2020, Vol. 41
, MIAO Yu-tian , HUANG Ting-lin , LIU Kai-wen , LIU Xiang , HUANG Xin , YANG Shang-ye , SI Fan , LI Cheng-yao
水圈生态系统中, 微生物群落及其代谢功能与通路复杂多样[1], 许多原核微生物和真核微生物均可代谢产生异嗅化合物[2], 如洋假鱼腥藻、丝状蓝藻、席藻和颤藻等40余种藻类[3, 4]、放线菌[5]及黏细菌[6].其代谢产生异嗅化合物种类繁多, 就目前报道的已达67种, 以2-甲基异冰片(2-methylisoborneol, 2-MIB)和土臭素(trans-1, 10-dimethyl-trans-9-decalol, GSM)[2, 7~14]研究最多, 即使痕量存在于水环境中, 也可产生强烈的异嗅味, 致使公众对水质安全颇为关注[2, 7, 10, 15~23], 系列水环境安全与健康事件频发, 影响全球的水生态系统与城镇供水安全[2].
近年来, 水环境异嗅问题备受关注[24], 成为供水领域的世界性热点问题, 尤其是水源水库[8, 25]和湖泊[26~28]等缓流水生态系统.据统计, 中国重点城市的111座自来水处理厂(DWTPs)均有异嗅事件发生[22], 且中国东部和南部地区异嗅化合物浓度更高, 以霉味/土味和沼泽/腐臭味为主[29]. 2015~2016年, 英国Plas Uchaf水源水库中的2-MIB和GSM浓度峰值达到60 ng·L-1和140 ng·L-1[8]. 2019年, 中国江苏省17个水源水库中有1/3水库的2-MIB超标[30].此外, 我国学者对典型水环境异嗅问题的调查表明, 在太湖及滇池等富营养化的湖泊之中异嗅问题尤为突出[31], 水源地已丧失供水功能.
迄今为止, 国内外学者在水环境异嗅化合物控制的研究上已有建树, 以物理和化学法为主, 应用广泛.微生物法对环境友好, 近年来研究热度逐渐增加[32~43].对于诸如水环境致嗅放线菌是否来源于陆生休眠孢子等问题尚不明确[2], 且存在异嗅化合物浓度影响因素、微生物产嗅机制等方面的系统性研究缺乏和水源水库异嗅问题研究不足等问题.基于此, 本文在回顾已有研究的基础上, 对异嗅化合物的种类、生物源、去除措施和形成机制进行较为全面地综述, 通过推进异嗅化合物的相关研究, 以期为我国供水安全提供科技支撑.
1 异嗅物 1.1 异嗅化合物种类目前, 存在于水环境中的异嗅化合物主要包括(图 1和表 1)：GSM、2-MIB、2-异丙基-3-甲氧基吡嗪(IPMP)、2-异丁基-3-甲氧基吡嗪(IBMP)、2, 3, 6-三氯苯甲醚(2, 4, 6-TCA)、三甲基胺、β-环柠檬醛和β-紫罗兰酮[5, 7, 9, 12, 17, 21, 22, 31, 44~50], 及有机硫化物、醛类、酚类、苯类、醚类、酯类、酮类和含氮杂环化合物等[21, 47].GSM表现为土味, 2-MIB表现为霉味, 常与土霉味嗅味事件相关[51], 是水体异味问题的关键化合物[14, 47].嗅阈值均低于10 ng·L-1[3, 28, 51, 52], 却可引起强烈的异嗅味, 为研究热点之一[53].Chiu等[16]于2012年对中国台湾29个水源水库调查研究证实, 2-MIB为水源水库异嗅事件的常见异嗅化合物, 这与Tsao等[11]的研究结果相同.同时, Sun等[22]的研究发现2-MIB引发的异味事件在地表水中发生率高达75%.此外, IPMP、IBMP和氯苯甲醚也是常见的土霉味物质[51].对于氯苯甲醚的研究, 主要集中在2, 4-二氯苯甲醚(2, 4-DCA)、2, 6-二氯苯甲醚(2, 6-DCA)和2, 4, 6-TCA[36, 42, 54, 55](阈值为0.03~4 ng·L-1)上.β-环柠檬醛表现为木头/青草味, β-紫罗兰酮表现为芳香/紫罗兰味[45], 也可引起感官不适, 常与GSM和2-MIB共存于水环境中[56].表现为沼泽/腐臭味的异嗅化合物则主要包括二甲基二硫化物(DMDS)、二甲基三硫化物(DMTS)[47]、二乙基二硫化物(DEDS)、二异丙基硫化物(DIPS)、二丙基硫化物(DPS)、双(2-氯异丙基)醚(BCIE)、二丁基硫醚(DBS)和吲哚等[29].其中, BCIE、DEDS和DMDS为腐臭类异味的3种典型化合物[26].Acero等[57]的研究认为在氯化作用中会有溴酚类异嗅化合物产生, 如2-溴苯酚和2, 6-二溴苯酚(气味阈值分别为30 ng·L-1和0.5 ng·L-1), 可使饮用水产生异嗅味.
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图 1 水环境中常见异嗅化合物分子结构 Fig. 1 Molecular structures of common taste and odor compounds in aquatic ecosystems |
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表 1 水环境中典型异嗅化合物 Table 1 Typical off-flavor compounds in aquatic ecosystems |
1.2 致嗅微生物种类及主要致嗅产物
早在1891年, Berthelot和Andre首次提出放线菌可代谢产生土/霉味物质[20]. 1964年, Gerber于放线菌培养基中得到GSM和2-MIB[68].因此, 土/霉味物质被认为最早来源于放线菌[2, 21, 34].放线菌广泛存在于地球生物圈内, 代谢产物常具有异嗅味[17], 即使在水环境中, 依旧为主要致嗅微生物[69].学者们深入研究后揭示, 链霉菌(Streptomyces)是放线菌中引发水生环境异嗅问题的主要贡献者[39, 64, 70], 包括灰色链霉菌(Streptomyces griseus)、土味链霉菌(Streptomyces odorifer)[67]和淡黄链霉菌(Streptomyces flaveolus)[48]等, 均可不同程度产生2-MIB和GSM, 且GSM为典型次级代谢产物.
随后, Jenkins等[23]采用气相色谱技术在蓝藻培养物及含藻饮用水源中发现了几种含硫有机异嗅化合物, 如DMDS、甲基硫醇、异丁基硫醇和N-丁基硫醇[37]. 1994年, Sugiura等[71]的研究中发现放线菌与藻类共生时, 异嗅事件频繁发生, 异嗅化合物浓度显著增高.因此, 人们将注意力转向藻类[20, 21, 25, 39, 68, 72].后续研究证明鱼腥藻(Anabaena spp.)[20, 25, 39, 61, 68, 69, 73]产生GSM, 席藻(Phormidium spp.)[61, 73]、小颤藻(Oscillatoria spp.)[8, 20, 25, 39, 61, 68]和洋假鱼腥藻(Pseudanabaena spp.)[3]产生2-MIB, 小球藻(Chlorella vulgaris)和水华鱼腥藻(Anabaena flos-aquae)代谢2, 4, 6-TCA的能力很强, 湖泊水体中致嗅贡献显著[54].铜绿微囊藻(Microcystis aeruginosa)在繁殖过程中可能产生β-环柠檬醛[46], 硅藻(diatom)被激活时也可产生异嗅化合物[20, 25, 39, 68].更有学者认为, 蓝藻才是水环境中异嗅化合物的主要贡献者[9, 45].Chiu等[16]指出2-MIB为蓝藻代谢产生的常见异嗅化合物.Tsao等[11]分别从澳大利亚和中国台湾地表水中分离得到21株致嗅蓝藻, 均可代谢合成GSM. Sun等[22]经调查湖泊和水源水库发现, 水样多数表现出藻源性异嗅味.日本学者对产生嗅味的30余个蓄水池、湖泊及水源水库的研究表明, 即使藻含量很低, 也会产生较浓异嗅味[20]. Suurnäkki等[65]检测100株蓝藻的致嗅能力时发现, 蓝藻的6个属21株菌均可产生GSM, 有两株同时产生2-MIB, 且在另外55株菌中发现其他异嗅化合物, 如非类胡萝卜素、β-紫罗兰酮和β-环柠檬醛. Zhang等[49]在太湖藻华暴发期采样, 发现IPMP和IBMP、β-紫罗兰酮和β-环柠檬醛主导异嗅事件.Wang等[29]的研究认为, 藻类暴发是硫醚浓度升高的原因.
此外, 一些黏细菌、阿米巴、真菌(如霉菌)、少数植物(如红甜菜)和倍足纲节动物亦可产生异嗅化合物[20, 25, 27, 39, 68]. 2018年, Zhang等[36]为分析产生三氯苯甲醚的菌株, 采用高通量技术对饮用水配水系统中的生物膜进行测序, 结果发现细菌：不动杆菌(Acinetobacter)、假单胞菌(Pseudomonas)、微杆菌(Microbacterium)与红球菌(Rhodococcus); 真菌：曲霉(Aspergillus)、根霉(Rhizopus)与青霉(Penicillium)为优势菌属.Lukassen等[6]的研究认为藻类、链霉菌和黏细菌主导再循环水产业养殖系统异嗅事件.
1.3 异嗅化合物分泌的影响因素水环境中的异嗅味直接影响群众对水质的美学评价, 是水质安全性评价的关键参数.因此, 了解环境、生物和化学因素对异嗅化合物分泌的影响至关重要(表 2).有研究表明, 异嗅化合物的分泌主要与水质参数[pH值、透明度、水温、溶解氧(DO)、氧化还原电位(ORP)及微量金属元素等]和共生微生物等具有相关性[2, 8, 14, 25, 27, 28, 30, 49, 54, 55, 71, 73, 74].当致嗅微生物处于培养状态时, 培养基配方、培养时长与温度等影响着异嗅化合物产量[44, 48, 58].此外, 自来水管网的管材、流速与余氯含量等与异嗅化合物浓度有一定的关系[36].在流域尺度上, 地表径流也会增加水生环境的异嗅化合物浓度[17].
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表 2 异嗅化合物浓度影响研究 Table 2 Effects of concentration of off-flavor compounds |
1.4 异嗅化合物富集措施
异嗅化合物为挥发性物质, 常以ng·L-1存在于水环境中, 易损失.因此, 在检测之前, 选择有效地富集措施尤为关键[34, 56].
目前报道的异嗅化合物富集措施有闭环吹脱技术(closed-loop stripping analysis, CLSA)、树脂吸附(resin adsorption, RA)、液液萃取法(liquid-liquid extraction, LLE)[47]、液液微萃取技术(liquid-liquid microextraction, LLME)、固相萃取法(solid phase extraction, SPE)[8]、固相微萃取技术(solid-phase microextraction, SPME)[10, 13, 28, 29, 35, 36, 44, 49, 53, 54, 65]、搅拌棒吸附提取技术(stir-bar sorptive extraction, SBSE)[78]、液相微萃取法(liquid-phase microextraction, LPME)、吹扫捕集(purge and trap, P&T)、静态顶空(static headspace, SH)和动态顶空(dynamic headspace, DH)等, 这些方法都具有不同程度的局限性, 如耗时长、灵敏度差及易造成二次污染等[7, 9, 21, 50, 59, 79, 80].随着萃取技术的提高, 顶空-固相微萃取技术(headspace solid-phase microextraction, HS-SPME)具有功能一体化, 容易操作、省时、萃取头循环使用、环境友好及自动进样等优点, 近年来被广泛使用[9, 56, 80].为了提高异嗅化合物检测的精确度与灵敏度, 也有学者将同位素标记法和LLE结合, 实现同时定量测定水环境中51种异嗅化合物[47].
1.5 异嗅化合物检测分析方法迄今为止, 对于嗅味化合物的检测方法主要有感官评价法[48, 62, 81]和仪器测定法[20, 21, 56, 79, 82, 83].还有一些基于非离子检测方法, 如溴代反应(bromine reaction)、酶联免疫检测(enzyme-linked immunosorbent assay)、化学发光反应(chemiluminescence reaction)、生物电子鼻(bioelectronic nose)及电子舌(electronic tongues)等, 但由于检测分析技术不够成熟, 并未得到广泛使用[7, 21, 79].
采用感官评价法评定异嗅味强度[84]的方法, 可了解水中异嗅化合物的物理特征, 快读便捷.包括嗅觉阈值检测法(NTO)、嗅觉层次分析法(FPA)[22, 26]及嗅味等级描述法(FRA).NTO与FPA检测原理相似, 最早用于检测嗅度[21, 76, 85, 86], 需提前稀释水样, 而FPA则不需要做任何预处理, 直接用文字表明水样气味特性和强度, 划分出层次即可.FRA法要求评估小组成员分别在20℃与煮沸稍冷时嗅水样味道, 并描述水样味道特征.在中国, FRA评价标准被分为六级, 美国七级, 而日本则为九级.但评估小组嗅觉、水体杂质及环境等因素影响评估结果, 存在较大误差, 无法实现量化表征.因此, 国内外大多采用仪器测定法[80, 82].
气相色谱-质谱联用技术(gaschromatography-mass spectrometry, GC-MS)是目前最常采用的检测方法[8, 9, 16, 22, 29, 35, 49, 60, 80, 87].Suurnäkki等[65]采用GC-MS研究蓝藻的致嗅特性, Zhang等[36]采用GC-MS检测2, 4, 6-TCA[54], Ma等[10]采用GC-MS鉴定出GSM和2-MIB是UV/Cl处理过程中产生的副产物.为了对此技术进行优化, Wu等[53]优化了提取温度、提取时间、解吸时间、超声处理温度、超声处理时间和GC/MS配置温度程序等参数, 使得在全扫描模式下分析的异嗅化合物达到1~300 ng·L-1的线性响应要求, 且对2-MIB和GSM的检测限降为1 ng·L-1, Chen等[13]通过优化SPME纤维、提取温度与时间、解吸时间、搅拌条件及NaCl剂量等, 使得GC-MS检测限达到0.1~73 ng·L-1, 接近异嗅阈值甚至更低.
此外, 有学者在火焰离子检测器(FID)模式的基础上, 采用气相色谱技术(gas chromatography spectrometry, GC)分析GSM和2-MIB(检测限为1.5 ng·L-1)[11, 14].当化学电离技术与飞行时间质谱结合时, 由于高电离效率, 异嗅化合物能被快速检测, 且不需要预富集操作[66].近年来, 又研究表明将分子印迹聚合物(MIPs)与荧光标记竞争性置换技术运用于测定异嗅化合物的方法兼具可视性、快速和灵敏的特点, 具有极高的应用价值和发展前景[59].
1.6 异嗅化合物去除措施异嗅化合物的有效去除一直持有研究热度.常用的物理方法有吸附剂吸附法及紫外线辐射法(UV)[32, 33], 粉末状活性炭吸附剂(PCA)应用广泛[15].Kim等[35]将PAC和膜法联合使用, 18~20 mg·L-1优级PAC在60 min内便可将初始浓度为50 ng·L-1、200 ng·L-1的2-MIB和GSM去除至检出限以下, 且当温度从5℃升至20℃时, 异嗅化合物的去除率倍增.有研究发现, 葡萄籽油可使初始浓度为360 ng·L-1的GSM降低81%~83%[60], 具有发展潜力.UV可降解GSM和2-MIB[10], 但有副产物生成, 造成二次污染[32].
化学处理方法则是通过氧化作用降低水环境中异嗅化合物浓度[9, 23, 33].pH值为7.0~10.0时, 臭氧对2-MIB的去除效果显著[88].TiO2电化学降解法快速、有效[32].高铁酸钾在1 min之内可将DMTS去除92.5%, 效果远高于高锰酸钾(10 min去除率为74.6%)[38].过氧乙酸使部分致嗅微生物失活[39].Xu等[3]发现H2O2具有降解2-MIB的能力.Bu等[40]证实掺硼金刚石对GSM和2-MIB的降解效率高于其他活性阳极, 相较硝酸盐或高氯酸盐电解质, 硫酸盐电解质对异嗅化合物的降解速率更快.电流密度变大、pH降低有利于2-MIB和GSM的降解.但当氧化剂为Cl2和ClO2等时, 去除效果不佳[10, 41].由于异嗅化合物的挥发性、低浓度, 使得化学平衡难以实现.所以化学处理方法的去除效果并不理想[34].
因此, 有学者联合使用物理和化学处理方法.Ma等[10]采用UV/Cl法, 5min内异嗅化合物的去除率达90%, 紫外线强度升高, 游离氯剂量加大和酸性环境都可以显著加快去除速度并提高去除率.Luo等[42]利用UV/过硫酸盐工艺, 酸性环境下过硫酸盐剂量增加, 可显著去除2, 4, 6-TCA.而相较单独使用真空UV, 真空UV/过二硫酸盐会将GSM和2-MIB的去除率分别提高76%和74%[43].此外, 生物处理法也是去除水环境中异嗅化合物的有效措施[9], 相较以上方法更加经济与环保. 1974年, 有学者发现枯草芽孢杆菌和蜡状芽孢杆菌可去除GSM[32]. Sumitomo[89]的研究又发现假丝酵母菌具有去除2-MIB的能力.随后, 假单胞菌[90]、肠杆菌[91]、甲基科菌、草酸杆菌科细菌[92]、黄杆菌、球形芽孢杆菌、中华根瘤菌及寡养单胞菌[32]等陆续被证明能够降解GSM和2-MIB.有研究表明, 生物膜是移动床生物膜反应器反应过程中去除异嗅化合物的关键[5].针对异嗅化合物的控制, 需要继续创新, 挑战依旧艰巨.综上而言, 生物处理法最有应用前景, 但技术不成熟, 降解机制不明确, 仍需深入研究[32].
2 致嗅放线菌研究 2.1 致嗅放线菌分离培养迄今为止, 大多数致嗅放线菌是使用根据放线菌代谢特性和营养需求设计的培养基从环境样品中筛选得到的[93].高氏一号培养基被广泛使用, 常与重铬酸钾溶液联合使用, 选择性分离放线菌菌株[25, 44, 48, 74, 94~97].其他常见配方还有腐殖酸琼脂培养基[93]、国际链霉菌计划培养基2[44, 94, 97, 98]、基础盐培养基[27]和黄豆粉培养基[99~102]等(表 3).
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表 3 放线菌培养基类型 Table 3 Type of actinomycetes medium |
2.2 致嗅放线菌鉴定：形态学和分子生物学
放线菌具有菌丝体, 菌落呈放射状, 属革兰氏阳性菌[93].大多数学者利用放线菌选择培养基, 根据菌落形态和致嗅特性初步判定, 再利用基因测序技术进行鉴定, 筛选出致嗅放线菌[17]. 1997年, Heuer等[105]利用大肠杆菌基因位点的226~243片段作为放线菌特异性正向引物243F, 采用温度梯度凝胶电泳(TGGE)和变性梯度凝胶电泳(DGGE)评估放线菌在微生物群落中的相对丰度.Klausen等[28]采用16S rRNA鉴定放线菌菌株, 但由于特异性引物太宽而效果不佳.因此针对放线菌生物学和分类鉴定, 该学者提出必须使用特异性较窄的基因探针.随后, 放线菌高效特异性引物被陆续报道(表 4).Auffret等[106]建立实时定量PCR方法, 采用Streptomyces特异性引物AMgeoF/R, 鉴定致嗅放线菌菌种, 为Lindholm-Lehto等[39]的研究提供了科学依据.陈娇等[44]使用特异性引物S-C-Act-235-a-S-20F与S-C-Act-878-a-A-19R对放线菌菌种进行鉴定.随着基因测序技术的发展, 高通量测序的出现, 为放线菌产生异嗅化合物的相关基因的研究带来了希望, 将丰富对致嗅放线菌的存在、活性及潜在环境作用的认识[2].
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表 4 放线菌测序的特异性引物类型 Table 4 Specific primer types for actinomycete sequencing |
2.3 常见的致嗅放线菌类别
近年来, 水环境致嗅放线菌被陆续报道, 以链霉菌为主[67, 106, 108], 诺卡氏菌其次[28].表 5中为水生态系统中被发现的部分致嗅放线菌, 因栖息环境的物理、化学及生物等因素的不同, 致嗅能力也具有差异性.
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表 5 致嗅放线菌与异嗅物1) Table 5 Odor-producing actinomycetes and odorants |
2.4 典型水环境中致嗅放线菌分布情况
有研究发现, 几乎所有水环境中都广泛存在着放线菌, 如水循环养殖系统、湖泊、溪流、海洋和水源水库等典型水环境(表 6). 1994年, 日本学者便在富营养的Kasumigaura湖中发现两株具有致嗅能力的链霉菌[71], 又有研究人员于水产养殖场养鱼池沉积物中分离得到半链霉菌(Strreptomyces halstedii), 在麦芽糖为唯一碳源时, 半链霉菌致嗅能力加强[58], Auffret等[106]也分离出了两株可产生GSM和2-MIB的链霉菌.Klausen等[28]在溪流中发现了致嗅放线菌; 陈娇等[44]从水源水库中分离得到的40株放线菌均具有致嗅作用, Park等[69]则分离得到60株具有致嗅作用的放线菌均为链霉菌, 而在Zuo等[25]的研究中认为, 水源水库水样异嗅化合物浓度与沉积物相关, 这些研究均为水环境放线菌的存在提供了证据.在热带地区, 年均适宜的温度促使水环境异嗅事件频发[113].关于水环境放线菌的来源研究中, Asquith等[70]的研究猜测, 地表径流的冲刷作用, 大大丰富了水环境放线菌生物多样性及生物密度, 为水环境放线菌的来源提出了新的研究方向.而对于海生红球菌(Rhodococcus marinonascens)、海藻气单胞菌(Aeromicrobium marinum)、链霉菌(Streptomyces)、放线菌(Actinoplanetes)和微单孢菌属(Micromonospora)的生境更像是水环境[2, 17, 114].然而, 水生态系统相对于陆生环境中放线菌数量却显得极少[115].
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表 6 水环境致嗅放线菌研究 Table 6 Research on odor-producing actinomycetes in water ecosystems |
3 致嗅微生物分泌异嗅化合物的产嗅机制
对于2-MIB的生物合成途径, 多数学者认为与磷酸(GPP)有关, 且放线菌和蓝藻具有相似性[110, 112, 116].Komatsu等[110]将Markoy模型、蛋白质家族与基因序列比对相结合, 发现在GPP甲基转移酶和单萜环化酶的作用下, GPP即可转化为2-MIB.合成机制具体如下：①异戊烯基焦磷酸(isopentenyl diphosphate, IPP)和二甲基烯丙基焦磷酸(dimethylallyl diphosphate, DMAPP)在GPP合成酶催化下合成GPP, GPP又经GPP甲基转移酶转化为甲酰化GPP; ②Mg2+和单萜环化酶(2-MIB合成酶)将甲酰化GPP环化生成2-MIB.后有学者认为微生物可将死亡蓝藻细胞裂解后释放的二甲基磺基丙酯(dimethyl-sulfoniopropi-onate, DMSP)转化为DMDS, 部分DMDS继续被转化生成甲硫醚和甲硫醇.厌氧条件下, 微生物可直接将DMSP转化生成甲硫醇.此外, 胡萝卜素在溶解氧的诱导下可产生β-环柠檬醛[31].这为Zhang等[46]的研究提供了一定的科学依据, 证明了β-环柠檬醛浓度与胡萝卜素浓度的正相关性.Zhang等[54]进一步研究后提出, 苯酚邻甲基转移酶(CPOMTs)可将2, 4, 6-三氯苯酚(2, 4, 6-TCP)催化生成2, 4, 6-TCA, 在此过程中, 依赖性活性腺苷甲硫氨酸(SAM)和非依赖性SAM的CPOMTs均起到重要作用, 胞外CPOMTs也参与了2, 4, 6-TCA的生物合成.Tsao等[11]采用qPCR技术研究21株蓝藻, 证实基因geoA主导GSM的合成[6].在Suurnäkki等[65]的实验中同样验证了GSM和2-MIB的合成与基因geoA相关, 蓝藻中的geoA和2-MIB合成酶的基因序列与放线菌、变形杆菌的萜烯合成酶基因序列具有同源性.
Citron等[117]在放线菌中发现了55种GSM合成酶、23种2-MIB同系物合成酶及98种其他倍半萜烯环化酶同系物.Singh等[111]对Streptomyces peucetius基因组深入研究发现, 倍半萜烯合成酶基因表达时, 可使菌株产生大量GSM.Auffret等[106]对分离得到的7株致嗅链霉菌进行基因检测, 发现产生GSM的菌株中均发现geoA基因, 产生2-MIB的菌株中则发现tpc基因, Anuar等[27]的实验证实了这一结果.Lindholm-Lehto等[39]的研究也发现, 当GSM浓度较高时, geoA基因含量相应较高, 进一步确定基因geoA在GSM合成过程中的作用.此外, 有学者指出, 链霉菌的次生菌丝发育过程中会产生GSM和2-MIB[2].综上, 胞内部分异嗅化合物生物合成途径如图 2所示.然而, 菌体具体的泌嗅机制尚需进一步研究.
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图 2 细胞内部分异嗅化合物生物合成机制 Fig. 2 Biosynthesis mechanism of some taste and odor compounds in cells |
对于缓解全球水环境问题而言, 对异嗅化合物的控制至关重要.深入研究异嗅化合物产生和释放机制的规律, 是优化水质和缓减全球水环境异嗅问题的关键.国内外学者在异嗅化合物的种类、来源、尤其是控制措施有了较为全面地研究, 而随着异嗅问题的深入促使人们对控制措施要求更高, 其发展潜力在于微生物法控制上.迄今对于异嗅化合物影响因素的研究逐渐增多, 但影响因素体系尚未形成.虽然部分异嗅化合物生物合成途径的研究取得突破性进展, 随着高通量技术的发展, 仍需深入研究, 丰富生物合成层面的知识.至今与水环境致嗅放线菌相关的研究, 仍存在大量问题, 亟待解决：①尚不明确水源水库致嗅放线菌是否完全来源于陆生环境, 对于异嗅味贡献报道缺乏; ②致嗅放线菌的快速有效鉴定方法缺乏、致嗅途径及系统发育树研究不足; ③致嗅放线菌与异嗅化合物的相关性尚未确定等.综上, 对于异嗅化合物的全面分析有助于水环境异嗅问题的治理, 后续研究应具有侧重点：①深入研究微生物控制异嗅化合物的机制, 实现应用价值; ②系统分析异嗅化合物影响因素, 从生境层面上调控异嗅味; ③探索致嗅微生物新的代谢途径, 基于合成生物学技术研发异嗅物分解工程菌, 用于控制工程系统中的异嗅物质.
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