生物工程学报  2023, Vol. 39 Issue (5): 1930-1948
http://dx.doi.org/10.13345/j.cjb.221024
中国科学院微生物研究所、中国微生物学会主办
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文章信息

袁英博, 周汶楷, 梁泉峰, 典龙阳, 苏田源, 祁庆生
YUAN Yingbo, ZHOU Wenkai, LIANG Quanfeng, DIAN Longyang, SU Tianyuan, QI Qingsheng
生物降解聚烯烃类塑料研究进展
Advances in biodegradation of polyolefin plastics
生物工程学报, 2023, 39(5): 1930-1948
Chinese Journal of Biotechnology, 2023, 39(5): 1930-1948
10.13345/j.cjb.221024

文章历史

Received: December 23, 2022
Accepted: March 9, 2023
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引用本文
袁英博, 周汶楷, 梁泉峰, 典龙阳, 苏田源, 祁庆生. 生物降解聚烯烃类塑料研究进展[J]. 生物工程学报, 2023, 39(5): 1930-1948.
YUAN Yingbo, ZHOU Wenkai, LIANG Quanfeng, DIAN Longyang, SU Tianyuan, QI Qingsheng. Advances in biodegradation of polyolefin plastics[J]. Chinese Journal of Biotechnology, 2023, 39(5): 1930-1948.
生物降解聚烯烃类塑料研究进展
袁英博 , 周汶楷 , 梁泉峰 , 典龙阳 , 苏田源 , 祁庆生     
山东大学 微生物技术国家重点实验室, 山东 青岛 266237
收稿日期：2022-12-23；接收日期：2023-03-09
基金项目：国家重点研发计划(2021YFC2103604)；国家自然科学基金(31961133014，32200081)
作者简介：典龙阳   山东大学微生物技术国家重点实验室(研究院)教授，博士生导师。山东省泰山学者青年专家，山东省海外优秀青年基金获得者，山东大学齐鲁青年学者。研究方向集中在利用合成化学和合成生物学手段，围绕塑料降解、生物质利用、天然产物和药物中间体及高附加值化学品开发等开展相关的基础与应用研究，在Chemical Reviews、Angewandte Chemie International Edition、ACS Catalysis等发表多篇研究成果，主持多项省部级研究课题，应邀担任包括科技部重点研发计划等项目评审专家;
苏田源    山东大学微生物技术国家重点实验室(研究院)长聘副研究员、硕士生导师。主要研究方向为塑料废弃物的生物降解及资源化利用，目前主持国家自然基金青年项目、山东省自然基金青年项目，并参与国家重点研发计划项目及国家自然科学基金委员会与欧盟委员会环境生物技术合作研究项目“合成塑料降解转化微生物菌群”。近年来在Frontiers in Microbiology、iScience、Chemosphere、Microbial Biotechnology等期刊发表学术论文20余篇，累计他引400余次.
摘要：聚烯烃类塑料是一类以C–C键为骨架的高分子材料，被广泛应用于日常生活的各个领域。由于具有稳定的化学性质并且难以被环境中的微生物快速降解，聚烯烃塑料废弃物在全球范围内持续积累，造成了严重的环境污染及生态危机。近年来，利用生物方法降解聚烯烃类塑料引起了研究人员的广泛关注。自然界丰富的微生物资源为生物降解聚烯烃类塑料废弃物提供了可能，已经有一些对聚烯烃塑料具有降解能力的微生物被陆续报道。本文总结了聚烯烃类塑料生物降解资源及生物降解机制的研究进展，提出了目前聚烯烃类塑料生物降解过程存在的问题，并对未来的研究方向进行了展望。
关键词：聚烯烃类塑料    生物降解    降解机制    废物处理    环境保护    
Advances in biodegradation of polyolefin plastics
YUAN Yingbo , ZHOU Wenkai , LIANG Quanfeng , DIAN Longyang , SU Tianyuan , QI Qingsheng     
State Key Laboratory of Microbial Technology, Shandong University, Qingdao 266237, Shandong, China
Received: December 23, 2022; Accepted: March 9, 2023
Supported by: This work was supported by the National Key Research and Development Program of China (2021YFC2103604) and the National Natural Science Foundation of China (31961133014, 32200081)
Corresponding author: DIAN Longyang, E-mail: longyang_dian@sdu.edu.cn;
SU Tianyuan, E-mail: sutianyuan@sdu.edu.cn.
Abstract: Polyolefin plastics are a group of polymers with C−C backbone that have been widely used in various areas of daily life. Due to their stable chemical properties and poor biodegradability, polyolefin plastic waste continues to accumulate worldwide, causing serious environmental pollution and ecological crises. In recent years, biological degradation of polyolefin plastics has attracted considerable attention. The abundant microbial resources in the nature offer the possibility of biodegradation of polyolefin plastic waste, and microorganisms capable of degrading polyolefin have been reported. This review summarizes the research progress on the biodegradation microbial resources and the biodegradation mechanisms of polyolefin plastics, presents the current challenges in the biodegradation of polyolefin plastics, and provides an outlook on future research directions.
Keywords: polyolefin plastics    biodegradation    degradation mechanisms    waste treatment    environmental protection    

塑料是一类通过加聚或缩聚反应聚合而成的高分子聚合物[1]，具有生产成本低、化学稳定性好、耐腐蚀、绝缘性强、生物安全性高等优点，被广泛应用于农业、工业、建筑业等多种行业，是人类生活和生产过程中不可或缺的重要基础材料。受2019全球新冠疫情的影响，塑料产量激增，尤其是聚烯烃类塑料。据统计，中国2020年1–2月的口罩产量高达1.1亿个，同比增加了450% [2]。除口罩外，一次性医疗用品以及一次性包装袋的使用在疫情期间大幅增加，由此产生的塑料废弃物在自然环境下难以被快速降解，造成的环境问题及生态危机日益加剧[3]。尤其是每年释放到海洋中的塑料垃圾会让数百万只海洋动物因误食而窒息死亡[4]；同时，塑料微粒还会随着食物链富集，进入人体血液，严重威胁人类健康[5]。据统计，2015年全球有79%的塑料垃圾未得到有效处理，按照目前趋势下去，到2050年，大约会有120亿t塑料垃圾被填埋或遗弃环境中[6]。

传统的塑料垃圾处理方法主要包括填埋法和焚烧法，填埋法占用土地、容易造成环境二次污染；焚烧法会排放大量CO2及二噁英等有害气体。传统处理方法耗能大、对生态环境造成严重的二次污染，因此急需开发更加安全绿色的塑料废弃物处置技术。目前从自然界中已经发现一些可以降解塑料废弃物的微生物和酶，尽管降解速率较低，但这预示着利用生物技术降解塑料废弃物是可行的[7]。利用生物技术降解塑料废弃物具有条件温和、绿色可持续等优点，近年来备受研究者的关注。本文综述了含C−C键的聚烯烃类塑料的物理化学特性、从不同环境中分离到的具有生物降解能力的微生物资源及潜在的生物降解机制，特别是针对聚烯烃类塑料难以被直接高效生物降解的难题，提出了结合物理化学处理方法的生物降解方案，以期促进生物技术在聚烯烃类塑料高效降解中的应用。

1 聚烯烃类塑料的分类及结构特点

根据化学结构的差异，塑料聚合物可以分为两大类(图 1)，一类是含有酯键的聚酯类塑料，主要包括聚对苯二甲酸乙二醇酯(polyethylene terephthalate, PET)和聚氨酯(polyurethane, PUR)，另外一类是以C−C键为骨架的聚烯烃类塑料，主要包括聚乙烯(polyethylene, PE)、聚苯乙烯(polystyrene, PS)、聚丙烯(polypropylene, PP)和聚氯乙烯(polyvinyl chloride, PVC)。聚烯烃类塑料的使用范围更为广泛，占据了全球塑料制品的77%[8]。

图 1 塑料的分类及市场占比 Fig. 1 Classification and market share of plastics.
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其中PE是全球生产量最高的塑料聚合物，由乙烯聚合而成，分子量从几万到几十万不等，分子式为(C2H4)n。PE无毒无味，手感似蜡，具有极强的化学稳定性及绝缘性，能够耐受大多数有机酸碱腐蚀。根据分子结构及密度，PE可以分为高密度聚乙烯(high-density polyethylene, HDPE)、低密度聚乙烯(low-density polyethylene, LDPE)及线性低密度聚乙烯(linear low-density polyethylene, LLDPE)。HDPE相比于LDPE和LLDPE有较高的耐温、耐油性，此外电绝缘性和抗冲击性好，主要应用于生产薄膜制品、日用品及工业用的各种中空容器、管材等；LDPE由于其密度较低，材质最软，主要用作地膜、工业包装膜、药品食品包装膜等；LLDPE是乙烯与少量高级α-烯烃在催化剂作用下聚合而成的共聚物，外观上与LDPE类似，透明性较差，具有低温下抗冲击强度高的优点，主要用于生产管材、电线电缆等。由于高密度和高结晶度，HDPE比LDPE和LLDPE更难被生物降解[9]。

产量居全球第二的是PP，其化学结构与PE类似，但不同之处在于PP侧链含有甲基(−CH3)，是一种半结晶性材料，疏水性好，结晶度高，具有优良的力学性能；熔融温度约为164–170 ℃，比PE高40%–50%；PP可以被浓硫酸和浓硝酸侵蚀，对其他化学试剂较为稳定。鉴于其良好的材料性能，PP广泛应用于各类商品包装、医疗用品、汽车配件等，被称为“包装塑料”，但PP很难被生物降解。另一方面，由于包装产品普遍使用周期较短，每年会产生大量的PP废弃物，例如疫情期间，全球每月使用和丢弃的口罩数量高达1 290亿个，其中核心材料熔喷布是以PP为原材料生产的，居民日常使用的口罩很难按照废弃医疗器械流程处理，遗弃到环境中将造成严重的环境问题。

PVC是由氯乙烯单体在过氧化物、偶氮化合物等引发剂或在光、热作用下按自由基聚合反应机理聚合而成的聚合物，具有稳定的物理化学性质，广泛应用于雨衣、建材和塑料盒等。由于PVC中含有氯和多种添加剂[10] (双酚A、邻苯二甲酸盐等)，当PVC填埋或焚烧时会产生氯化氢或氯化二噁英等环境污染物[11]。

PS是由苯乙烯单体经自由基加聚反应合成的聚合物，其线性主链的骨架原子连接有苯环。PS具有透明度高、刚度大、玻璃化温度高等特点，常被用于一次性餐具、儿童玩具等，但PS在高温下会产生有毒单体苯乙烯，世界卫生组织已经将PS列入3类致癌物。发泡PS比重低，易于漂浮于水面，是主要的海洋漂流物，海洋生物误食后会对消化系统造成伤害甚至死亡。

聚烯烃类塑料都含有高键能C−C键构成的稳定碳链骨架，不同材料的结构差异在于侧链基团，赋予了其不同的材料性能及应用范围。同时，聚烯烃类塑料的结构特点使其普遍难以被生物降解，主要原因是：(1) 聚烯烃类塑料化学结构稳定，主要由高键能的−C−C−和−C−H−共价键构成，缺乏易被氧化和水解的基团[12]；(2) 高分子量长链结构使其无法进入微生物细胞内被胞内酶降解；(3) 高度疏水性及高结晶度使其难以与细胞或酶接触发生生物降解反应[13]。

2 生物降解聚烯烃类塑料的微生物

塑料的生物降解是指在一定的条件下，通过细菌、真菌、甚至是某些昆虫的生理活动使塑料中的化学键断裂，分子量逐步变小，最终被同化吸收并支持细胞生长的过程。虽然聚烯烃类塑料化学结构稳定，难以被一般的微生物降解，但目前已经陆续发现一些微生物具有降解PE、PP、PS、PVC等聚烯烃类塑料的能力。

2.1 真菌对聚烯烃类塑料的降解

真菌可以在各种土壤条件下生长，并通过孢子扩散繁殖。一般来说，真菌对塑料等污染物具有较强的降解能力，这主要是由于真菌具有强大的酶系统、吸附能力以及产生天然生物表面活性剂的能力，这些特性使真菌能够利用塑料底物作为碳和电子的来源，为细胞提供物质和能量。近年来，已经发现很多对聚烯烃塑料具有降解作用的真菌(表 1)。

表 1 降解聚烯烃类塑料的真菌 Table 1 Fungal degradation of polyolefin plastics
Plastic type Fungi Source Pretreatment method Reaction condition Degradation result References
PE Aspergillus clavatus JASK1 Landfill soil UnpretreatedLDPE films (bags) Shaken flasksincubated for90 days Weight loss: 35.0% [14]
Collectotrichum fructicola Culture collection of the Institute of Excellence in Fungal Research (CEFR) LDPE microplastic granules made of LDPE film 90 days at 25 ℃ Weight loss: 48.8% [15]
Penicillium citrinum Plastic waste dump yard UnpretreatedLDPE films 90 days at 28 ℃ Weight loss: 38.8% [16]
Cephalosporium National Collection of Industrial Microorganism (NCIM) Nitric acid treated HDPE film 20 days at 28 ℃ Weight loss: 7.2% [17]
Aspergillus terreus BAYF5 Rhizosphere soil PE films 60 days at room temperature Weight loss: 58.5% [18]
Trichoderma viride RH03 Landfill soil LDPE films 45 days at 28 ℃ Weight loss: 5.1% [19]
Alternaria alternata FB1 Sea UnpretreatedLDPE film 28 days at 28 ℃ Molecular weight: decreased 95.0% [20]
Aspergillus flavus PEDX3 Galleria mellonella gut Unpretreated LDPE film 28 days at 25 ℃ Reduction in the molecular weight [21]
Meyerozyma guilliermondii ZJC1 Larvae of Plodia interpunctella Unpretreated PE film 60 days at 30 ℃ Weight loss: 13.9% [22]
Aspergillus tubingensis VRKPT1 and A. flavus VRKPT2 Coastal area of India Unpretreated HDPE film 12 weeks at 30 ℃ Weight loss: 6.0% and 8.5%, respectively [23]
PVC Chaetomium globosum André Tosello Culture Collection (ATCC) Unpretreated PVC film 28 days at 28 ℃ Weight loss: 9.0% [24]
Phanerochaete chrysosporium PV1 Soil Unpretreated PVC film 7 weeks at 28 ℃ Molecular weight reduction [25]
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大多数降解聚烯烃塑料的真菌是从土壤中分离得到的，尤其是长期存放塑料垃圾的填埋场。例如在垃圾填埋场分离得到的棒曲霉(Aspergillus clavatus) JASK1与未经处理的LDPE膜孵育3个月后可使LDPE的重量损失35.0%[14]，红外光谱(fourier transform infrared spectrometer, FTIR)检测到新的氧化基团，扫描电镜(scanning electron microscope, SEM)观察到LDPE膜明显破损，孵育4周后检测到2.3 g/L矿化产物CO2；同样来自垃圾填埋场的桔青霉(Penicillium citrinum)，在相同条件下孵育3个月后LDPE膜的重量损失达38.8% [16]。除PE外，从土壤中还分离到小部分可以降解PVC的真菌，例如黄孢原平革菌(Phanerochaete chrysosporium) PV1与PVC膜孵育7周后，PVC膜的分子量明显降低[25]，SEM和FTIR也分别观察到PVC膜表面破损并产生新的官能团。

海洋中存在大量塑料垃圾及微塑料颗粒，同时具有丰富的微生物资源。2022年，孙超岷研究员在山东青岛汇泉湾分离得到1株降解PE的海洋真菌链格孢菌(Alternaria alternata) FB1[20]，该菌与PE膜孵育120 d后会产生明显的生物膜，通过SEM观察到PE膜表面出现破损，FTIR检测到新的官能团−OH，气相-质谱(gas chromatograph-mass spectrometer, GC-MS)检测到降解产物二甘醇胺；此外，在印度沿海地区分离到塔宾曲霉(Aspergillus tubingensis) VRKPT1和黄曲霉(Aspergillus flavus) VRKPT2两株真菌，与HDPE培养1个月后，失重率分别为6.0%和8.5%[23]。

近年来，昆虫摄食聚烯烃塑料引起了研究人员的关注，最具代表性的包括鞘翅目(Tenebrionidae)的黄粉虫(Tenebrio molitor)[26]、黑粉虫(Tenebrio obscurus)[27]、大麦虫(Zophobas morio)[28]和鳞翅目(Pyralidae)的大蜡螟(Galleria mellonella)[29]、印度谷螟(Plodia interpunctella)[30]等，很可能此类昆虫的肠道微生物对聚烯烃塑料的生物降解发挥了重要作用。2022年，东北林业大学张国财研究团队在蜡虫的肠道中分离得到了1株季也蒙迈耶氏酵母(Meyerozyma guilliermondii) ZJC1 (MgZJC1)和1株粘质沙雷氏菌(Serratia marcescens) ZJC2 (SmZJC2)[22]，并对蜡虫体内的PE生物降解机制进行了预测(图 2)：首先蜡虫通过咀嚼将塑料变成碎片进入到肠道，在肠道微生物的作用下降解成小分子，菌株MgZJC1可以降解PE产生有机酸和醛；菌株SmZJC2可以降解PE产生有机酸、盐酸盐和苯酚。作者通过多种手段证明了这2株菌能够降解PE，首先与MgZJC1及SmZJC2孵育60 d后PE的重量损失分别达到13.9%和3.6%；SEM观察到PE膜发生明显破损；FTIR也显示有新的官能团产生，如−C=O和−C−O−C等；利用凝胶渗透色谱(gel permeation chromatography, GPC)确定了PE的分子量明显降低[22]。同样在蜡虫肠道中分离到的A. flavus PEDX3在与HDPE培养28 d后也有明显的分子量降低，FTIR显示出现新的−OH，−C=O和−C−O−C等含氧基团[21]。

图 2 MgZJC1和SmZJC2降解聚乙烯示意图[22] Fig. 2 Scheme of polyethylene degradation by strains MgZJC1 and SmZJC2[22].
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2.2 细菌对聚烯烃类塑料的降解

除真菌外，来自垃圾填埋场、废水、污泥等环境中的细菌也被报道具有聚烯烃塑料的生物降解能力(表 2)。细菌的生长速度较真菌快，遗传操作手段丰富，代谢途径清晰，更加便于聚烯烃塑料生物降解机制的研究。

表 2 降解聚烯烃类塑料的细菌 Table 2 Bacterial degradation of polyolefin plastics
Plastic type Bacteria Source Pretreatment method Reaction condition Degradation result References
PE Rhodococcus ruber C208 PE agricultural
waste in soil		 Unpretreated LDPE
film		 8 weeks at 37 ℃ Weight loss: 7.5% [31]
Rhodococcus sp. Three forest soils Preoxidized LDPE film 30 days at 30 ℃ Confirmation of
adherence		 [32]
Spingobacterium moltivorum IRN19 Plastic dump soil UV-irradiated LDPE
film		 4 weeks at 30 ℃ Weight loss:
26.8%±3.04%		 [33]
Stentrophomonas sp. Plastic debris in
soil		 Unpretreated LDPE
film		 30 days at 28 ℃ Change in chemical
properties		 [34]
Stentrophomonas
pavanii		 Solid waste dump
site		 Modified LDPE 56 days at 30 ℃ Confirmed by FTIR [35]
S. marcescens Soil LLDPE powder
made of LLDPE film		 70 days at 30 ℃ Weight loss: 36.0% [36]
Bacillus gottheilii and
Bacillus cereus		 Soil UV-pretreated PE
powder		 40 days at 30 ℃ Weight loss: 6.2% and 1.6%, respectively [37]
Alcanivorax
borkumensis		 Mediterranean Sea Unpretreated
LDPE film		 7 days at 30 ℃ Weight loss: 3.5% [38]
Pseudomonas
aeruginosa PAO1		 ATCC Unpretreated
LDPE film		 120 days at 37 ℃ Weight loss: 20.0% [39]
Enterobacter sp. D1 Guts of wax moth
Galleria mellonella		 Unpretreated PE film 31 days at 37 ℃ Morphology changes [40]
Enterobacter
asburiae YT1 and
Bacillus sp. YP1		 Guts of plastic-
Eating waxworms		 Unpretreated LLDPE
film		 60 days at 30 ℃ Weight losses of
6.1% and 10.7%
after incubation
with E. asburiae YT1
and Bacillus sp.
YP1, respectively		 [41]
Raoultella ornithinolytica
MP-1		 Solid waste dump
site		 Unpretreated
PE film		 30 days at 28 ℃ Weight loss: 4.4% [42]
PS Pseudomonas sp. Soil Unpretreated
high-impact PS
films		 30 days at 30 ℃ Weight loss: more
than 10.0%		 [43]
Bacillus sp. Soil Unpretreated
high-impact PS
films		 30 days at 30 ℃ Weight loss: 23.7% [43]
Exiguobacterium sp.
strain YT2		 Guts of the larvae
of Tenebrio molitor
Linnaeus		 Unpretreated
styrofoam PS films		 60 days Weight loss: 7.4%
Mw decrease:
11.0%		 [44]
Exiguobacterium
sp. strain YT2		 Degraded plastic
waste		 High-impact PS 30 days at 30 ℃ Weight loss: 12.4% [45]
R. ruber C208 Unpretreated
styrofoam PS films		 8 weeks at 28 ℃ Weight loss: 0.8% [46]
PP Stenotrophomonas
panacihumi PA3-2		 Soil Unpretreated PP
powder		 90 days at 37 ℃ Mw decreased [47]
R. rhodochrous
ATCC 29672		 ATCC PP film with
pro-oxidant additives		 180 days Changes in ATP levels [48]
Bacillus flexus Plastic dumping
site		 UV-pretreated PP
film		 1 year Weight loss: 2.5% [49]
Pseudomonas sp. WZH-4 Guts of insect larvae Unpretreated PP 30 days at 37 ℃ Weight loss: 4.57% [50]
Bacillus cereus Mangrove
sediments		 UV-pretreated PP
granules		 40 days at 30 ℃ Weight loss: 3.6% [37]
Sporosarcina
globispora		 Mangrove
sediments		 UV-pretreated PP
granules		 40 days at 30 ℃ Weight loss: 11.0% [37]
Bacillus sp. Municipal compost
waste		 Unpretreated PP
powder		 15 days at 37 ℃ Weight loss:
10.0%–12.0%		 [51]
Psychrobacillus sp. Plastic dumping
site		 Unpretreated PP 60 days at 30 ℃ Weight loss: 6.0% [52]
Pseudomonas sp. Plastic dumping
site		 Unpretreated PP 60 days at 37 ℃ Weight loss: 4.6% [53]
Staphylococcus sp. Plastic dumping
Site		 Unpretreated PP 60 days at 37 ℃ Weight loss: 5.0% [54]
PVC P. citronellolis Collection of
microorganisms
and cell cultures		 PVC film with 30% additives 30 days at 30 ℃ Weight loss: 19.0% [55]
Alteromonas sp. BP-4.3 Marine Unpretreated PVC film 60 days at 30 ℃ Weight loss: 1.8% [56]
Achromobacter denitrificans Ebl13 Compost Thermo-oxidative pretreated LDPE film 30 days at 30 ℃ Weight loss: 12.3% [57]
Klebsiella sp. EMBL-1 Guts of insect larvae Unpretreated PVC film 90 days at 30 ℃ Weight loss: 19.6% [58-59]
表选项

相比于真菌，降解聚烯烃类塑料的细菌种类更多，降解底物的范围更广，真菌的生物降解研究大多集中在PE，而细菌对PE、PP、PS和PVC等不同类型的聚烯烃塑料都有降解作用。土壤中降解聚烯烃类塑料的细菌以芽孢杆菌(Bacillus sp.)为主，如土壤中分离得到的Bacillus sp.与高抗冲聚苯乙烯(high-impact polystyrene, HIPS)膜孵育30 d后可使PS膜失重23.7%[43]；在红树林土壤中分离到的蜡样芽孢杆菌(Bacillus cereus)对经过紫外线照射预处理的PP粉末和PE粉末均具有降解作用，失重率在40 d内分别达到3.6%和1.6%，与B. cereus同时分离得到的B. gottheilii对经过紫外线照射处理的PE粉末也有降解作用，40 d内失重率达6.2%[37]，对于PE的降解率明显低于上述HIPS，除了不同菌株间降解能力差异，另外一个重要原因是HIPS是PS和聚丁二烯的混合物，相比于纯的PE更易降解。此外，还有一些芽孢杆菌如Bacillus sp. MYK2[60]、B. amyloliquefaciens BSM-1和B. amyloliquefaciens BSM-2等也被分离鉴定具有降解不同类型聚烯烃类塑料的能力[61]。除芽孢杆菌外，来自土壤的鸟氨酸拉乌尔菌(Raoultella ornithinolytica) MP-1在60 d内可使PE失重达4.4%，膜表面出现明显破损[42]；假单胞菌(Pseudomonas sp.) WZH-4[53]、葡萄球菌(Staphylococcus sp.)[54]、冷杆菌(Psychrobacillus sp.) LICME-ZWZR-10[52]等可以降解PP；克雷伯菌(Klebsiella pneumoniae)[62]、莴苣不动杆菌(Acinetobacter dijkshoorniae)[63]可以降解PE；戈登氏菌(Gordonia sp.)能降解PS，降解率为2.7%−7.7%[64]。最近，本课题组在山东东营石油污染的土壤中筛选到1株鸟氨酸拉乌尔菌(Raoultella sp.) DY2415，该菌对PS和紫外线照射处理过的PE (UVPE)都具有明显的降解作用，60 d内可以使UVPE失重达8.0%，通过FTIR观察到新的官能团如−OH、−C=O等，SEM显示塑料膜表面有大量细菌附着并产生明显裂缝。

海洋也是聚烯烃塑料降解细菌的重要来源。2018年，研究者在海洋沉积物中分离得到了红球菌(Rhodococcus)菌株36，其在40 d内可以使PP失重达6.4%[65]。R. ruber C-208可以以每周0.9%的降解速率降解PE，同时，该菌株在培养56 d后，PE的失重率达7.5%[31]；2022年同济大学马杰团队在湖底沉积物中分离得到了1株降解PS的B. cereus CH6，孵育50 d后，该菌对0.5 g PS的降解率达10.7%，并且研究发现在降解过程中细菌胞外蛋白质浓度和酯酶活性出现由低到高的变化[66]。此外，还有很多细菌对聚烯烃类塑料具有降解作用，例如海洋细菌食烷菌(Alcanivorax sp.) 24可以在34 d内使LDPE的分子量从122 kg/mol降低到83 kg/mol[67]；A. borkumensis与未经处理的LDPE膜孵育7 d后失重率达3.5%。

昆虫肠道中分离到了一系列可以降解聚烃类塑料的细菌，2018年研究者在黄粉虫中分离到产酸克雷伯氏菌(Klebsiella oxytoca)、费格森埃希菌(Escherichia fergusonii)及芽孢杆菌(B. cereus) Reborn，这3株菌的共培养物对PE、PP、PS、PVC都具有降解作用[50]。2019年，Ren等使用以PE为唯一碳源的培养基从G. mellonella幼虫肠道中筛选到了肠杆菌(Enterobacter sp.) D1[40]，PE膜经过菌悬液处理14 d后表面出现凹陷、裂纹、羰基和醚官能团，培养上清液经色谱分析也发现了与PE生物降解相关的代谢物；2020年，研究发现昆虫肠道分离到的不动杆菌(Acinetobacter sp.) NyZ450和Bacillus sp. NyZ451与PE膜共培养30 d后，检测到PE膜分子量明显下降，FTIR也显示PE膜表面被氧化[68]；2022年研究者在昆虫肠道中分离得到了1株可以降解PVC的变栖克雷伯菌(Klebsiella sp.) EMBL-1，经过90 d培养后，PVC膜的重量损失达19.6%，分子量也明显下降[58-59]。

3 聚烯烃塑料的生物降解机制

聚烯烃塑料的生物降解是一个复杂的过程，可以简单概括为在生物及非生物因素的共同作用下，塑料聚合物的分子量逐渐下降，产生的小分子降解物进入细胞内参与物质代谢，最终转变为碳源和能量供细胞生长。通常将生物降解过程分成4个阶段：定殖、解聚、同化和矿化(图 3)[69]。

图 3 聚烯烃塑料的生物降解过程[69] Fig. 3 The biodegradation process of polyolefin plastics[69].
图选项
3.1 定殖

定殖是指微生物在塑料表面形成生物膜的过程。生物膜的形成是塑料生物降解的第1阶段，同时也是必要阶段[70]。微生物通过分泌多糖等物质可以牢固附着在塑料表面，例如具有PE降解活性的R. ruber在与PE共培养时形成了三维的“蘑菇状”生物膜[31]。生物膜的形成可以降低塑料的浮力和疏水性[71]，增加微生物分泌的酶与塑料底物的接触面积，该阶段塑料膜表面可能会出现劣化、孔洞等。此外，一些非生物因素也会引起塑料表面物理化学性质的变化，如紫外线照射、机械损伤和极端温度等。经过非生物因素处理后的聚烯烃塑料更加有利于微生物降解，例如来自链霉菌(Streptomyces sp.) K30的乳胶清除蛋白LcpK30对紫外线照射处理过的PE (UVPE)和PP (UVPP)有明显降解效果，具体表现为FTIR观察到UVPE和UVPP膜上有新的官能团产生，如−OH和−C=O；SEM显示酶处理后的UVPE和UVPP膜明显变粗糙，出现裂纹和凹坑；GPC也观察到分子量的下降[72]。2022年有研究者利用漆酶介质体系氧化UVPE膜，并通过GC-MS检测到降解产物油酸和鲨烯[73]。

3.2 解聚

形成生物膜后塑料表面会发生劣化，微生物会通过分泌胞外酶催化聚烯烃塑料化学键的断裂，降低其分子量并释放小分子物质。目前认为氧化还原反应是破坏聚烯烃塑料C−C主链的主要方式，虽然具体反应机制仍不清楚，但该过程往往伴随自由基的产生，漆酶、锰过氧化物酶、烷烃羟化酶等均被报道对聚烯烃塑料具有降解作用(表 3)。

表 3 对聚烯烃类塑料具有降解作用的酶 Table 3 Enzymes capable of degrading polyolefin plastics
Enzyme Isolated source Plastic type Incubation time Degradation effect References
Manganese peroxidase P. chrysosporium PE film 12 days Molecular weight decreased [74]
Soybean peroxidase Soybean HDPE film 2 hours – [75]
Laccase R. ruber C208 LDPE film 30 days Weight loss: 2.5% [76]
Alkane hydroxylase Pseudomonas sp. E4 LDPE film 80 days Weight loss: 19.3% [77]
Demetra The saliva of Galleria
mellonella larvae		 PE film 90 min Detect 2-ketones [78]
Ceres The saliva of Galleria
mellonella larvae		 PE film 90 min Detect 2-ketones [78]
Hydroquinone peroxidase Azotobacter beijerinckii
HM121		 PS film 20 min Molecular weight decreased [79]
Alkane hydroxylase A. johnsonii JNU01 PS powder 7 days Confirmed by SEM and FTIR [80]
Laccase Botrytis aclada and
Bacillus subtilis		 UVPE film 24 hours Detect oxygen-containing
products		 [73]
Lignin peroxidase P. chrysosporium PVC film 4 weeks Weight loss: 31% [81]
表选项

真菌中具有聚烯烃类塑料降解能力的酶主要是过氧化物酶和氧化酶等[82-84]。例如哈茨木霉(Trichoderma harzianum)中的漆酶和过氧化物酶在PE降解过程中发挥了重要作用，在与PE膜孵育10 d后，质量分别减少了0.5%和0.6%[85]，同时FTIR检测到了羧酸、醛、芳烃、醇、酯、醚和烷基卤化物等特征基团。2022年江南大学吴敬团队发现来源于杏鲍菇(Pleurotus eryngii) 的过氧化物酶对紫外线照射过的PE具有降解作用，通过GC-MS检测到了醛、酮、醇和酸等降解产物，利用SEM和FTIR观察到PE膜表面变得粗糙褶皱并且有新的官能团产生[86]。在大肠杆菌中异源表达并纯化来源于白腐真菌(Gelatoporia subvermispora)的锰过氧化物酶，与经过紫外线照射的PE膜孵育后，膜表面出现刻蚀，亲水性也明显增强，同时检测到了醛、酮等小分子化合物[87]。除PE外，来自P. chrysosporium的木质素过氧化物酶对PVC具有降解效果，经纯酶处理后，PVC的重量减少31.0%[81]。细菌中也发现了一些对聚烯烃塑料具有降解作用的酶，如Pseudomonas sp. E4 AlkB家族的烷烃羟化酶，在80 d内将低分子量聚乙烯(low molecular weight polyethylene, LMWPE)中19.3%的碳转化为CO2[77]。2022年，来自西班牙最高科学理事会生物学研究中心的Federica Bertocchini团队发现G. mellonella的唾液能够快速氧化并降解PE。通过对唾液成分的深入研究，最终发现了2种属于苯酚氧化酶家族的酶Demetra和Ceres，在室温条件下短时间内对PE膜表现出了降解活性，同时通过FTIR检测到了新官能团C−O−C和C−COO[78]。

氧化酶对聚烯烃塑料的生物降解普遍遵循自由基氧化原理。例如漆酶将电子从有机物转移至分子氧，而血红素过氧化物酶使用H2O2作为底物进行氧化还原反应，这一过程会产生活性氧自由基(羟基自由基·OH和超氧阴离子自由基O2–·)[88]。自由基在化学上也称“游离基”，是含有1个不成对电子的原子团，由于化学键中的电子必须成对出现，因此自由基需要夺取其他物质的1个电子，使自己成为稳定的物质，这种现象称为“氧化”。2021年，研究人员通过量子化学模拟和计算探究了羟基自由基·OH和超氧阴离子自由基O2−·对PE的氧化作用。结果表明C6模式底物上的氢原子可以转移到·OH上，形成烷烃自由基R·和1分子水，这种氢原子的转移会引发烷烃的裂解反应，R·与O2反应后产生过氧自由基ROO·，通过计算ROO·与烷烃RH和R·反应的活化能，发现这2种反应都可以在常温常压下自发进行，进一步氧化产生ROOH和ROO− (图 4)[89]。

图 4 烷烃氧化反应途径[89] Fig. 4 The oxidation reaction pathway of alkanes[89].
图选项

以P. chrysosporium的锰过氧化物酶(MnP)为例[84]，MnP的活性位点包括Asp179、Ala79、Ser78、Arg42、Glu35、Glu143、Glu39、Asn81、His46、Phe45、Pro142，丙酸血红素和2个水分子(图 5A)，在含有H2O2的酸性条件下发挥催化作用。H2O2作为氧化剂通过均裂释放出2个·OH，其中1个·OH与MnP上的氢结合，另1个·OH与Fe2+结合，释放出1分子的水(图 5B)；然后铁-卟啉配合物发生均裂生成卟啉-π-阳离子自由基和羟基铁中心自由基，其中卟啉-π-阳离子自由基与羟基铁中心自由基的氢结合，羟基铁中心自由基上丙酸中的氧与水中氢共价结合生成氧锰自由基(图 5C)；随后PE进入MnP的活性位点，与氧锰自由基的氧反应生成氧铁-MnP自由基复合物(图 5D)；最后PE在MnP的作用下分解成小分子，如烷烃、烷烃自由基等(图 5E)。

图 5 Phanerochaete chrysosporium中潜在的PE生物降解机制[84] Fig. 5 Potential mechanisms of PE biodegradation in Phanerochaete chrysosporium [84].
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3.3 同化

同化是指聚烯烃塑料解聚过程产生的小分子物质被细胞吸收并参与生化代谢的过程。聚烯烃塑料分子量降低后增加了被细胞膜上受体识别的可能性，可以通过细胞膜转运进细胞内参与物质代谢。聚烯烃类塑料结构与烷烃相似，可以参照烷烃的同化途径[90]。以图 5为例，PE生物解聚过程中产生的烷烃和烷烃自由基，可以进入细胞并发生一系列氧化反应，它们会依次氧化为醛(图 5I)和羧酸(图 5K)，最后与辅酶A反应生成酰基辅酶A经β氧化生成乙酰辅酶A，进入矿化阶段。

3.4 矿化

矿化是聚烯烃塑料生物降解的最后一步，是指塑料解聚后的分子在微生物细胞内经过物质代谢，最终氧化生成CO2、N2、CH4等物质或积累生物质的过程[91]。以图 5为例，同化过程将进入细胞内的烷烃代谢生成乙酰辅酶。接着进入矿化阶段，乙酰辅酶A会通过三羧酸循环矿化成CO2和生物质(图 5O)，完成整个聚烯烃塑料的生物降解过程。

4 总结与展望

聚烯烃类塑料作为1种由C−C键骨架构成的高分子聚合物，具有多样的化学结构及物理特性，且难以被生物降解。虽然目前已经发现一些微生物甚至昆虫具有聚烯烃塑料的降解能力，但目前生物降解效率仍然较低，其中涉及的酶和底物较多，具体的生物降解机制及关键酶仍有待明确。探究聚烯烃类塑料的生物降解机制并提高其生物降解效率是未来的主要研究方向。以下3个问题需要重点关注：(1) 目前生物降解聚烯烃塑料的表征手段虽然很多，但标准模糊，大部分仍停留在定性层面，如通过电子显微镜观察微生物对于塑料膜的侵蚀时，通常局限在塑料膜局部的表面变化，难以衡量对于塑料整体的降解效果，并且受主观因素影响较大。另外，一些环境因素及塑料添加剂也会干扰对于聚烯烃塑料生物降解效果的判断。针对该问题，应建立更加系统规范的聚烯烃塑料生物降解表征方法，通过多个维度衡量微生物对于聚烯烃塑料的生物降解能力，如宏观层面的塑料形态变化、物理性质变化、重量损失和微观层面的物质组成变化等，尤其是采取合适的降解产物分析方法，明确生物降解过程产生的小分子降解物，这将有利于后续生物降解机制的研究。(2) 聚烯烃类塑料化学结构高度稳定、疏水性强，这些材料特性决定了单纯依靠微生物作用可能难以实现高效的生物降解。针对该问题，通过一些绿色的物理化学方法，对聚烯烃塑料进行预处理，如增加材料的比表面积、疏水性，甚至通过化学氧化技术添加羟基等更易水解的化学基团，可以明显提升聚烯烃塑料的生物可降解性。再与生物降解技术有机结合，引入对应的生物降解途径，优化整个降解工艺，提高整体的降解效率。(3) 目前聚烯烃塑料的生物降解研究主要集中在降解微生物的筛选鉴定方面，具体的生物降解机制，尤其是发挥作用的关键酶，还有待深入研究。针对该问题，应结合基因组、转录组、代谢组等多组学技术，深入分析聚烯烃降解微生物的遗传特性，挖掘生物降解相关的关键酶。从分子层面解析聚烯烃塑料的生物降解机制，并通过蛋白质工程技术提高关键酶的催化活性，进而推动聚烯烃塑料的绿色高效循环利用。

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