微生物学通报  2023, Vol. 50 Issue (4): 1559−1575

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文章信息

刘晓林, 崔庆锋, 杨正明, 魏士平, 张群
LIU Xiaolin, CUI Qingfeng, YANG Zhengming, WEI Shiping, ZHANG Qun
石油中长链烷烃微生物降解及分子机制研究进展
Microbial degradation and molecular mechanism of medium and long-chain alkanes in petroleum: a review
微生物学通报, 2023, 50(4): 1559-1575
Microbiology China, 2023, 50(4): 1559-1575
DOI: 10.13344/j.microbiol.china.221229

文章历史

收稿日期: 2022-12-14
接受日期: 2023-02-15
网络首发日期: 2023-03-07
石油中长链烷烃微生物降解及分子机制研究进展
刘晓林1,2 , 崔庆锋1,3 , 杨正明1,3 , 魏士平2 , 张群1,3     
1. 中国石油勘探开发研究院提高采收率研究中心, 北京    100083;
2. 中国地质大学(北京)海洋学院, 北京    100083;
3. 提高石油采收率国家重点实验室, 北京    100083
摘要: 中长链烷烃是石油烃中的重要组成部分,由于其疏水性强、黏度大、化学活性低、难降解,是地下原油黏度大、石油采收率低、泄漏后长期污染生态环境的重要原因,因此成为提高石油采收率和石油污染环境治理中的重要降解目标。微生物降解中长链烷烃作为一种新型高效的绿色技术日益受到重视。本文总结了微生物降解中长链烷烃的间期适应与转运过程,与转运过程相关的膜蛋白,微生物好氧与厌氧降解的代谢途径,以及好氧降解过程中的基因调控机制,并对微生物降解中长链烷烃的研究方向提出了展望,以期为后续的相关研究工作提供参考。
关键词: 中长链烷烃    微生物    好氧降解    厌氧降解    基因调控    
Microbial degradation and molecular mechanism of medium and long-chain alkanes in petroleum: a review
LIU Xiaolin1,2 , CUI Qingfeng1,3 , YANG Zhengming1,3 , WEI Shiping2 , ZHANG Qun1,3     
1. Research Center of Enhanced Oil Recovery, PetroChina Research Institute of Petroleum Exploration & Development, Beijing 100083, China;
2. School of Ocean Sciences, China University of Geosciences (Beijing), Beijing 100083, China;
3. State Key Laboratory of Enhanced Oil Recovery, Beijing 100083, China
Abstract: Medium and long-chain alkanes are key parts of petroleum hydrocarbons. They are the main cause of the high viscosity of underground crude oil, low oil recovery, and long-term pollution of the ecological environment after leakage due to their strong hydrophobicity, high viscosity, low chemical activity, and difficulty in degradation. As a result, they are important degradation targets in terms of improving oil recovery and treating petroleum pollution in the environment. As a new and efficient green technology, microbial degradation of medium and long-chain alkanes has gained popularity. This paper summarized the interphase adaptation and transport process of medium and long-chain alkanes in microbial degradation, membrane proteins related to the transport process, metabolic pathways of aerobic and anaerobic microorganism degradation, gene regulation mechanism in aerobic degradation, and putative gene regulation mechanism in anaerobic degradation. Furthermore, prospects for the research direction of microbial degradation of medium and long-chain alkanes were presented, serving as references for subsequent related research.
Keywords: medium and long-chain alkanes    microbe    aerobic degradation    anaerobic degradation    gene regulation    

石油被誉为“工业的血液”,是人类现代社会的动力燃料、化工原料和重要战略物资。中长链烷烃是指碳链长度大于C12的直链烷烃,是石油中最丰富的碳氢化合物。由于其疏水性强、黏度大、化学活性低、难降解,是石油开采时采收率低、泄漏后长期污染生态环境的重要原因。在石油常规开采中,中长链烷烃是石蜡的主要组分(约为80%−95%),当其运移到生产井附近,在较低的温度和压力下会使原油黏度变大甚至凝固,导致石油采收率降低;在气驱、化学驱等3次采油的过程中,原油中的轻质组分容易被气体或化学驱介质抽提,中长链烷烃、沥青质等重质组分保留下来,中长链烷烃围绕沥青质穿插缠绕,使其分子量变大,进一步引起原油黏度增加,降低了石油的采收率[1]。石油在开采、炼制、运输、存储和利用中时有泄漏,中长链烷烃由于常温常压条件下多为固体、难自然降解,容易对生态环境造成长期污染。因此,在提高石油采收率和石油污染环境治理中,中长链烷烃都是重要的降解目标和降解难点。

近年来,中长链烷烃的微生物降解引起了广泛关注[2-3]。烃降解菌中具备与烷烃识别、转运以及降解相关的代谢系统[4-6]。其中,微生物对中长链烷烃的特异性识别与转运是实现有效降解的前提,调控因子调控功能基因的高效表达是实现有效降解的关键。因此,为了更好地认识微生物降解中长链烷烃过程中的限速步骤,提高微生物降解中长链烷烃的降解效率,本文总结了近年微生物转运降解石油烃的代谢途径、该过程中涉及的识别、转运蛋白以及相关的功能基因与酶等领域最新的研究进展,梳理了底物与基因之间的诱导关系和基因与基因之间的调控机制。

1 降解中长链烷烃的微生物

在自然选择下,油藏和石油污染环境广泛存在着能够以石油烃为唯一碳源进行生命活动的烃降解微生物。它们分布范围极其广泛,在土壤、海洋、油藏以及其他极端环境均有存在。据报道,现已发现细菌、真菌、藻类等200多种微生物具备石油烃降解能力,其中,细菌是石油烃降解的主力军[7]。Abena等[8]分析了石油污染土壤中微生物的多样性,细菌在烃降解微生物群落占主导地位,而细菌中又以变形菌门数量最多,其次为放线菌门和酸杆菌门;烃降解真菌则以子囊菌门和接合菌门为主。

国内外的研究人员筛选到许多能够降解中长链烷烃的细菌。例如,Adlan等[9]将筛选得到的Geobacillus kaustophilus N3A7、NFA23和G. jurassicus MK7及G. stearothermophilus AD11、AD24培养3 d后,能够几乎完全降解C38–C40长链烷烃;Kong等[10]筛选到的烃降解菌Acinetobacter pittii SW-1能够耐受低温、重金属、高盐度等极端环境,在实际应用中具有更大潜力;嗜冷菌Oleispira antarctica RB-8可用于解决南极洲的原油泄漏问题,但其无法利用大于C24的长链烷烃[11];其他具有代表性的烃降解菌及降解效果如表 1所示。作者[30-31]曾研究了新疆中低温油藏内部的烃降解菌,发现Pseudomonas aeruginosa L6-1[30]能以中长链烷烃为主的稠油为唯一碳源生长,培养3 d后将原油降解为平均粒径1−8 μm的水包油乳状液;P. aeruginosa 709[31]则能够在厌氧或高压等极端条件下生长并产生生物表面活性剂。同时,室内岩心模拟实验结果表明,P. aeruginosa WJ-1和P. aeruginosa 709通过原位生产生物表面活性剂使原油采收率分别提高9.23%和7.04%,证实了利用降解中长链烷烃的微生物提高原油采收率的可行性和应用潜力[31-32]

表 1 可降解中长链烷烃的微生物及降解效果 Table 1 Microbial strains and degradation effects of medium and long alkanes degraded
Microbial category Strains Gene Substrate Condition Degradation rate References
Bacteria Rhodococcus sp. CH91 alkB C16−C36
alkanes
25−50 ℃, 0.4% (W/V) substrate, 140 r/min, 15 d 27.40%−98.90% [12]
A. pittii SW-1 alkB
almA
ladA1
ladA2
C18−C36
alkanes
30 ℃, pH 7.0−8.0, 500 mg/L substrate, NaCl 0−3% 13.37%−91.25% [10]
Alcanivorax sp. Est-02 alkB C14−C28
alkanes
25−35 ℃, pH 7.0, 0.5% (W/V) substrate, NaCl 10%, 2 weeks 40%−65%
C24 60%
C28 65%
[13]
Gordonia sp. SoCg alkB C12−C36
alkanes
30 ℃, 200 r/min, 10 mmol/L substrate, 62 h C16 88.90%
C30 98.10%
[14]
Mycobacterium vanbaalenii PYR-1 alkB C12−C28
alkanes
30 ℃, 200 r/min,
1% substrate, 60 d
18%−68% [15]
P. aeruginosa GOM1 alkB1
alkB2
alma
C12−C38
alkanes
30 ℃, 0.5% substrate, 30 d 96% [16]
Achromobacter xylosoxidans ZKNU01 C15−C32
alkanes
37 ℃, 106 CFU/mL cell, 0.5 kg/L substrate, 150 r/min, 7 d 82.06% [17]
Shewanella sp. LZ02 C15−C32
alkanes
35 ℃, 106 CFU/mL cell, 0.5% (W/V) substrate, 7 d 62.90% [18]
Bacillus sp. NG80-2 C15−C36
alkanes
65 ℃, pH 7.2, 1% (W/V) substrate, 180 r/min C28 22.80%
C16 55.60%
[19]
Enterobacter cloacae C15−C40
alkanes
35 ℃, pH 7.0, 170 r/min,
1% substrate, 30 d
98%±1% [20]
Rhizobium sp. C19−C30
alkanes
25 ℃, 150 r/min, 2 mL cell, 200 µL substrate, 7 d C25−C30 50%
C20−C21 75%
C22−C24 60%–70%
[21]
Sphingomonas sp. C19−C30
alkanes
25 ℃, 150 r/min, 2 mL cell, 200 µL substrate, 7 d C19 40%
C25−C30 35%
C20−C24 75%–55%
[21]
Variovorax sp. C19−C30
alkanes
25 ℃, 150 r/min, 2 mL cell, 200 µL substrate, 7 d 37%−70% [21]
Smithella sp. assA C16−C20
alkanes
21 ℃, 0.90 mmol substrate, darkness, stationary [22]
Desulfatibacillum sp. assA C16−C20
alkanes
21 ℃, 0.90 mmol substrate, darkness, stationary [22]
Aromatoleum sp. HxN1 masD C16 28 ℃, 50–70 r/min, 13−58 µL substrate, 6–8 weeks [23-24]
Fungi Cladosporium sphaerospermum C14−C38
alkanes
28 ℃, 30 d, 1 g substrate 97.19% [25]
Aspergillus parasiticus NRRL: 3386 C10−C33
alkanes
30 ℃, 150 r/min, 1% substrate, darkness C10−C18 47%−77%
C11, C13, C19−C26 80%
C27−C33 90%
[26]
Talaromyces sp. Oil 0.6 mg/g cell, 14 d, 300 g soil, 1 220 mg/kg oil, 10% moisture content C5−C20 50% [27]
Penicillium citrinum NIOSN-M126 Oil 28 ℃, 80 r/min, 13.35% (W/V) oil, 23 d C20−C29 Average 95.37% [28]
Aspergillus flavus NIOSN-SK56S22 Oil 28 ℃, 80 r/min, 13.35% (W/V) oil, 23 d C30 100% [28]
Candida tropicalis MW488263 C14, C15, C17, C22
alkanes
30 ℃, 1% (V/V) substrate, > 15 d C14 59.50%, C15 59.10%, C17 45.60%, C22 51.40% [29]
Rhodotorula mucilaginosa MW488265 C14, C15, C17, C22
alkanes
30 ℃, 1% (V/V) substrate, > 15 d C14 45.50%, C15 39.30%, C17 33.30%, C22 40.20% [29]

尽管某些单一菌株对中长链烷烃有较好的降解能力,但是也存在活性不稳定等诸多应用限制[33]。将2种或多种微生物以一定比例复合构成混菌体系,利用微生物之间协同作用可提高对中长链烷烃的降解率[34]。Ghorbannezhad等[35]在评价真菌-细菌混合体系降解石油烃的效果时发现,通过顺序混合培养方式制备的真菌-细菌混菌体系要比传统混合培养效果更好,即先将真菌与底物培养一段时间后再加入细菌会取得更高的降解率。需要注意的是,混菌体系在构建时一定要根据菌株的自身生理特性进行合理复配。例如,菌株Kocuria rosea k-24在和Microbacterium sp. M-08复配时,由于两株菌对底物的利用是竞争关系,从而使得复合菌群对石油烃的降解效果反而弱于单株菌[36]

2 微生物对中长链烷烃的吸收与转运过程 2.1 微生物对中长链烷烃的吸收过程

微生物可直接吸收的直链烷烃只能是分子量低且有一定水溶性的短链烷烃,对于疏水的中长链烷烃,其分子量越大,在水中的溶解度越小、越难吸收[37]。因此,微生物想要利用中长链烃代谢产生能量,需要间期适应过程扩大与烷烃的接触,并将其转运到细胞内[38]

在吸收过程中,微生物主要通过分泌生物表面活性剂来增加与烷烃的接触。G. sihwaniensis菌株lys1-3在降解石油烃时可分泌脂肽类生物表面活性剂,改善细胞表面的疏水性,使细胞更容易与碳氢化合物进行接触[39]。王卫强等[40]筛选到的Pseudomonas sp. W12#可产生糖脂类生物表面活性剂,能够降低油水之间的界面张力,使油以胶束形式形成小油滴稳定地分散在水相中,促进微生物的利用。此外,生物表面活性剂还可洗脱吸附在固体颗粒上的石油烃,使这一部分中长链烷烃的生物可利用性增加[41]。当然,也存在某些微生物不需要表面活性剂的帮助即可直接接触中长链烷烃并将其转运至细胞内。例如,Goswami等[42]筛选到的Pseudomonas M1可直接与碳氢化合物接触。关于生物表面活性剂的种类、生产以及在提高采收率和治理石油烃污染上的应用在其他最新的综述中已有体现[43-45],这里不再赘述。

以上研究表明,表面活性剂的产生对微生物降解中长链烷烃具有重要作用。然而,目前对于产表面活性剂相关功能基因的研究还不充分,应进一步加深对相关基因调控关系的认识,并利用基因敲除等技术手段不断发掘新的功能基因。同时,还需进一步深入研究微生物细胞表面结构对微生物直接吸收中长链烷烃的影响,采用数学建模等技术定量研究微生物对中长链烷烃的感应趋化运动机制等问题。

2.2 微生物对中长链烷烃的转运过程

微生物与中长链烷烃接触后可通过被动运输、主动转运或胞吞等方式实现外膜转运,将石油烃转运到细胞内,然后再通过内膜转运到达降解位点进行降解[41]。例如,Thalassolituus oleivorans MIL-1利用被动运输和主动转运组合的方式吸收烷烃[46];Liu等[39]研究发现戈登氏菌属能够以胞吞的形式将石油烃转移到细胞内,运输到胞内后以包涵体为单位进行降解。微生物对中长链烷烃的转运过程往往需要膜蛋白的参与,革兰氏阴性菌中常见的外膜转运蛋白主要有FadL蛋白家族和TonB复合物,内膜转运主要有TRAP转运系统[47-50]

研究表明,FadL蛋白家族在中长链烷烃的选择性吸收中发挥重要作用。例如,在P. aeruginosa SJTD-1中FadL蛋白参与了正十八烷的跨膜转运[51]T. oleivorans MIL-1在底物为C28的长链烷烃生长时,FadL的同源蛋白表达量增加了9倍[46]。Liu等[48]最近在A. venetianus RAG-1中发现同属FadL蛋白家族的新型跨膜转运蛋白AltL具备典型β筒状结构,但是有更长的细胞外环,使RAG-1能够运输疏水性更强、体积更大的底物;AltL主要负责C20−C38长链烷烃和C18−C28脂肪酸的转运,同时还参与C16−C18中链烷烃的转运过程。Wang等[52]A. dieselolei B-5发现3个属于FadL蛋白家族的外膜蛋白OmpTs,不同的OmpTs可以选择性地运输不同链长的直链烷烃:OmpT-1转运C28−C36长链烷烃,OmpT-2转运C16−C24中链烷烃,OmpT-3转运C8−C12的短链烷烃。

其他研究表明,外膜蛋白TonB系统对革兰氏阴性菌的烷烃转运具有重要作用。TonB系统由TonB、ExbB和ExbD这3个质膜蛋白构成,可以帮助革兰氏阴性菌从外界摄取营养,同时具备烷烃外膜转运的能力;同时,TonB依赖性受体可特异性识别环境中存在的烷烃,如T. oleivorans MIL-1在以C28烷为底物时TonB依赖性受体的合成量增加了7倍,并利用质子动力将底物从外界环境转运至细胞周质[46, 49]

李梅青[50]首次验证了烃降解菌中具有烷烃转运能力的内膜转运系统,在菌株A. dieselolei B5中存在TRAP内膜转运系统中的3个蛋白,其中底物连接蛋白DctP对C12与C16具有良好的亲和性,可与中长链烷烃特异性结合,使得菌体具备中长链烷烃内膜转运的能力。这也是首次验证了TRAP转运系统可转运烷烃,对研究烷烃的内膜转运机制具有重要意义。TRAP转运系统是一种不依赖ATP的二级转运系统,该系统在进行底物转运时借助某些离子(如H+、Na+)的电化学梯度,将底物和离子一起跨膜双向转运[53]

对于中长链烷烃跨膜转运的研究主要集中于细菌,对真菌的研究相对较少。对跨膜蛋白结构、功能和机制方面的认识仍然不甚清楚,特别是底物通过跨膜蛋白的扩散机制以及跨膜蛋白与底物的结合位点等问题。作为连接吸附和降解的必要中间过程,跨膜转运相关的研究进展相对缓慢。然而,随着技术的进步,可利用单分子荧光成像技术与分子动力学模拟等技术对中长链烷烃转运过程进行动态表征,观察跨膜蛋白在不同时间的结构与行为,更加全面地认识跨膜转运过程。

3 微生物对中长链烷烃的代谢机制

中长链烷烃转运至胞内后开始诱导相关功能基因的表达,开启微生物对中长链烷烃代谢降解过程。其代谢机制主要利用功能基因表达产生的酶催化中长链烷烃发生羟基化、脱氢、脱羧等反应,将中长链烷烃逐级降解为低分子量物质,并产生微生物自身代谢所需的能量[54]。其中,在污染的土壤、水体等表层环境以好氧代谢为主,在油藏、深海沉积物和深层污染土壤中主要以厌氧方式进行代谢[55-58]图 1对微生物对中长链烷烃吸收、转运、代谢过程做了简要概述。

图 1 细菌吸收、转运和代谢中长链烷烃过程示意图 Figure 1 Schematic diagram of bacterial uptake, transport, and degradation of medium and long alkanes.
3.1 好氧代谢机制

3.1.1 好氧代谢途径及关键基因与酶

在好氧代谢中,微生物主要通过末端氧化、次末端氧化、ω-氧化和β-氧化等方式降解中长链烷烃[59]。具体代谢途径可参考图 2[60]。好氧微生物通过加氧酶羟基化反应启动对烷烃的代谢,加氧酶部分还原O2产生高活性氧,进而将羟基引入烃分子,该反应是好氧降解过程中的关键步骤[61]

图 2 微生物好氧降解中长链烷烃代谢途径[60] Figure 2 Metabolic pathways of medium and long chain alkanes by microbial aerobic degradation[60].

吴慧君等[62]最近总结了与石油烃降解的关键基因和相关的酶,其中关键的基因有alkBalmAladA,与中长链烷烃降解相关的酶主要有烷烃羟化酶AlkB、AlkM、AlmA、LadA和单加氧酶细胞色素P450s (cytochrome P450s, CYPs)。其中,烷烃羟化酶AlmA不仅可以末端氧化烷烃,还可以次末端氧化开启长链烷烃的降解[63]。此外,Nie等[64]发现Dietzia sp.中普遍存在红素氧还原蛋白(rubredoxin, Rd)与烷烃羟化酶AlkB融合的独特基因编码方式,可显著扩大AlkB的烷烃底物利用范围,如果将该融合方式应用于具有alkB基因的革兰氏阴性菌中,可使革兰氏阴性菌获得先前所不具有的利用长链烷烃的能力。

在一株菌中往往存在多种烷烃氧化酶,可确保菌株利用不同碳链长度的烷烃[50]。因此,在某些菌种中往往存在末端氧化和次末端氧化两种代谢途径共存的情况,菌种根据底物的不同,酶类差异化表达,开启不同的羟基化过程。例如,Gregson等[46]观察到T. oleivorans MIL-1在底物为C14时特异性表达一种AlkB2同源的烷烃单加氧酶,与其他单加氧酶类协同开启烷烃的末端代谢;在底物为C28时则表达一种酯酶和Baeyer-Villiger单加氧酶(Baeyer-Villiger monooxygenases,BVMO,BVMO酶类可催化羰基酮转化为酯)[65],以次末端氧化的途径开启烷烃的降解。

经单加氧酶羟基化后形成的烷醇被进一步氧化成脂肪酸,脂肪酸经β氧化后形成乙酰辅酶A,最后进入三羧酸循环或其他代谢途径被降解为CO2[66]。其中,脂肪酸的代谢也是烷烃降解过程中的重要一环。像长链脂肪酸(long chain fatty acid, LCFA)如果代谢不及时,积累过多会对细胞产生一定毒性[67]

3.1.2 好氧代谢中的基因调控

研究发现,不同链长的烷烃可诱导不同的加氧酶基因表达,烷烃的氧化产物烷醇和脂肪酸对加氧酶基因的表达也可起到诱导作用[68-70];调控因子作为激活剂或阻滞剂影响加氧酶基因的转录[71]。降解中长链烷烃的基因调控机制研究主要集中于好氧降解过程,如细菌Pseudomonas sp.、Acinetobacter sp.、Dietzia sp.和真菌Yarrowia sp.,具体调控关系可参考图 3[49, 70, 72-73],有关厌氧降解过程中的基因调控机制报道较少。

图 3 Pseudomonas aeruginosa SJTD-1、Alcanivorax dieselolei B5和Dietzia sp. DQ12-45-1b菌株降解中长链烷烃的代谢调控机制[49, 70, 72-73] Figure 3 Metabolic regulation mechanism of Pseudomonas aeruginosa SJTD-1, Alcanivorax dieselolei B5 and Dietzia sp. DQ12-45-1b strains degradation of medium and long chain alkanes[49, 70, 72-73]. ㊀:抑制 ㊀: Inhibition.

Wentzel等[74]在之前的文章中综述了菌株P. putida GPo1、A. baylyi ADP1和Acinetobacter sp. M-1中的长链烷烃代谢调控机制:P. putida GPo1中烷烃羟化酶基因alkB的表达由底物正构烷烃诱导,由调控因子AlkS调控,但是同属的不同菌株P. putida AP1中却存在不同的调控方式;A. baylyi ADP1中加氧酶基因alkM的表达由C7−C18链烷烃诱导,AraC家族的调控因子AlkR调控alkM的转录,并且AlkR是否激活alkM基因取决于C6以上长链烷烃是否存在;在Acinetobacter sp. M-1中存在2种独立的烷烃羟化酶AlkMa和AlkMb,分别由不同碳链长度的正构烷烃诱导,并受独立的调控蛋白调控。

近年来,Ji等[72, 75]利用P. aeruginosa SJTD-1菌株进一步丰富了Pseudomonas sp.代谢长链烷烃的调控机制。LysR家族中的CrgA蛋白是alkB2基因的调控因子,CrgA蛋白可与alkB2基因启动子区的2个位点bs1和bs2特异性结合,抑制alkB2基因的转录,调节菌株SJTD-1对中长链正链烷烃的利用;其中,bs1和bs2属于镜像结构(IIR),其结构完整性对CrgA与alkB2基因有效结合起到至关重要的作用;此外,中长链脂酰基辅酶A (十六烷基辅酶A和十八烷基辅酶A)可通过正反馈作用影响CrgA与alkB2基因靶区的结合,解除CrgA的抑制作用[72]。同时,CrgA蛋白还参与了菌株内其他烷烃降解基因almA2ladA1ladA2和自身编码基因的转录过程[75]

邵宗泽团队[50, 52, 76]详细研究了菌株A. dieselolei B5降解中长链烷烃的代谢调控网络:当细胞外存在中长链烷烃时,外膜蛋白OmpS作为感受器感应烷烃刺激,激活编码甲基受体趋化蛋白基因mcp的表达,趋化复合物的偶联蛋白CheW通过一定机制诱导cyo基因表达,产生的Cyo末端氧化酶可解除调控因子AlmR对almA基因和ompT-1基因转录的抑制作用,使烷烃羟化酶AlmA和膜转运蛋白OmpT-1大量表达,中长链烷烃由细胞膜外转运至胞内,与烷烃趋化MCP系统结合后,细胞即开展对烷烃的趋化运动,转运至胞内的中长链烷烃通过TRAP内膜转运系统转运到特定位置后开始降解过程,这可能是目前对烃降解菌代谢调节网络最全面的研究。其中,CheW蛋白在OmpS外膜蛋白到Cyo末端氧化酶的信号传递中起关键作用,其编码基因位于mcp基因簇中,CheW1经C8−C24的短中链烷烃诱导产生,CheW2经C24−C32的长链烷烃和支链烷烃诱导产生,MCP系统对于菌种B5的烷烃趋化是不可或缺的[52];Cyo末端氧化酶抑制调控因子AlmR的同时也影响编码膜转运蛋白基因ompT-2ompT-3的表达,所以cyo基因缺失会导致B5不能利用长链烷烃(C28−C36)及姥鲛烷,但不影响其对C8−C24烷烃的利用[76]。此外,王万鹏等[77]还探究了A. hongdengensis A-11-3中AlmA的诱导表达机制:almA1基因在C28−C32的长链烷烃诱导下表达,almA2基因在C24-C34长链烷烃以及支链烷烃的诱导下表达。

Liang等[70-73]研究了菌株Dietzia sp. DQ12-45-1b内两种烷烃降解基因alkW1cyp153的调控机制;烷烃羟化酶alkW1由C16−C40烷烃诱导产生,细胞色素P450家族烷烃羟化酶CYP153蛋白由碳原子数 < C14的烷烃诱导产生。alkW1基因的表达由TetR家族的AlkX调控因子调节,无中长链烷烃存在时,AlkX以一对二聚体的形式与烷烃羟化酶基因alkW1上游的反向重复序列结合,抑制alkW1基因的表达,但也会维持一定的AlkW1和AlkX蛋白浓度;当存在中长链烷烃时,代谢形成的LCFA以正反馈调节机制影响AlkX与alkW1的结合,LCFA可与AlkX的C端结合并改变其构象,将其从alkW1-alkX启动子中释放,这也是首次发现TetR家族转录调控因子(TetR family transcriptional regulators, TFRs)在中长链烷烃的代谢中发挥调控作用[70]。细胞色素P450家族中的烷烃羟化酶CYP153蛋白的表达由位于cyp153基因簇上游的调控因子CypR调节,该调控因子属于AraC蛋白家族,这种调控是许多放线菌中所共有的,CYP153蛋白可与烷烃羟化酶AlkW1协同降解中长链烷烃[73]

在酵母菌Yarrowia lipolytica中,与中长链烷烃降解相关的P450ALK基因转录由底物诱导,受Yas1p-Yas2p-Yas3p系统调控,当中长链烷烃存在时,Yas3p从细胞核中转出,与内质网上的受体结合后固定在内质网上,Yas1p和Yas2p在细胞核中形成络合物,与P450ALK基因启动子中的ARE1结合并启动转录;当不存在中长链烷烃时,Yas3p在细胞核中与Yas2p结合,抑制转录[78]

正如前文所说,菌体内往往存在多种中长链烷烃分解代谢酶和复杂的代谢网络,以适应中长链烷烃的胁迫环境并保持高效利用。在烷烃降解过程中,存在多个单加氧酶发挥作用和多种调控模式,一种酶可能受几种不同的调控因子调控,一个调控因子也可能有多个靶标[71]。因此,以菌属为划分,梳理构建具有高效降解能力菌种的趋化、吸收、转运、调控和降解的全局信号传递网络是有意义的。除调控关系外,也应深入研究蛋白分子的结构、调节位点和分子间具体的诱导调节机制等信息,为功能基因的外源表达、降低下游工业生产成本及高效工程菌株的构建奠定基础。

3.2 厌氧代谢机制

油藏、深层污染土壤和深海沉积物等环境都是厌氧环境,厌氧代谢也是中长链烷烃降解中的重要部分。相较于好氧代谢,只有少数的研究报道了厌氧条件下微生物降解中长链烷烃的机理。一些兼性或专性厌氧细菌能够与除氧气之外的电子受体耦合厌氧降解石油烃,例如硝酸盐、硫酸盐、Fe3+、Mn4+、CO2[79]。其中,以硝酸盐-氮作为电子受体时,原油的降解速度仅次于有氧降解过程[80]。周蕾等[81]根据离解C‒H键所需能量的不同,将中长链烷烃的厌氧初始活化方式划分成3类,每一类按照不同的途径进行活化:当C‒H键的离解能 < 355 kJ/mol时,以脱氢羟基化方式活化;当C‒H键的离解能在355–430 kJ/mol时,以延胡索酸加成的方式活化;当C‒H键的离解能 > 430 kJ/mol时,活化方式存有一定争议。

目前,已知能够厌氧降解中长链烷烃的细菌主要集中在变形菌门和厚壁菌门,它们主要通过延胡索酸加成的方式激活C3−C50中长链烷烃的降解,如图 4所示[82]。同时,最近的研究发现放线菌门和暗黑菌门(Atribacteria)的细菌也可厌氧降解中长链烷烃。Liu等[79]从胜利油田获取样品,在55 ℃条件下构建了降解C21−C30长链烷烃的嗜热菌培养体系,经900多天培养后,放线菌Candidatus Syntraliphaticia以及产甲烷菌成为优势菌群,Candidatus Syntraliphaticia中存在完整的烷基琥珀酸合成酶ASS基因操纵子以及其他与降解相关的基因,碳原子数为偶数的长链烷烃经延胡索酸激活后,被进一步氧化成二氧化碳、氢气、甲酸盐和醋酸盐,最后被产甲烷菌转化为甲烷,而碳原子数为奇数的长链烷烃降解较少。Liu等[83-84]还总结了暗黑菌门的环境分布和功能特征,基因组学分析表明菌株Atribacteria JS1内有编码延胡索酸加成酶的操纵子,但是不存在脂肪酸代谢相关的基因,其产生的脂肪酸需排出胞外被其他微生物降解,其厌氧代谢机制还需进一步研究。

图 4 延胡索酸加成反应激活微生物厌氧代谢中长链烷烃[82] Figure 4 Fumarate addition activates microbial anaerobic metabolism of long-chain alkanes[82].

微生物还可通过脱氢羟基化和羧化反应开启中长链烷烃的厌氧降解。So等[85]利用同位素标记法探究了Desulfobacteriaceae Hxd3降解烷烃时的初始活化方式,即在饱和链烷烃的C3位置利用外源无机碳源通过羧基化反应引入羧基,随后脱下与羧基相邻的2个末端碳原子形成脂肪酸,脂肪酸通过β氧化等其他代谢途径最终被矿化为CO2

研究人员对中长链烷烃的厌氧降解过程关注度越来越高,但是由于菌种需要严格的厌氧环境,以及中间代谢产物存在浓度低、时间短等问题,对微生物厌氧降解的具体调控机制仍不明晰。随着技术的进步,可通过打造分析设备齐全的厌氧手套操作系统,保证分析流程的全过程厌氧,并对样品采取预处理(如衍生化法预处理样品,可提高挥发性脂肪酸检测的灵敏度)和在线分析方法等来解决中间代谢产物的瞬时性等问题。

4 展望

微生物对中长链烷烃的降解已经成为目前的研究热点。相关研究围绕微生物降解中长链烷烃的吸收转运、功能基因和代谢机制等方面广泛开展,其中细菌的研究最为深入,真菌次之,对其他微生物的研究相对较少。中长链烷烃作为一种难降解的复杂有机物,现有的微生物降解技术在实际应用中效果还非常有限,主要存在以下问题:(1) 单一菌种降解作用较弱,复合菌群菌种之间的协同效应亟须研究;(2) 微生物降解中长链烷烃的代谢机制需要进一步研究;(3) 微生物的降解效果受环境因素影响大。

基于以上问题,实现微生物高效降解中长链烷烃可从3个方面开展:(1) 加强筛选中长链烷烃降解菌株/菌群,构建稳定高效降解中长链烷烃的复合菌剂。高效稳定降解中长链烷烃的菌株/菌群是微生物降解中长链烷烃的核心和根本。通过筛选高效稳定降解中长链烷烃的微生物,探索不同微生物之间和微生物与环境之间的协同互作效应;依据实际应用环境,有针对性地设计环境适应性高且高效稳定降解中长链烷烃的微生物菌群/菌剂。(2) 深入微生物降解中长链烷烃的机理机制研究,开发更加高效的基因工程菌株/菌剂。应用宏基因组、宏转录组、蛋白组、代谢组等组学技术与分子生物学技术,明确好氧与厌氧条件下微生物降解中长链烷烃的关键基因和关键酶,解析微生物降解中长链烷烃的代谢途径和调控机制,利用基因工程技术构建高效降解中长链烷烃的基因工程菌株/菌剂。(3) 联合应用物理学、化学、生物修复多个学科技术,研发高效降解中长链烷烃技术。降解不同环境的中长链烷烃时,可结合具体问题和难题,交叉应用物理学、化学、生物修复等不同学科的最新技术,互为补充,形成高效降解中长链烷烃技术,如利用固定化技术可显著提高细菌对原油的降解率[86],利用低电压电解水产氧气可加快烃类的生物降解速率[87]

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