed that most of the crude oil isolates harbored bigger genome size than PAO1 strain except SJTD-1. However, SJTD-1 possessed about 200 more CDSs than PAO1 even with similar genome size. These results suggested that genome expansions might confer survival advantages for these isolates living in crude oil.

FIGURE 3

We then analyzed the group of shared and unique genes between IMP66, IMP67, IMP68, and PAO1. All the shared and unique genes were determined and visualized by EDGAR platform. The four bacteria shared a conserved core genome comprising of 5,376 CDSs. PAO1 possessed 168 distinct singletons, which was the largest number of singletons among these four strains. Interestingly, there were 592 unique genes shared by IMP66, IMP67, and IMP68 (Figure 3B). The largest portion (46 genes) of the 592 unique genes belonged to mobilomes (prophages and transposons, category X) according to the COG annotation. Other major unique genes were in category K (transcription), category L (replication, recombination, and repair), and category M (cell wall/membrane/envelope biogenesis), respectively. A small portion of genes were related to energy production and conversion, amino acid transport, and metabolism, etc (Figure 3C). The mobilome genes are commonly associated with HGT as they often carry transposons, phage fragments, and IS elements. These results suggested that HGT might commonly occur in these three strains. To further investigate HGT events in these three strains, the MicroScope platform was used to compare the genomes of IMP66, IMP67, IMP68 with PAO1. We selected IMP66 genome as the representative reference (the same results were obtained when using IMP67 or IMP68) of these three strains against the query genome of strain PAO1. A total of 23 conserved RGPs were identified in IMP66 (Figure 4A and Table 2). Most of these regions contained the sequences related to transposable elements and were flanked by genes coding for tRNAs or phage-structures (Table 2), indicating the possible horizontal gene acquisition. Although most of these regions encoded proteins of unknown functions, some RGPs with important genomic information were identified. RGP9 carried an insertion sequence at its 5′-end (IS3 family, DBX28_13665) and harbored CDSs with predicted functions that belonging to type I-F CRISPR-Cas system. RGP10 is a 48.9 kb genomic element with 56 CDSs including the genes for IS3 family transposases and a possible alkane degradation cluster with a long-chain alkane hydroxylase (almA, DBX28_14785). A noteworthy plasticity region with an aromatic ring-hydroxylating dioxygenase (DBX28_20755) was found in RGP11 (Table 2), which might provide competitive advantages for these three strains living in crude-oil areas. A large number of RGPs detected in IMP66, IMP67, and IMP68 indicated a complex exchange pattern of genetic material segments that occurred during the chromosome reshaping process throughout the evolutionary history.

FIGURE 4

Analysis of Genes Involved in Alkane Degradation

Alkane hydroxylases (AHs) catalyzes the first step of alkane degradation and is involved in an AH system that includes electron transport protein rubredoxin and rubredoxin reductase. In the present study, whole-genome screening was used for identification of alkB, P450, almA, and ladA from IMP66, IMP67, and IMP68 through blasting against the reported AHs. We observed that the three strains carried up to 9 AH-encoding genes including two alkB homologous genes, three P450 homologous genes, two almA homologous genes and two ladA homologous genes (Figure 4B). As the “persistent genome” is generally considered to be more stable replicons, it is not surprising that most of the alkane hydroxylase genes were located in the “persistent genome”. Interestingly, two alkane hydroxylases were found in the “shell genome” (an almA gene in RGP10, and a cytochrome P450 gene in RGP14) (Figure 4B and Table 2). RGP10 was later identified to be a possible alkane degradation cluster that contained the genes coding for alkane hydroxylase (almA, DBX28_14785), SDR family oxidoreductase (DBX28_14685, DBX28_14840), phytanoyl-CoA dioxygenase (DBX28_14740, DBX28_14775), NAD(P)/FAD-dependent oxidoreductase (DBX28_14785, DBX28_14815), and NAD-dependent alcohol dehydrogenase (DBX28_14795). Almost all the genes in RGP10 were related to metabolism and energy production (Table 3). These results suggested that these three strains might have the ability to degrade and utilize a broad-spectrum of alkanes for growth.

Difference in RLs Production Among the Three Strains is Not at Genomic Level

Our previous study described that three strains produced different amounts of RLs. IMP67 and IMP68 produced much larger amounts of RLs than IMP66 (Das et al., 2014). The pan-genome analysis found that IMP67 and IMP68 possessed sereval unique genes compared to IMP66 (Figure 3B). These unique genes belong to type II secretion system, lipopeptide, non-ribosomal peptide synthetase, and hypothetical protein, which were not reported to be directly relevant to the RLs biosynthesis. Moreover, the average nucleotide identity (ANI) value between IMP66 and IMP68 is 99.99%. These results implied that the reason for the different yield of RLs of these strains is unlikely at genomic level.

To find the reason for the difference in RLs production among three strains, we then detected the QS signal molecules (C4-HSL and C12-HSL) by LC-MS/MS quantification, which positively regulate the expression of rhlAB, the two key RLs synthesis genes. After 24 h incubation, IMP67 and IMP68 produced significantly higher (3–10-folds) level of the two QS signal molecules (C4-HSL and C12-HSL) than that of PAO1, yet IMP66 produced trace amount of C4-HSL as well as C12-HSL, which might explain the reason for the less RLs production in IMP66 (Figure 5A). In addition, we used the rhlAB:gfp fluorescence-based reporter plasmid pTH22 to track the intracellular rhlAB transcription level. GFP fluorescence intensity was normalized by the whole cell protein concentration. IMP67 and IMP68 presented higher rhlAB expression than that of IMP66 and PAO1 (Figure 5B), which is correlated with the RLs and QS signal molecule production. Moreover, rhlAB-gfp intensity was significantly improved when IMP67 and IMP68 were grown in the nutrient broth (NB) medium (Figure 5B), which is a known RLs-promoting medium, suggesting that rhlAB transcription is related to the nutrient in the growth medium.

FIGURE 5

Rhamnolipids can enhance the bioavailability of crude oil, and therefore are crucial for microbial consortium living in the crude oil environment. To investigate whether these three strains possess the ability to produce RLs when grown with crude oil, especially n-alkanes, as the sole carbon source. We used again the rhlAB:gfp to determine the transcription of rhlAB when they cultured with alkane (tetracosane, C24; eicosane, C20) as the sole carbon source. The three strains displayed noticeable growth indicating by total protein concetration, and rhlAB transcription after 72 h incubation in the two types of n-alkanes, while they exhibited similar growth rate and growed better in C20 than C24 (Figures 6A,B). Altogether, our data indicated that high yield of RLs in IMP67 and IMP68 was due to the transcriptional regulation, not at genomic level.

FIGURE 6

Discussion

The environmental microorganism P. aeruginosa possesses great ecological flexibility, which enables it to thrive in diverse ecological niches. Under different environmental settings, P. aeruginosa may use different strategies to survive, such as forming biofilms in high-temperature oilfields (Orphan et al., 2000). It was widely believed that crude oil is a harsh habitat for microbes because of its high toxicity and hydrophobicity. However, growing evidence has revealed the presence of living microbes in crude oil (Yoshida et al., 2005). It has been reported that microbial communities in the crude oil phase were dominated by Pseudomonas, accounting for 96.84–98.87% of the total OTUs (Yamane et al., 2008; Cai et al., 2015), which exhibited significant survival advantages over other genera. In this study, we reported that three P. aeruginosa strains isolated from crude oil, IMP66, IMP67, and IMP68, exhibited excellent capabilities in n-alkane utilization and RLs production, which are two important factors for P. aeruginosa strains to maintain sustainable growth in the crude oil environment. Our work provides a better understanding of the adaptability of P. aeruginosa in crude oil.

Alkanes are the main components of petroleum hydrocarbons, and therefore, alkane degradation ability is critical for microorganisms to survive in petroleum-contaminated environments. Alkane hydroxylases are the key enzymes for aerobic degradation bacteria. Previous reports have identified AHs in several P. aeruginosa strains cultured from petroleum-contaminated environments (Zhang et al., 2011; Liu et al., 2012; Das et al., 2015). Compared to these P. aeruginosa strains, IMP66, IMP67, and IMP68 possess more AHs related genes, especially the coding genes for long-chain alkane monooxygenase, almA, and ladA. The coexistence of multiple alkane hydroxylases including alkB, P450, almA, and ladA in one bacterium is quite common in Marinobacter strains (Sun et al., 2018), Dietziasp. DQ12-45-1b (Nie et al., 2011), Amycolicicoccus subflavus DQS3-9A1T(AEF42720) (Nie et al., 2013), Acinetobacter spp. ADP1 (Barbe et al., 2004), and was also observed in many Alcanivorax isolates (Wang et al., 2010; Wang and Shao, 2012). However, nine AH genes identified in one single chromosome is rare in P. aeruginosa species. These alkane hydroxylases were demonstrated to be complementary and cover expanded substrate ranges, thereby enhancing the adaptive ability of P. aeruginosa in crude oil.

Gene duplication and horizontal gene transfer are common evolutionary processes that generate novel genes or functions for rapid adaptation (Kondrashov et al., 2002; Gogarten and Townsend, 2005). In this study, alkB, P450, almA and ladA paralogs with multiple copies were found in IMP66, IMP67, and IMP68. Moreover, a P450 and an almA gene were found integrated into the chromosome presumably through HGT (Figure 3B). These paralogs may exhibit different functions. For example, alkW1 and alkW2 are two paralogous genes encoding AlkB rubredoxin fusion proteins, which has been reported originally in Dietzia sp. DQ12-45-1b. It was shown that AlkW1 could hydroxylate n-alkanes ranging from C14 to C32, whereas AlkW2 was not expressed in this condition (Nie et al., 2011). Results from the whole genome comparison and pan-genome analysis have indicated that many flanking mobile genetic elements exist in the genomes of IMP66, IMP67, and IMP68. For instance, a 50 kb-long alkane metabolism-related genomic island (RGP10) containing an almA-like (50% identity) alkane monooxygenase along with several IS3 transposases is found in these three strains (Tables 2, 3), suggesting that horizontal gene transfer may occur during their life cycles.

Pseudomonas aeruginosa is previously demonstrated to use QS to regulate the synthesis of RLs, which are critical for P. aeruginosa to emulsify hydrocarbons and make them easier for uptake and assimilation. However, RLs also act as costly extracellular public goods. When the population growth was limited by restricted nutritional conditions, the regulation of QS-controlled public-good related genes may exhibit a metabolic prudent manner (Xavier et al., 2011). This could be a reasonable explanation for the fact that IMP67 and IMP68 expressed huge amount of QS signal molecules (Figure 5A), yet produced just a little higher amount of RLs than PAO1 (Das et al., 2014). Another plausible explanation could be that both RLs and exopolysaccharides share the common sugar precursors catalyzed by AlgC (Olvera et al., 1999; Wang et al., 2014). It could be a tactic for P. aeruginosa to synthese proper amount of RLs even with excess AHLs, and balance the synthesis of RLs and exopolysaccharides.

In conclusion, our results have showed that the alkane degradation ability and the superior AHL synthesis ability are two promoting factors for P. aeruginosa adaption in crude oil environment, which may also confer these three new isolates potential engineering performance in bioremediation of crude-oil pollution as well as microbial-enhanced oil recovery. Moreover, the native type I-F CRISPR-Cas system possessed by IMP66, IMP67, and IMP68 could be further established as a highly efficient, in situ genome-editing technique to modify the strains for future applications in such as industrial RLs production.

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