2022, Vol. 43
2. 厦门大学环境与生态学院, 福建省海陆界面生态环境重点实验室, 厦门 361102
2. Fujian Provincial Key Laboratory for Coastal Ecology and Environmental Studies, College of the Environment and Ecology, Xiamen University, Xiamen 361102, China
现代工业的发展使得塑料制品的使用急剧增加, 全球塑料的产量从1950年的150 t剧增到2020年的3.67亿t[1], 由此也造成了大量废旧塑料垃圾的产生.这些合成多聚物由于其化学结构稳定而难以被降解, 在环境中经过风吹日晒而裂解, 形成稳定的小颗粒, 一般把直径5mm以下的塑料碎片、颗粒和纺织纤维统称为微塑料(microplastics).微塑料污染已经成为一个全球性的环境问题.
聚乙烯(polyethylene, PE)是全球生产量最高的塑料[2], 主要用途包括制作薄膜、包装材料和电线电缆等, 它是一种以乙烯为原材料聚合加工得到的热塑性树脂材料, 其结构式为: —[—CH2—CH2—]n—.根据聚合方法、相对分子质量和碳链结构的不同, 聚乙烯可被分为低密度聚乙烯(low density polyethylene, LDPE)、高密度聚乙烯(high density polyethylene, HDPE)、线性低密度聚乙烯(linear low density polyethylene, LLDPE)和低分子量聚乙烯(low molecular weight polyethylene, LMWPE)等.
合成塑料的产生, 仅有不到80年的历史, 原来普遍认为尚不足以进化出能降解塑料的微生物或酶, 近年来有报道则显示自然环境中存在着对塑料具有降解能力的微生物.尽管分解速率缓慢, 但已有研究预示从自然环境中获得原本认为难以降解的塑料的降解微生物是可能的.由于反应条件温和且产物无污染, 微生物降解逐渐被认为是可行的处理微塑料的方法[3].本文综述了当前生物降解PE的研究现状, 总结和分析PE降解菌株的种类、来源和降解机制并讨论了降解效果的量化方法, 以期为科学评估微塑料降解效果及微塑料污染去除提供参考.
1 PE生物降解效果的表征由于PE的结构特性, 生物可利用率较低, 现有降解菌普遍分解缓慢, 降解现象不够明显.为了确定微生物对PE的生物降解程度, 一方面可通过监测微生物在以PE作为唯一碳源生长时的生长动力学变化, 来判断对PE的降解能力; 另一方面通过分析PE材料的性质变化, 如表面官能团变化、机械性能、表面形貌、疏水性/亲水性、相对分子质量分布和降解产物等, 评价生物降解的效果[4].目前常用的方法包括观察法、失重法、CO2生成量法、傅里叶变换红外光谱仪(fourier transform infrared spectrometer, FTIR)法、凝胶渗透色谱(gel permeation chromatography, GPC)法和气相色谱-质谱联用(gas chromatograph-mass spectrometer, GC-MS)法等, 以上方法也可以从定性和定量两种角度进行区分.
1.1 定性法 1.1.1 显微镜观察法观察法通常使用原子力显微镜(atomic force microscope, AFM)和扫描电子显微镜(scanning electronic microscope, SEM)两种仪器.AFM可观察微生物降解前后PE本身材质的变化, 包括空洞、凹坑的出现和粗糙度的变化等; SEM在此基础上还可观察微生物在PE表面的定殖情况, 如生物膜的形成和细胞形态大小等.因此通过与对照组比较, 根据PE的变化和微生物的定殖情况, 能够直观地判断接种微生物后PE表面是否发生降解现象.
Zahra等[5]通过SEM发现培养100 d后Aspergillus fumigatus和A. terreus这2株真菌在紫外线照射后的LDPE膜表面形成了明显的生物膜.Esmaeili等[6]的研究也显示, A. niger F1与预先照射紫外线的LDPE膜共培养后, SEM显示其菌丝黏附并穿透紫外线照射的LDPE膜表面.同一研究中细菌Lysinibacillus xylanilyticus S7-10F也在LDPE膜表面形成了生物膜.Awasthi等[7]通过SEM图象观察到, 菌株Klebsiella pneumoniae CH001培养60 d后HDPE膜表面粗糙, 有许多裂缝和凹槽, 而对照组则保持光滑, AFM图像也观察到表面拓扑结构的类似变化, 证明该菌株能够利用PE并形成凹槽.Auta等[8]接种Bacillus属细菌与PE粉末共培养40 d后, PE表面变得粗糙并出现许多孔洞、侵蚀、裂缝和凹槽.Yang等[9]观察到在经高压灭菌的LDPE膜表面生长的Enterobacter asburiae YT1和Bacillus sp. YP1分别为0.2 μm× 0.8 μm的棒状细胞和0.5μm×3μm的不规则短棒状细胞.
1.1.2 FTIR法FTIR法是利用傅里叶变换红外光谱仪检测微生物处理前后PE样品差异, 通过官能团对应的吸收峰的变化判断PE结构是否因聚合物链断裂或氧化产物形成发生变化.
Skariyachan等[10]在3 375.46 cm-1处发现了由于C—H键断裂形成的额外的峰, 认为可能是由于生物降解导致的长链烃的分解.经Aspergillus flavus PEDX3处理后的HDPE微塑料颗粒与对照样品相比, 在3 500~3 100 cm-1范围内出现对应羟基(—OH)的宽吸收峰, 还在1 113 cm-1和1 647~1 716 cm-1处分别出现对应醚基(C—O—C)和羰基(—C O)的吸收峰, 3种官能团的出现为HDPE微塑料颗粒的生物氧化提供了证据[11]. Skariyachan等[12]报道了处理后LDPE条由于羰基的形成在光谱的1 700 cm-1和1 600 cm-1处出现新的吸收峰.Chen等[13]利用衰减全反射红外光谱(ATR-FTIR)和光谱图像对在自然环境中暴露的PE降解程度进行建模分析, 结果表明可利用1 720 cm-1处的羰基指数表征降解程度, 为预测原位暴露微塑料降解程度提供有效帮助.而随着生物降解的进行, 氧化聚合物被微生物利用, 羰基指数也将降低[14].因此使用FTIR检测羰基峰的形成对阐明PE降解机制有着重要的作用[15].
1.1.3 GPC法GPC法通过测定数均相对分子质量(Mn)、重均相对分子质量(Mw) 和相对分子质量分布(MWD), 反映PE相对分子质量的变化, 根据相对分子质量的损失判断PE长链结构是否解聚.
Yamada-Onodera等[16]通过3个月对PE相对分子质量的监测验证了Penicillium simplicissimum YK对PE的降解作用, 当用热硝酸处理过的PE作为唯一碳源时, 相对分子质量高于100 000的PE会被真菌降解为较低的相对分子质量. Zhang等[11]利用高温凝胶等效色谱(HT-GPC)对经Aspergillus flavus PEDX3处理过的HDPE微塑料颗粒样品进行检测发现, 与对照样品的相对分子质量(Mw=222 003和Mn=55 135)相比, 处理后样品的Mw(89 801)和Mn(26 064)分别下降了132 202和29 069, 差异明显.同样的, 用菌株Enterobacter asburiae YT1和Bacillus sp. YP1培养的PE样品使用HT-GPC测定培养60 d后的相对分子质量(Mw/Mn)和相对分子质量分布(MWD), 与对照相比, 显示出明显的负趋势.二者培养的PE样品Mw/Mn分别为82 500/24 700和78 200/23 900, 降解组与对照相比减少了6% ~13%[9].然而值得注意的是, 降解现象的发生也可能导致相对分子质量增加, 这是由于降解菌株优先降解PE样品中低相对分子质量的部分, 致使剩下的PE样品的相对分子质量因生物降解而增加[17, 18].
1.1.4 GC-MS法GC-MS法是利用气相色谱-质谱联用仪对PE降解过程中的代谢产物进行分析.
Albertsson等[18]利用Arthrobacter paraffineus降解高温处理后的LDPE膜, 3.5 a后对照组(非生物降解样品)中检测出一元和二元羧酸和酮酸的同系物, 而这些产物在生物降解样品中则完全被吸收, 只发现疑似醇类的物质.Abraham等[19]用曲霉菌处理LDPE膜, 90 d后LDPE膜上鉴定出烷烃、芳香烃、饱和脂肪酸和不饱和脂肪酸等无毒化合物.Ambika等[20]利用气相色谱法洗脱大量挥发性和半挥发性化合物, GC-MS分析鉴定了Achromobacter Denitrificans s1对LDPE的降解产物包括醛、酮、酯和羧酸.分别对经Alternaria alternata FB1处理60 d和120 d的PE膜进行检测, 结果表明处理60 d的样品产物碳数范围在12~30之间, 主要产物碳数为27, 占比为51.24%, 而时间延长到120 d后产物碳数在3~27之间, 具有4个碳的二甘醇胺是Alternaria alternata FB1降解PE膜的主要产物, 占所有产物的93.28%[3].
1.2 定量法 1.2.1 失重法通过测定PE材料在微生物降解前后的质量损失, 并与对照进行比较, 可以直观地以百分比判断微生物对PE的降解效果, 有利于横向比较不同菌株的降解能力, 失重法是目前使用较多的一种方法.
将Bacillus subtilis接种在经紫外线照射且加入表面活性剂的PE膜上, 30 d内减重9.26%[21].菌株Pseudomonas sp. AKS2可直接降解未经预处理的LDPE膜, 45 d内可降解(5±1)%的原始材料, 若添加矿物油改变疏水性, 效果增强至(14±1)%[22].在15 d的培养过程中, 接种Streptomyces albogriseolus LBX-2菌株的PE粉末的重量损失显著增加.此外, LBX-2对低相对分子质量的PE粉末降解效果更好[23].若是在同一实验体系内, 由于降解对象的初始状态且培养条件相同, 失重法是比较不同降解菌株降解效果的最简单直接的方法.如对于HDPE的降解, Pseudomonas sp.(15%)比Arthrobacter sp.(12%)表现出更好的降解效果[24].Devi等[25]从沿海塑料垃圾倾倒地区土壤中分离出248株细菌并运用失重法进行比较, 最终得到了10株有效的HDPE降解菌.
1.2.2 CO2生成量法结合已有研究可知, 微生物不论是好氧还是厌氧降解PE, CO2的产生都是PE被完全降解的标志.有研究通过测量CO2产生量来表征降解效果. Shah等[26]测定Fusarium sp. AF4降解PE膜, 4周后CO2产量为1.85 g ·L-1, 显著高于空白对照的0.45 g ·L-1. Abraham等[19]的研究显示, Aspergillus nomius JAPE1和Streptomyces sp. AJ1降解LDPE, 4周后的CO2产生量则分别为2.85 g ·L-1和4.27 g ·L-1.
定性法可以直观地表现微生物的生长或PE的表形变化, 但是判断某一菌株是否为PE降解菌, 通常需要多种方法确认.且由于定性法难以量化, 无法对不同降解菌的降解能力进行横向比较.定量法虽然可以在同一体系中针对不同降解菌的降解能力进行比较, 但由于降解对象本身质量很轻, 操作过程中易造成较大误差, 且不同体系使用的PE材质不同(是否含有添加剂)、反应条件不同(温度、时间), 无法简单地凭借重量的失去判断降解能力地强弱.因此PE降解菌降解能力的量化仍然比较困难.降解效果的表征虽然可以直观体现降解菌对PE的降解能力, 表征PE的生物降解尚需要几种方法结合, 如SEM结合失重和FTIR法, 从微生物的生长状态结合PE材料的变化, 较全面地评价降解菌对PE的降解能力.
2 PE的微生物降解PE的微生物降解最早可追溯到20世纪70年代, 通过检测CO2的生成, 发现由C14标记合成的PE能被微生物成功降解, 并最终矿化为CO2和H2 O, 但是过程极为缓慢[27].近年来, 相继有不同的研究从土壤、海洋和昆虫肠道等生境中筛选出对PE具有降解能力的微生物[25, 28], 表 1和表 2分别归纳了目前为止报道的PE降解细菌和降解真菌类群.如表中所示, 已筛选出的PE降解细菌分属4个门, 其中变形菌门γ-变形菌纲和放线菌门放线菌纲所包含的降解菌种类最多.相较于细菌, 真菌对于PE的降解速率似乎更高[29].真菌能以菌丝体结构在不同基质上生长, 延伸到其他微生物难以生长的地方[2, 30], 成为真菌降解PE的优势之一[4].已报道的PE降解真菌分属子囊菌门和担子菌门下共4个纲, 包括链格孢菌、曲霉菌、镰孢菌和青霉菌等多个属, 其中与曲霉菌相关的研究较多.
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表 1 PE降解细菌类群 Table 1 Bacterial taxa capable of PE degradation |
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表 2 PE降解真菌类群 Table 2 Fungal taxa capable of PE degradation |
Harshvardhan等[37]从阿拉伯海沿岸海水中分离出60株海洋细菌并从中筛选出3株(Kocuria palustris M16、Bacillus pumilus M27和Bacillus subtilis H1584)能够在以PE为唯一碳源的培养基中生长的菌株, 30 d共培养后, PE失重分别为1%、1.5%和1.75%, FTIR光谱也显示了酯、酮、乙烯基和内部双键的形成.Sangale等[46]在印度西海岸红树林根际土壤中得到的109株真菌中, 从重量变化和拉伸强度变化两个角度, 在不同pH条件下筛选出了Aspergillus terreus MANGF1/WL (pH=9.5)和Aspergillus sydowii PNPF15/TS (pH=3.5)两株PE降解真菌.相较于海洋环境, 更多的PE降解菌筛选自土壤环境.Watanabe等[38]从田地土壤中筛选到3株能够降解LDPE的菌株, 分别属于Bacillus circulans、B. brevies和B. sphaericus. Muhonja等[29]从富含塑料垃圾的土壤中分离具有LDPE降解能力的细菌和真菌, 培养16周后, 其中曲霉菌(Aspergillus oryzae A5, 1)对LDPE膜的降解效率最高, 减重(36.4±5.53)%. Soleimani等[40]从含有塑料垃圾的城市垃圾填埋场土壤中筛选可降解LDPE膜的放线菌, 获得链霉菌属(Streptomyces)、诺卡氏菌属(Nocardia)和红球菌属(Rhodococcus)这3个属共17株降解菌, 其中以链霉菌降解效果更好, 经60 d培养, Streptomyces sp. IR-SGS-T10对LDPE膜重量降低速率为1.58 mg ·(g ·d)-1. Hou等[51]从污水处理厂长期受塑料污染的活性污泥和废水中分离到54株能以PE农用地膜为唯一碳源进行生长的细菌, 其中两株假单胞菌(Pseudomonas knackmussii N1-2和P. aeruginosa RD1-3)能有效降解PE地膜, 在8周的培养周期中, PE地膜分别失重(5.95±0.03)%和(3.62±0.32)%.此外, 昆虫肠道也是一个特殊的筛选来源. Yang等[9]以PE作为唯一碳源在食用PE膜的印度谷螟幼虫肠道中分离出Bacillus sp. YP1和Enterobacter asburiae YT1两株细菌, 共培养28 d后LDPE膜的物理性能、化学结构和相对分子质量等均发生改变. Zhang等[11]将从蜡蛾肠道分离出的Aspergillus flavus PEDX3与HDPE微塑料共培养28 d后, FTIR检测到新的官能团出现, 相对分子质量降低, 且两个与降解相关的基因表达上调.
有研究表明, 将PE膜进行预处理, 如紫外线照射或添加表面活性剂, 可以增强降解菌对PE的降解效果.这是由于紫外线可以使聚乙烯链发生解聚, 生成烷烃、烯烃、酮、醛等低相对分子质量产物[9], 而表面活性剂可以提高PE的水溶性, 从而提高它的生物可利用性.Montazer等[35]以紫外线处理过的LDPE膜为降解对象, 利用Sphingobacterium moltivourum IRN11和Acinetobacter pitti IRN19对其进行降解, 结果表明二者均可在LDPE膜表面产生表面活性剂并定殖, 培养4周后Acinetobacter pitti IRN19降解了(26.8±3.04)%的PE薄膜. Dwicania等[52]发现将铜绿假单胞菌(Pseudomonas aeruginosa)和短杆菌(Brevibacterium sp.)混合培养, 在25℃, pH=7的条件下, 30 d后失重测定, 去除了2% ~7%的经紫外线预处理的LLDPE. Sowmya等[49]比较了PE膜经高压灭菌、膜表面消毒和紫外线照射3种处理后, 木霉菌(Trichoderma harzianum)对其的降解效果, 3个月后该菌株对这3种处理的PE的降解效率分别是23%、13%和40%, 其中漆酶和锰过氧化物酶2种降解酶的活性在第10周达到最高.将LDPE片和HDPE片预先用表面活性剂Tween-80处理, 再经Penicillium oxalicum NS4和Penicillium chrysogenum NS10降解, 取得较好的降解效果, 90 d后菌株NS4对LDPE和HDPE的降解率分别为36.60%和55.34%, 菌株NS10的降解率为34.35%和58.60%[14].
3 PE生物降解影响因子和降解机制PE的生物降解受化合物、微生物和环境因子这3方面的影响.从化合物角度, PE的分子量、表面官能团(羰基、酯、乙烯基和碳碳双键)组成和疏水/亲水性等影响其生物可利用性, 一般来说带氧化基团和亲水性较好的PE更易被微生物降解[53].从微生物角度, 微生物能否顺利在PE表面定殖, 形成生物膜, 并分泌相应的酶对降解至关重要.而温度、pH和培养时间等因素则影响PE的微生物降解效率.
微塑料PE的微生物降解通常包括4个阶段[3, 4, 29, 50, 54]：劣化(deterioration)、解聚(depolymerization)、同化(assimilation)和矿化(mineralization).劣化阶段是指PE在外因作用下(如紫外线照射或微生物分泌的胞外酶)物理化学性质发生改变[55, 56], 在此过程中引入羰基, 随后被氧化形成羧酸[54, 57]; 解聚阶段PE在微生物分泌的特定酶的作用下进一步被分解成低聚物、二聚体和单体, 即易于吸收的低相对分子质量的片段[2, 58]; 这些解聚产物中的一些水溶性短链中间体被受体识别, 跨膜转运到微生物体内参与多种代谢活动, 并将这些物质代谢为碳或能量来源[3, 56], 该过程被称为同化; 最终微生物通过有氧或无氧代谢将这些短链低相对分子质量化合物转化为CO2、H2 O和CH4从细胞中排出, 由此产生的能量供给细胞生长[55, 56, 59].由此可见, 劣化和解聚是PE降解的重要步骤, 只有特定的微生物才能分泌相应的降解相关酶类, 以达到降解的目的.目前已报道的参与劣化与解聚阶段的PE降解酶类群包括[11, 60~64]：单加氧酶(如烷烃单加氧酶)、过氧化物酶(锰过氧化物酶、大豆过氧化物酶和谷胱甘肽过氧化物酶等)和漆酶等. Jeon等[65]发现烷烃单加氧酶基因直接参与热解制备的低分子量PE的生物降解, 而红氧还蛋白酶和红氧还蛋白还原酶通过相关电子的转移间接参与.Santo等[61]发现从Rhodococcus ruber C208中分离出的漆酶具有高热稳定性和铜诱导性, 与PE共培养两周后, 羰基指数增加且PE分子量减少, 证明漆酶参与了劣化与解聚过程.Zhao等[66]使用大豆过氧化物酶对HDPE进行改性, 在处理过的HDPE表面检测到羟基和羰基基团, 表明酶的处理显著增加了材料亲水性.
尽管在PE的微生物降解机制方面已经有研究, 但总体而言, 当前的研究还不够深入, 对于降解中间产物的产生和微生物利用途径仍然未知.
4 结论与展望相对分子质量高、疏水性强和化学性质稳定等特点使环境中的微塑料难以自行降解, 尽管越来越多的PE降解菌被筛选出, 但是由于降解现象不明显和分解速率普遍比较缓慢等原因, 降解菌的筛选难度还比较大, 本文认为, 在降解菌筛选方法上可以进行适当的改进, 如原位环境的定向驯化、稳定同位素探针技术(stable isotope probe, SIP)结合实验室培养和基于合成微生物组的高效降解菌群构建等方法, 可以提高从环境中分离降解菌的效率.此外, 对PE降解菌的特性研究、降解条件的优化和降解机制的研究, 对于提高降解菌的降解效率, 并为其成为潜在的修复菌源也具有很重要的意义.综上, 进一步开展对PE具有高效降解能力的微生物筛选和特异性研究仍然很有必要.
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