顧浩天 袁永達(dá) 張?zhí)熹? 滕海媛 常曉麗 王冬生
摘要:蚯蚓是土壤生態(tài)系統(tǒng)中最大的動物區(qū)系,在維持土壤理化結(jié)構(gòu)、提高土壤肥力、促進(jìn)有機(jī)質(zhì)降解及營養(yǎng)礦化循環(huán)、改善微生物群落等方面有重要生態(tài)作用。此外,蚯蚓可以抵御高濃度土壤污染,對無機(jī)物和有機(jī)物污染均有良好的修復(fù)效果,利用其修復(fù)污染土壤具有環(huán)境友好、高效、低成本等優(yōu)勢。蚯蚓對土壤污染物的吸收積累、轉(zhuǎn)化降解作用較為復(fù)雜且與多種生物、非生物因素相關(guān)。本文綜述了蚯蚓修復(fù)土壤污染的潛能、影響因素及機(jī)理,同時展望蚯蚓修復(fù)的應(yīng)用前景。
關(guān)鍵詞:蚯蚓;土壤;污染修復(fù);重金屬;有機(jī)污染物;機(jī)理
中圖分類號:X53 ??文獻(xiàn)標(biāo)志碼: A
文章編號:1002-1302(2021)20-0030-10
收稿日期:2021-01-08
基金項(xiàng)目:上海市科技興農(nóng)重點(diǎn)推廣項(xiàng)目[編號:滬農(nóng)科推字(2018)第4-14號]。
作者簡介:顧浩天(1993—),男,遼寧阜新人,碩士,研究實(shí)習(xí)員,主要從事農(nóng)藥生態(tài)毒理效應(yīng)及環(huán)境風(fēng)險、農(nóng)業(yè)害蟲生理分子調(diào)控機(jī)理相關(guān)研究,E-mail:guhaotian@saas.sh.cn;共同第一作者:袁永達(dá)(1972—),男,上海人,碩士,副研究員,主要從事農(nóng)藥學(xué)、農(nóng)業(yè)昆蟲與害蟲防治相關(guān)研究。
通信作者:王冬生,碩士,研究員,主要從事農(nóng)藥學(xué)、農(nóng)業(yè)昆蟲與害蟲防治相關(guān)研究。E-mail:zb3@saas.sh.cn。
近年來,土壤污染隨著城鎮(zhèn)化、工業(yè)化的發(fā)展日趨嚴(yán)峻,已成為全球普遍關(guān)注的熱點(diǎn)問題[1]。土壤污染物會對生物的生長、繁殖、多樣性等造成負(fù)面影響,甚至通過食物鏈的富集級聯(lián)威脅人類健康[2]。Zeng等調(diào)查了我國1 781個農(nóng)田土壤樣點(diǎn)的5 597個樣品復(fù)合污染的狀況,結(jié)果表明:我國農(nóng)田土壤受污染的比例為22.1%,其中1.23%的土壤樣品為重度污染[3]。因此,消除或減輕土壤污染問題迫在眉睫[4]。
常見的土壤污染修復(fù)技術(shù)有物理、化學(xué)和生物修復(fù)技術(shù)[5]。生物修復(fù)技術(shù)旨在利用微生物、植物、動物降解或清除污染物[6]?,F(xiàn)階段,生物修復(fù)的研究大多關(guān)注微生物修復(fù)[7]、植物修復(fù)[1],而利用動物修復(fù)土壤的研究相對較少。蚯蚓修復(fù)旨在利用蚯蚓降解土壤污染物[8],可作為土壤生物修復(fù)的替代技術(shù)[5]。蚯蚓是土壤生態(tài)系統(tǒng)中生物量最大的無脊椎動物,因其具有較強(qiáng)的環(huán)境適應(yīng)性、生殖力及對有機(jī)污染物表現(xiàn)出極強(qiáng)的耐性和抗性等優(yōu)勢[9],被廣泛地應(yīng)用于土壤污染生物修復(fù)中。蚯蚓掘穴、取食、呼吸、排泄等生理活動不僅可以改良土壤結(jié)構(gòu)、理化性質(zhì),還可以改變土壤污染物的遷移轉(zhuǎn)化、生物有效性及空間分布[10-11]。蚯蚓修復(fù)的機(jī)理主要為通過取食或表皮從土壤中吸收有毒有害物質(zhì)[12];通過增加土壤微生物活性、營養(yǎng)物質(zhì)生物有效性、植物生長等方式提高微生物修復(fù)或植物修復(fù)效率[13]。
已有學(xué)者研究了蚯蚓對土壤重金屬[14]、多環(huán)芳烴(PAHs)[15]、農(nóng)藥[16]等的降解修復(fù),但其中相關(guān)機(jī)理仍不清楚。鑒于此,本文綜述了蚯蚓對污染土壤的修復(fù)作用與機(jī)理。同時對蚯蚓修復(fù)技術(shù)的應(yīng)用前景進(jìn)行了展望,以期為未來研究蚯蚓修復(fù)技術(shù)提供參考和依據(jù)。
1 利用蚯蚓修復(fù)污染土壤
1.1 消除土壤中重金屬污染
蚯蚓對土壤中的重金屬污染具有修復(fù)潛力[17]。蚯蚓腸組織中的黃色細(xì)胞(chloragogenous tissue)可以吸收較高濃度的重金屬[18],且隨著環(huán)境中銅(Cu)和鎘(Cd)濃度升高,其在蚯蚓體內(nèi)的含量也逐漸升高[19]。Lavelle研究表明,蚯蚓黏液中含有—COOH、—NH2、—CO等活性基團(tuán),能夠絡(luò)合、螯合重金屬,提高土壤中重金屬的活性[20]。Suthar等發(fā)現(xiàn)Eisenia fetida體內(nèi)可積累高濃度的重金屬,如鉛(Pb)、Cd、鉻(Cr)、Cu[21]。Suthar利用E. fetida處理釀酒廠淤泥后發(fā)現(xiàn)體內(nèi)組織中鋅(Zn)、錳(Mn)、Cu含量顯著升高,而環(huán)境中的含量分別減少32%、31.8%、43.5%[22]。與上述結(jié)論一致,Wu等報道E. fetida處理Cd污染土壤后,消除了土壤環(huán)境中17.60% Cd含量[23]。
蚯蚓對重金屬的生物積累作用也與重金屬種類及形態(tài)相關(guān)。Cd、汞(Hg)、Zn因其生物富集系數(shù)(BAF)顯著>1.0而易被蚯蚓吸收積累[24]。Nannoni等發(fā)現(xiàn)在蚯蚓Allolobophora rosea和Nicodrilus caliginosus中,幾種重金屬的生物富集系數(shù)從大到小依次為Cd > Zn > Cu>As=Pb=Sb[25]。lvarez等發(fā)現(xiàn)單甲基汞(monomethyl-mercury,MeHg)的生物富集系數(shù)遠(yuǎn)高于Hg[26],表明甲基化汞由于其高脂溶性更易被蚯蚓吸收積累[27]。蚯蚓對不同重金屬的吸收也取決于重金屬的生物有效性[28]。重金屬的生物有效性受多種環(huán)境因素影響,如重金屬的溶解性、與土壤有機(jī)質(zhì)絡(luò)合情況[29]等。
生物富集也與土壤性質(zhì)、暴露時間及蚯蚓種類相關(guān)。Davies等報道E. fetida對Pb的吸收量與暴露時間成正比,且外源環(huán)境中Pb濃度越高,蚯蚓對Pb的積累速率越快[30]。Nannoni 等證明蚯蚓對重金屬的積累能力受土壤有機(jī)碳、碳酸鹽含量等理化性質(zhì)的影響[31]。Wang等在我國湖南省原位污染的土壤中評估了Metaphire californica、Amynthas homochaetus、Amynthas pecteniferus、Amynthas heterochaetus幾種蚯蚓對重金屬吸收積累能力的差異,并發(fā)現(xiàn)生物富集系數(shù)從大到小為Cd (10.6~18.8) > Zn (1.15~1.75)>Cu (1.01~1.35)>Pb (0.56~0.95)[32];且相同生態(tài)類群的蚯蚓對重金屬的生物富集系數(shù)接近[31-32],不同生態(tài)類群蚯蚓對重金屬污染的耐受性不同[33-34]。此外,重金屬在不同食料中的分布影響其在不同生態(tài)類群蚯蚓體內(nèi)的積累,表明食物選擇性和生態(tài)位分化對生物富集重金屬的影響[35]。因此,評估重金屬對蚯蚓生物積累的影響時,要考慮多種蚯蚓生態(tài)類群及重金屬在環(huán)境中的本底濃度。利用蚯蚓修復(fù)重金屬污染的研究結(jié)果如表1所示。
1.2 消除土壤中的有機(jī)污染物
土壤有機(jī)污染因其持久性、蓄積性、“三致”效應(yīng)、毒性效應(yīng)等特點(diǎn)而引起研究者們的廣泛關(guān)注。我國農(nóng)田土壤除受到傳統(tǒng)有機(jī)污染物如多氯聯(lián)苯(PCBs)、多環(huán)芳烴(PAHs)、農(nóng)藥、石油烴等污染外,還受到多種新型有機(jī)污染物如抗生素、酞酸酯(PAEs)、全氟化合物(PFASs)、微塑料等的污染[38]。研究表明,蚯蚓能夠促進(jìn)土壤中多種有機(jī)污染物的降解(表2),在土壤有機(jī)污染生物修復(fù)方面具有廣闊的應(yīng)用前景。
蚯蚓可以通過增加土壤通氣性促進(jìn)PCBs降解微生物的擴(kuò)散分布,同時增加土壤中的碳、氮含量,改良土壤微生物群落[39]。蚯蚓生物堆肥處理使基質(zhì)中總PCB含量降低55%~66%,而蚯蚓體內(nèi)PCBs水平顯著增加,表明PCBs主要被蚯蚓吸收富集[40]。Luepromchai等進(jìn)行土壤柱試驗(yàn),采用蚯蚓+投菌法可以消除9 cm表層土柱中約50% PCBs,而單獨(dú)采用投菌法或添加蚯蚓僅能消除3 cm表層土柱中的PCBs。因此,蚯蚓不僅促進(jìn)了PCBs降解菌的傳播擴(kuò)散,同時為微生物的生長和活動提供了適宜的環(huán)境條件,有利于土壤中PCBs污染的修復(fù)[41]。
蚯蚓通過取食含有有機(jī)污泥或廢塑料的土壤而富集多溴聯(lián)苯醚(PBDEs)[42]。有報道稱PBDEs在蚓糞中含量比非根際土壤中低40%[43],表明PBDEs主要被蚯蚓吸收富集。外源添加Pb可以降低PBDEs的生物有效性,同時減少蚯蚓腸道吸收土壤中的PBDEs。Li等報道高劑量Pb會破壞蚯蚓細(xì)胞膜而增加滲透性[44];Zhang等發(fā)現(xiàn)250 mg/kg Pb暴露誘導(dǎo)更多BDE209被蚯蚓表皮吸收;而當(dāng)Pb濃度為 500 mg/kg 時,蚯蚓體內(nèi)吸收的BDE209含量下降[45]。
蚯蚓可以通過表皮被動吸收土壤中可溶態(tài)的PAHs,或經(jīng)主動攝食由腸道吸收消化土壤中的PAHs[46-47]。蚯蚓通過排泄易降解的碳來增加微生物活性,進(jìn)而促進(jìn)土壤中PAHs的降解;同時通過取食、腸道消化吸收、形成土壤團(tuán)聚體等間接影響微生物群落的結(jié)構(gòu)和活性[15]。此外,蚯蚓活動不斷混合擾動土壤,改善土壤的需氧條件進(jìn)而加劇PAHs的降解過程[46]。E. fetida可以消除土壤中93%的蒽含量[48];在多環(huán)芳烴污染土壤中分別消除約91%芘、99%菲、91%蒽、16%苯并[a]芘(BaP)、43%熒蒽[49]。Hernández-Castellanos等發(fā)現(xiàn)培育112 d,蚯蚓Pontoscolex corethrurus從滅菌土壤中消除了26.6 mg/kg苯并[a]芘;而將Pontoscolex corethrurus添加到非滅菌土壤中,約有36.1 mg/kg BaP被消解,表明土壤微生物對BaP也具有降解作用[50]。因此,蚯蚓不僅增加了PAHs的生物有效性,同時提高了微生物對PAHs的消除能力[49,51]。蚯蚓對PAHs的生物積累也取決于蚯蚓種類,Parrish等研究發(fā)現(xiàn)蚯蚓E. fetida、Lumbricus terrestris可分別從PAHs污染土壤中吸收0.204、0.084 μg/g的PAHs[52]。
蚯蚓能夠促進(jìn)除草劑、殺蟲劑等多種農(nóng)藥的降解[53-54]。Shan等研究蚯蚓Metaphire guillelmi糞便對二氯苯酚(2,4-DCP)、三氯苯酚(2,4,6-TCP)、五氯苯酚(PCP)的吸附能力,結(jié)果發(fā)現(xiàn)0、7、30 d后蚓糞對3種農(nóng)藥的吸附能力依次為PCP>2,4-DCP>2,4,6-TCP[55]。此外,蚯蚓也可以通過取食活動改變氯酚類化合物的生物有效性及環(huán)境歸宿[54]。Schreck等發(fā)現(xiàn)添加蚯蚓消除了土壤中80%以上的毒死蜱、氯氟氰菊酯、滅菌丹、甲霜靈、腈菌唑等農(nóng)藥[56]。蚯蚓通過取食、轉(zhuǎn)運(yùn)土壤表面除草劑及促進(jìn)土壤與除草劑吸附結(jié)合,從而降低除草劑的淋溶風(fēng)險[57]。Lumbricus terrestris和 Aporrectodea caliginosa不僅可以轉(zhuǎn)運(yùn)污染物,還能改變莠去津的礦物化和吸附性。培育86 d后,蚯蚓將莠去津的礦物化成分14CO2—C含量從15.2% 降低至117%。同時,蚯蚓促進(jìn)土壤中結(jié)合態(tài)莠去津形成[58]。不同種蚯蚓對莠去津的生物積累能力不同。當(dāng)暴露于 4.25 mg/kg 莠去津污染的土壤中,深棲類蚯蚓Metaphire guillelmi的生物富集系數(shù)約是表?xiàng)怑. fetida的5倍,可能由于不同蚯蚓對受試物的吸收機(jī)制不同。M. guillelm的生物富集途徑主要是腸道吸收,而E. fetida則主要通過表皮吸收途徑[59]。同理,內(nèi)棲類Amynthas robustus和表?xiàng)怑. fetida可以顯著消除土壤中雙對氯苯基三氯乙烷(DDT)污染,且 A. robustus具有相對較高的消除潛力[60]。
蚯蚓修復(fù)的效率與石油的組成、濃度、土壤中微生物群落、蚯蚓種類相關(guān)。蚯蚓活動混合土壤、增加土壤透氣性,從而促進(jìn)石油消解過程。Schaefer等用3種不同生態(tài)型的蚯蚓E. fetida、Allolobophora chlorotica、Lumbricus terrestris修復(fù)石油烴污染土壤,石油烴的降解速度明顯加快,且L. terrestris的降解效果優(yōu)于其他2種蚯蚓[61]。Chachina等采用蚯蚓、光合細(xì)菌、固氮細(xì)菌及真菌聯(lián)合修復(fù)方法后土壤中機(jī)油消除效率高達(dá)99.9%,是無蚯蚓對照組的4~10倍[62]。同理,只使用植物修復(fù)對消除土壤中柴油污染效果不佳,而添加蚯蚓后表層土中柴油含量減少43%,深層土中柴油量減少52%[63]。蚯蚓對總石油烴(TPH)的降解速度約為90 mg/d[64]。對中濃度石油烴污染(<4 000 mg/kg)可考慮使用蚯蚓進(jìn)行原位修復(fù)處理[61]。但當(dāng)石油烴濃度高于 4 000 mg/kg 時,投放蚯蚓的死亡率上升,對生存生長造成不利影響[65]。因此,須注意利用蚯蚓來修復(fù)石油污染的土壤具有一定局限性。
2 蚯蚓修復(fù)過程及機(jī)理
蚯蚓對土壤有機(jī)物及重金屬污染均有良好修復(fù)效果。蚯蚓對污染物的吸附、積累、消除作用較為復(fù)雜且與多種因素相關(guān)[8]。蚯蚓修復(fù)過程包括對污染物的吸收、轉(zhuǎn)化及降解,其中涉及的機(jī)理分為內(nèi)在和外在機(jī)理。內(nèi)在機(jī)理包括改善土壤理化性質(zhì)、刺激土壤微生物生長、影響微生物活性和代謝、提高植物吸收率等。外在機(jī)理包括蚯蚓生理活動,蚯蚓對污染物形態(tài)、遷移及生物有效性的影響等。如蚯蚓可以直接通過掘穴活動有氧氧化DDT脫氯化氫后形成DDE,同時腸道厭氧還原DDT或DDE形成DDD或DDMU[2,2-雙-(對氯苯基)-1-氯乙烯];也可以通過
間接改變土壤理化性質(zhì),刺激本土DDT降解菌的生長和活性,從而促進(jìn)DDT污染的消除[73]。
2.1 蚯蚓吸收積累污染物
蚯蚓從環(huán)境中吸收污染物到自身體內(nèi)的過程稱為蚯蚓生物積累[5]。蚯蚓通過腸道消化吸收或表皮接觸吸附等途徑從環(huán)境中吸收污染物。在腸道消化吸收中,含污染物的土壤經(jīng)E. fetida取食后被腸道消化,從而被其他組織吸附(攝食作用);在表皮接觸吸收中,污染物的電化學(xué)勢降低而被蚯蚓體壁吸收,隨后在蚯蚓體內(nèi)各器官組織間遷移(擴(kuò)散作用)[12]。
污染物類型是決定蚯蚓生物積累效率的關(guān)鍵因素[8]。Sizmur 等調(diào)查了Lumbricus terrestris對土壤中As、Cu、Pb和Zn遷移性和形態(tài)的影響,結(jié)果表明蚯蚓對重金屬含量、存在形態(tài)的影響與土壤性質(zhì)和重金屬類型相關(guān)[74]。Sizmur等調(diào)查了表?xiàng)怑. veneta、深棲類L. terrestris、內(nèi)棲類A. chlorotica對金屬遷移率和生物有效性的影響。結(jié)果表明,與無蚯蚓對照組相比,蚯蚓顯著增加了滲濾液中重金屬含量及離子活性[75]。其中,L. terrestris處理效果最為顯著,分別將滲濾液中As、Cu、Pb、Zn濃度提高了267%、393%、190%、429%[75]。A. rosea、E. fetida、L. mauritii、N. caliginosus通過體內(nèi)金屬硫蛋白(MT)結(jié)合吸收重金屬污染物[76]。金屬硫蛋白是調(diào)節(jié)蚯蚓腸道內(nèi)必需及非必需金屬離子動態(tài)的一類小分子蛋白,其分子量約6~7 ku[77],可以與Cu2+、Zn2+、Mn2+等重金屬離子結(jié)合[78]。此外,土壤理化性質(zhì)是影響重金屬吸附-解吸附行為及生物有效性的重要因素。Huang等報道土壤pH值與重金屬生物有效性顯著負(fù)相關(guān)[79]。施用糞肥增加土壤可溶性有機(jī)碳(DOC)含量,可促進(jìn)根圍土壤的重金屬生物有效性增加[80]。Wen等發(fā)現(xiàn)蚯蚓活動通過提高土壤pH值、水溶性重金屬組分和可溶性有機(jī)碳(DOC)含量,進(jìn)而增加了重金屬的遷移性和生物有效性;同時蚯蚓活動還促進(jìn)了微生物種群增加、小麥生長,增加了小麥中重金屬含量[81]。Liu等發(fā)現(xiàn),隨著暴露時間增加溴敵隆逐漸被蚯蚓富集,且生物富集因子(BSAFs)隨著土壤中溴敵隆濃度的增加而減小;造成這種現(xiàn)象的原因可能是當(dāng)疏水性有機(jī)物在土壤和沉積物中濃度較高時,生物體內(nèi)可利用的結(jié)合位點(diǎn)達(dá)到了飽和,并且產(chǎn)生了不可逆解吸現(xiàn)象[82]。
2.2 蚯蚓轉(zhuǎn)化降解污染物
蚯蚓轉(zhuǎn)化是蚯蚓修復(fù)的重要機(jī)理之一,指通過蚯蚓和微生物的聯(lián)合作用將可生物降解的固體廢物轉(zhuǎn)化為肥料的過程[5]。蚯蚓通過腸道酶和微生物轉(zhuǎn)化污染物,并將其以蚓糞形式排泄到土壤中[83]。Yang等報道蚯蚓和微生物消除土壤中687%菲污染,同時通過氧合作用將菲轉(zhuǎn)化為原兒茶酸[84]。Gu等發(fā)現(xiàn)M. guillelmi可以將四溴雙酚A(TBBPA)轉(zhuǎn)化為代謝產(chǎn)物或固定為殘留物。TBBPA進(jìn)入蚯蚓體內(nèi)后經(jīng)循環(huán)系統(tǒng)轉(zhuǎn)移至其他組織,在環(huán)帶組織中被迅速轉(zhuǎn)化為毒性較低的二甲基四溴雙酚A醚,隨后以蚓糞排出體外。M.guillelmi對TBBPA的解毒作用主要是通過在消化道產(chǎn)生 O-甲基化代謝產(chǎn)物及形成結(jié)合態(tài)產(chǎn)物實(shí)現(xiàn)的[85]。蚯蚓對污染物的轉(zhuǎn)化需體內(nèi)多種酶共同參與[86]。蚯蚓體內(nèi)轉(zhuǎn)化代謝污染物的酶主要有羧酸酯酶、谷胱甘肽硫轉(zhuǎn)移酶、細(xì)胞色素P450等[83]。Sanchez-Hernandez等發(fā)現(xiàn)L. terrestris腸道微環(huán)境是土壤羧酸酯酶活性的主要來源,且蚓糞能增加土壤中酯酶活性。蚯蚓處理12周后,土壤羧酸酯酶活性相比對照組高2~4倍,表明蚯蚓通過增加土壤酶活性,從而促進(jìn)農(nóng)藥污染土壤的修復(fù)[87]。
蚯蚓降解指蚯蚓利用腸道微生物或體內(nèi)酶系統(tǒng),如細(xì)胞色素酶(CYP450)、過氧化物酶(POD)、羧酸酯酶(CarE)、谷胱甘肽硫轉(zhuǎn)移酶(GST)降解不同的污染物[5]。研究表明CYP450能夠通過芳香環(huán)斷裂的方式降解PAHs、PCBs、農(nóng)藥等有機(jī)污染物[88]。Sanchez-Hernandez等報道A. caliginosa通過Ⅰ相酶CYP450和Ⅱ相酶GST解毒酶共同降解土壤毒死蜱污染[89]。體內(nèi)酶的氧化與水解是典型的相Ⅰ反應(yīng),反應(yīng)產(chǎn)物與內(nèi)源分子結(jié)合進(jìn)行相Ⅱ反應(yīng)[83]。E. fetida可以將PAHs代謝為相Ⅱ共軛物,代謝產(chǎn)物通常具有更高的水溶性和生物有效性,因此更易被蚯蚓和植物吸收消除[90]。
蚯蚓堆肥主要通過蚯蚓和微生物間的互作固定并氧化有機(jī)質(zhì),是降解有機(jī)污染物的有效方法之一[91]。添加蚯蚓后增加了污染物的異化分解代謝,且蚯蚓分解的小分子碳、分泌的黏液、排泄的蚓糞等均有助于土壤微生物的生長繁殖[55]。在土壤和堆肥的混合物中添加蚯蚓后,菲的礦化程度從68%提高到86%,表明蚯蚓+堆肥處理顯著增加了污染物的異化代謝,從而降低菲污染[92]。Dendrobaena veneta+堆肥也增加了土壤中微生物對柴油污染的消解[93]。E. fetida+堆肥處理60 d后亞甲藍(lán)消除率高達(dá)98%[94]。
2.3 與蚯蚓相關(guān)的微生物
蚯蚓腸道微生物與蚯蚓是互利共生關(guān)系,兩者間的互作可以共同促進(jìn)土壤有機(jī)質(zhì)及污染物的降解,也會影響彼此種群、群落的變化及分泌的酶[95]。微生物降解污染物主要利用微生物分泌的胞外酶降解以及污染物被微生物吸收到細(xì)胞內(nèi)由胞內(nèi)酶降解[96]。如腸道細(xì)菌Bacillus licheniformis菌株KX657843分泌的胞外聚合物(EPS)可以吸附Cu(Ⅱ)、 Zn(Ⅱ),且最大吸附能力分別為58.82、5245 mg/g[97]。近年來,E. fetida腸道中需氧和厭氧細(xì)菌群落相繼被研究發(fā)現(xiàn),需氧菌為Aeromonas、Bacillus、Photobacterium、Pseudomonas、Shewanella等菌屬,厭氧菌為Aeromonas、Bacillus、Shewanella、Paenibacillus、Clostridium、 Cellulosimicrobium、Streptomyces、Chloroflexi等菌屬[98]。其中,Bacillus、Clostridium、Pseudomonas、Streptomyces和Shewanella等可以降解有機(jī)污染物[99-100];Pseudomonas、Alcaligenes和Acidobacterium等可以降解烴類及其他有機(jī)污染物[101-102];Pseudomonas、Acidobacterium、Penicillium、Mucor、Aspergillus等可提高PAHs的消除率[103-104];腸道細(xì)菌Hydrogenophaga和Streptomyces可以降解DDT[73]。Zhang等從Pheretima tschiliensis腸道中分離出真菌Trichoderma brevicompactum QYCD-6,并發(fā)現(xiàn)該微生物對多種重金屬脅迫具有耐受性和良好的消除效果,其中對Pb(Ⅱ)的消除效率最高,達(dá)到97.5%[105]。
添加蚯蚓能夠提高土壤微生物的活性和分解能力,進(jìn)而促進(jìn)污染物的降解[69]。E. fetida+A. caliginosa+稻秸稈聯(lián)合處理后,Pseudomonas、Luteimonas、Rhodanobacter、Sphingomonas、Gemmatimonas、Flavobacterium、Leifsonia等細(xì)菌群落生物量顯著增加,而菲濃度顯著下降,表明蚯蚓+秸稈通過增加微生物群落多樣性及生物量促進(jìn)菲的降解[106]。Lin等報道蚯蚓堆肥處理42 d后微生物群落結(jié)構(gòu)發(fā)生顯著變化,PCP被微生物顯著降解。采用系統(tǒng)進(jìn)化及序列分析鑒定其屬于六大細(xì)菌家族:鞘脂桿菌(Sphingobacteriaceae)、黃桿菌(Flavobacteriaceae)、假單胞菌(Pseudomonadaceae)、細(xì)菌TM7、黃單胞菌(Xanthomonadaceae)、Opitutaceae及四大真菌家族[毛霉菌(Mucoraceae)、Tremellaceae、Trichocomaceae、Hypocreaceae][69]。Liu等報道蚯蚓能夠刺激除草劑2-甲基-4-氯苯氧乙酸(MCPA)降解菌的生長和活性,細(xì)菌Xanthomonadaceae在該過程中起主要降解作用[4]。添加蚯蚓顯著刺激了變形菌門微生物的生長,且不同類型的變形菌門均能將萘、蒽、菲等多環(huán)芳烴污染物完全礦化降解[107-108]。
蚯蚓可以通過取食直接調(diào)節(jié)細(xì)菌群落[109],也可以將土壤中高分子有機(jī)物分解為小分子化合物,繼而被污染物降解菌作為碳源利用[110]。Tiunov等在L. terrestris的洞穴中發(fā)現(xiàn)紅球菌(Rhodococcus)與固氮菌(Azotobacter)[111],且Rhodococcus可以將蒽、菲、芘、熒蒽等化學(xué)污染物作為碳和能量的唯一來源[112]。
綜上所述,蚯蚓自身的生命活動及腸道內(nèi)功能微生物可共同促進(jìn)外源污染物的降解轉(zhuǎn)化,但兩者在該過程中的貢獻(xiàn)難以衡量[83]。因此,未來研究應(yīng)側(cè)重解析污染物消解過程所涉及的機(jī)理。代謝組學(xué)、毒理基因組學(xué)、轉(zhuǎn)錄組學(xué)等有助于闡明蚯蚓-微生物對污染物降解的作用機(jī)理[8]。
3 小結(jié)與展望
蚯蚓修復(fù)作為一種環(huán)境友好的技術(shù)近年來發(fā)展迅速,本文綜述了蚯蚓對不同類型土壤污染物的修復(fù)潛力及機(jī)理。筆者認(rèn)為以下方面將成為該領(lǐng)域研究的重點(diǎn):(1)不同類型污染物對蚯蚓的毒性風(fēng)險研究;(2)蚯蚓響應(yīng)污染物脅迫的機(jī)理研究;(3)利用分子生物及基因工程技術(shù)改造蚯蚓基因,篩選培育超累積高耐性的蚯蚓品種;(4)采用強(qiáng)化策略提高污染物的生物有效性;(5)采用微生物+動物或微生物+動物+植物的協(xié)同修復(fù)策略;(6)利用不同生態(tài)群的蚯蚓品種來消除不同類型的污染物?,F(xiàn)階段蚯蚓修復(fù)技術(shù)仍處于實(shí)驗(yàn)室模擬階段,如何將這項(xiàng)技術(shù)切實(shí)有效地應(yīng)用在實(shí)際污染場地,在大田進(jìn)行推廣、應(yīng)用、示范,亟需開展大量的研究。
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