丁巧蓓, 晁元卿, 王詩忠, 陳燕玫, 仇榮亮
(中山大學(xué)環(huán)境科學(xué)與工程學(xué)院,廣州 510275)
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根際微生物群落多樣性在重金屬土壤修復(fù)中的研究
丁巧蓓, 晁元卿*, 王詩忠, 陳燕玫, 仇榮亮*
(中山大學(xué)環(huán)境科學(xué)與工程學(xué)院,廣州 510275)
摘要:植物根際微生物在植物修復(fù)重金屬污染土壤時(shí)分泌的激素、鐵載體、ACC脫氨酶、黃酮類化合物和酚酸類等有機(jī)物具有增強(qiáng)植物生長、促進(jìn)植物根際對(duì)重金屬吸收、轉(zhuǎn)運(yùn)和積累的作用,同時(shí)促進(jìn)適應(yīng)相應(yīng)根際環(huán)境的功能微生物群落的建立. 文章結(jié)合作者課題組的研究結(jié)果,概述根際微生物在植物修復(fù)重金屬污染土壤過程中的作用,總結(jié)了根際細(xì)菌、真菌、古菌在植物修復(fù)中的作用,進(jìn)一步分析了土壤污染類型、改良劑、根際植物的種類等對(duì)根際微生物活動(dòng)的影響,對(duì)今后植物修復(fù)重金屬污染土壤過程中與根際微生物作用相關(guān)的研究進(jìn)行了展望.
關(guān)鍵詞:重金屬污染土壤; 植物修復(fù); 根際微生物; 微生物群落
2014年《全國土壤污染狀況調(diào)查公報(bào)》[1]指出,我國土壤環(huán)境狀況總體不容樂觀,耕地點(diǎn)位超標(biāo)率為19.4%.污染類型以無機(jī)型為主,占全部超標(biāo)點(diǎn)位的82.8%,主要無機(jī)污染物包括Cd、Hg、As、Cu、Pb、Cr、Zn、Ni等重金屬.在全國24個(gè)省份(城市)調(diào)查的所有污染農(nóng)作物種植地中,重金屬污染土壤占總面積80%,全國遭受不同程度重金屬污染的耕地面積已接近2 000萬hm2,約占耕地總面積1/5.嚴(yán)重的土壤重金屬污染對(duì)生態(tài)環(huán)境和人體健康構(gòu)成巨大威脅,已經(jīng)成為制約區(qū)域土地可持續(xù)開發(fā)利用和影響生態(tài)安全的關(guān)鍵因素.因此,對(duì)土壤重金屬污染的修復(fù)治理勢在必行.
較之傳統(tǒng)物理化學(xué)方法,植物修復(fù)因治理過程原位、治理成本低、管理與操作簡易以及環(huán)境美學(xué)兼容等不可替代的優(yōu)勢而受到重視[2]. 植物修復(fù)包括植物提取和植物穩(wěn)定過程.植物提取主要是通過植物根系吸收可溶性重金屬并將其轉(zhuǎn)移、貯存到植物莖葉,然后收割莖葉以達(dá)到去除土壤重金屬目的(圖1A).植物穩(wěn)定主要是通過植物根系的吸收、螯合、沉淀及還原作用將高毒性形態(tài)轉(zhuǎn)化為低毒形態(tài),將重金屬固定在根系和根際土中[3].同時(shí)植物根際分泌物能通過改變根際環(huán)境的pH或Eh,使有毒重金屬的形態(tài)發(fā)生改變,如在根際菌的作用下可將Cr6+還原為Cr3+,從而降低Cr的毒性(圖1B).然而在實(shí)際應(yīng)用中,植物修復(fù)技術(shù)具有修復(fù)周期長、對(duì)重金屬深層污染修復(fù)困難、植株成活率低的缺點(diǎn)[4],修復(fù)過程中常利用化學(xué)修復(fù)與微生物修復(fù)技術(shù)以輔助植物修復(fù),其中微生物修復(fù)技術(shù)中的根際微生物在植物修復(fù)中發(fā)揮重要作用.
根際微生物是指在植物根系直接影響的土壤范圍內(nèi)生長繁殖的微生物,能通過定殖、競爭、共生和調(diào)節(jié)土壤的營養(yǎng)動(dòng)態(tài)等活動(dòng)影響植物的生長[5].根際微生物對(duì)于重金屬污染土壤植物修復(fù)效率的提高具有重要的輔助作用[6],主要通過合成、分泌植物促生物質(zhì)增加植物生物量,同時(shí)通過功能酶、鐵載體、有機(jī)酸及生物表面活性劑等物質(zhì)活化土壤重金屬(圖1C),從而增強(qiáng)植物提取效果[7].此外,根際微生物還能通過生物吸附和富集作用、溶解和沉淀作用、氧化還原作用改變重金屬離子在土壤中的貯存形式,以緩解重金屬對(duì)植物的毒害[8].
圖1 根際微生物在植物修復(fù)重金屬污染土壤的作用機(jī)制
本文系統(tǒng)介紹了在植物修復(fù)過程中根際微生物的種類、影響根際微生物修復(fù)重金屬污染的因素、關(guān)于根際微生物在植物修復(fù)重金屬研究的主要觀點(diǎn)與不足,對(duì)根際微生物在植物修復(fù)重金屬污染土壤中的研究現(xiàn)狀進(jìn)行分析,并對(duì)今后植物修復(fù)重金屬污染土壤過程中根際微生物作用的相關(guān)研究進(jìn)行了總結(jié)和展望.
1根際微生物在植物修復(fù)過程中的群落多樣性表征
大多數(shù)根際微生物對(duì)重金屬產(chǎn)生敏感性反應(yīng),即根際微生物接觸重金屬后會(huì)產(chǎn)生應(yīng)激作用,通過分泌有機(jī)酸類物質(zhì)與重金屬結(jié)合,抑制重金屬對(duì)植物的毒害[9].利用微生物群落的敏感性反應(yīng)輔助植物修復(fù)重金屬污染土壤,不僅克服單一功能微生物修復(fù)過程中微生物難存活與難收集的缺點(diǎn),還可改善植物修復(fù)效果,具有廣闊的應(yīng)用前景[10].有研究證實(shí),不同類型微生物在植物根際的功能不同[11],主要體現(xiàn)在根際細(xì)菌是通過化學(xué)反應(yīng)改變重金屬的價(jià)態(tài)及存在形態(tài)(絡(luò)合態(tài)、脫烷基),從而降低重金屬毒性,根際真菌主要是輔助宿主植物獲取更多的礦質(zhì)營養(yǎng)[12],促進(jìn)寄主植物在重金屬污染土壤中的生長,增加其生物量,根際古菌雖存在于重金屬污染土壤但其主要功能并不明晰[13].
1.1根際細(xì)菌
根際細(xì)菌數(shù)量繁多,在根際分布較廣,且在根際的不同部位,細(xì)菌數(shù)量和類型分布也有差異.有研究[14]表明,相比于非根際區(qū),根際更適宜細(xì)菌的生長.VIK等[15]通過統(tǒng)計(jì)植物根際和根內(nèi)的操作分類單位(Operational Taxonomic Units, 簡稱OTUs),發(fā)現(xiàn)根際細(xì)菌多樣性以及根際細(xì)菌的數(shù)量和種類都顯著高于根內(nèi)細(xì)菌.BULGARELLI等[16]發(fā)現(xiàn)擬南芥根際中數(shù)量最多是變形菌門細(xì)菌,且以鏈霉菌科和叢毛單胞菌科細(xì)菌為主導(dǎo);LUNDBERG等[17]研究顯示,擬南芥根際菌以放線菌門和變形菌門為主,其中變形菌門豐度最高,主要以α、β、γ 3種綱的細(xì)菌為主.
在受污染土壤中,根際細(xì)菌的數(shù)量和種類最多,且以放線菌門、厚壁菌門和變形菌門為主(表1).這3種菌門的根際細(xì)菌耐重金屬性強(qiáng),在重金屬污染土壤中大量存在.重金屬脅迫條件對(duì)根際細(xì)菌的結(jié)構(gòu)與功能具有重要的影響.EISONBATY等[26]研究發(fā)現(xiàn)當(dāng)細(xì)菌暴露于有毒物質(zhì)或環(huán)境時(shí),外界氧化脅迫的壓力是使細(xì)胞死亡的一種常見機(jī)制.雖然該機(jī)制會(huì)減少土壤中根際細(xì)菌的數(shù)量、種類,影響根際細(xì)菌群落結(jié)構(gòu),但具有特定耐性功能的根際細(xì)菌群落仍能在重金屬脅迫環(huán)境條件下存活.MARGESIN等[27]在Pb/Zn污染土壤中發(fā)現(xiàn)有較多的可培養(yǎng)的、需氧的Pb/Zn耐性根際細(xì)菌.SOLIS-DOMINGUEZ等[28]研究發(fā)現(xiàn)叢枝菌根真菌(Arbuscular Mycorrhizal Fungi, 簡稱AMF)影響根際細(xì)菌群落在重金屬污染土壤中的修復(fù)作用,二者在植物根系進(jìn)行相互選擇性作用,表明根際細(xì)菌群落會(huì)受到其它類型微生物的影響.
表1 根際細(xì)菌分類情況
在重金屬污染土壤中,根際細(xì)菌對(duì)重金屬的抗性及解毒能力是影響植物修復(fù)的關(guān)鍵性因素[8].根際細(xì)菌與重金屬接觸后可發(fā)生離子交換、配位、螯合、沉淀等作用,使重金屬離子在細(xì)胞表面結(jié)合.同時(shí),根際細(xì)菌還能通過其自身的細(xì)胞代謝活動(dòng)來調(diào)節(jié)變價(jià)重金屬元素的氧化還原反應(yīng),使重金屬的價(jià)態(tài)及存在形態(tài)(絡(luò)合態(tài)、脫烷基)發(fā)生改變,從而降低重金屬毒性[29].BAI等[30]發(fā)現(xiàn)枯草芽孢桿菌BacillusSubtilisDBM能通過生物吸附、積累方式降低Pb的生物有效性,通過使Pb沉淀的作用方式緩解Pb的毒性作用.RAJKUMAR等[31]發(fā)現(xiàn)根際細(xì)菌Pseudomonassp. PsA4和Bacillussp.Ba32能通過催化反應(yīng)將高毒性的Cr6+還原成低毒性的Cr3+,從而減緩Cr對(duì)芥菜的重金屬毒性.此外,土壤中還存在一些氧化硫桿菌、氧化亞鐵桿菌等能直接或間接地通過氧化、還原、絡(luò)合、吸附或溶解作用去除重金屬,緩解植物根際的重金屬毒性效應(yīng)[31].可見,根際細(xì)菌在降低重金屬對(duì)植物的毒害,增強(qiáng)植物對(duì)重金屬的轉(zhuǎn)化和吸收等方面起著重要作用.
根際細(xì)菌是植物修復(fù)重金屬污染土壤的重要根際微生物,它能緩解重金屬對(duì)植物根際的毒害效應(yīng).根際細(xì)菌群落的多樣性能有效地表征植物修復(fù)重金屬污染效果.因此研究根際細(xì)菌分類情況、群落結(jié)構(gòu)特征及群落的功能,有利于篩選和利用在植物修復(fù)重金屬污染過程中起重要作用的根際細(xì)菌,強(qiáng)化植物修復(fù)重金屬污染土壤技術(shù).
1.2根際真菌
根際真菌是指分布在根際周圍的真菌,分為腐生真菌和菌根真菌[32].在重金屬污染土壤修復(fù)實(shí)踐中應(yīng)用較多的是菌根真菌,主要包括內(nèi)生菌根真菌、外生菌根真菌以及內(nèi)外生菌根真菌.
1.2.1內(nèi)生菌根真菌內(nèi)生菌根真菌在促進(jìn)植物生長和降低農(nóng)作物對(duì)重金屬吸收方面起到重要的輔助作用[33].LIU等[34]篩選出3種來自玉米根際能吸收和富集Cd的內(nèi)生菌根真菌,它們分泌的植物激素可將土壤中的多磷酸鹽沉淀吸附于細(xì)胞壁上,并且這些沉淀物能將Cd螯合固定在真菌細(xì)胞內(nèi),有利于緩解Cd對(duì)植物生長的毒害作用.另有研究表明,具有重金屬耐性的內(nèi)生菌根真菌可通過影響宿主羊茅屬植物對(duì)Cd、Zn、Cu等重金屬的吸收和轉(zhuǎn)運(yùn)機(jī)制,緩解重金屬對(duì)羊茅屬植物的毒害作用,與未接種內(nèi)生菌根真菌的植株相比,接種處理的植株具有較高的生物量和生長指數(shù)[35].
從枝菌根真菌(AMF)在重金屬污染修復(fù),尤其是與植物聯(lián)合修復(fù)方面的潛力已有較多的研究.AMF在重金屬污染土壤植物修復(fù)中的作用主要是輔助寄主植物獲得更多的礦質(zhì)營養(yǎng)和提高寄主植物對(duì)重金屬的抗性[36].在重金屬污染土壤中,植物因缺少必需的礦質(zhì)養(yǎng)分影響生長,而AMF的存在能幫助宿主植物獲取更多的礦質(zhì)營養(yǎng)[37].肖雪毅等[38]發(fā)現(xiàn)AMF能顯著增加金雞菊地上部和根系的P濃度,從而促進(jìn)金雞菊的生長并提高其生物量.AMF另可通過提高宿主植物P營養(yǎng)水平,使宿主植物根部中的重金屬與磷酸根形成難溶物質(zhì)[39],抑制重金屬向地上部轉(zhuǎn)運(yùn),減少植物地上部重金屬含量[40].此外,AMF能夠減輕重金屬對(duì)寄主植物的毒性,提高寄主植物的重金屬耐性[41].CHEN等[36]和DONG等[42]通過室內(nèi)盆栽實(shí)驗(yàn)發(fā)現(xiàn)AMF不僅能改善植物營養(yǎng)元素P的濃度,還能降低植物地上部Cd、Cu和As的濃度,減輕重金屬對(duì)寄主植物的毒性.在Cr污染土壤中AMF對(duì)車前草促生效應(yīng)顯著,減少車前草地上部分Cr的濃度,表明AMF對(duì)Cr的耐性能改變植物對(duì)重金屬的積累與吸收效應(yīng)[43].此外,AMF通過改變根際土壤微生物群落結(jié)構(gòu)及其物理化學(xué)性質(zhì)間接地影響重金屬形態(tài)變化.在Mn污染土壤中,AMF能改變根際土壤中Mn氧化和Mn還原細(xì)菌群落的組成,因而降低Mn的生物有效性,減緩植物受Mn的脅迫,促進(jìn)植物生長[44].
1.2.2外生菌根真菌外生菌根是高等植物營養(yǎng)根系和外生菌根真菌(Ectomycorrhizal Fungi, 簡稱ECMF)形成的共生體,在重金屬脅迫條件下可將重金屬固定于真菌細(xì)胞壁內(nèi),調(diào)節(jié)寄主植物細(xì)胞的生理、化學(xué)過程,阻止過多的重金屬進(jìn)入植物根部,以促進(jìn)寄主植物的生長,增強(qiáng)寄主植物對(duì)重金屬的耐受性[45].FINLAY等[46]發(fā)現(xiàn)根際ECMF優(yōu)勢種為擔(dān)子菌門或子囊菌門.
ECMF緩解植物重金屬毒害作用的機(jī)理主要是能對(duì)重金屬進(jìn)行吸收、轉(zhuǎn)運(yùn)和分布[46].Zn、Cu、Mn是植物和外生真菌所需的微量營養(yǎng)元素,通過吸收和氧化還原反應(yīng)調(diào)整它們?cè)谥参锔H的分布情況,促進(jìn)植物對(duì)這些元素的吸收、轉(zhuǎn)運(yùn),使植物生物量增加.過多的微量重金屬進(jìn)入植物根際時(shí)會(huì)對(duì)植物產(chǎn)生毒害.LANGER等[47]發(fā)現(xiàn)在ECMF的植物根際,隨著Zn濃度的增加,植物根際中Zn的轉(zhuǎn)運(yùn)系數(shù)下降.研究[48]表明,較之無菌根植株,接種了美味牛肝菌(Boletusedulis)菌根和混合菌根的植株體內(nèi)Cd以及根部Cu濃度均顯著降低.可能的原因是土壤中的重金屬破壞了無菌根植物根部的原生質(zhì)膜,損傷了原生質(zhì)膜的功能,導(dǎo)致重金屬離子大量流入根部并向枝葉轉(zhuǎn)移,而接種菌根可以減少對(duì)有害重金屬的過量吸收,有效緩解土壤中重金屬對(duì)植物的毒害,促進(jìn)植物在重金屬污染土壤中的生長,使植物生物量增加.
1.3根際古菌
古菌作為生命的第三種形式[49],主要存在和生活于極端環(huán)境中,如熱泉、鹽湖等,因而被歸為極端微生物.研究表明,重金屬污染土壤中也存在一定豐度和種類的古菌,如SANDAA等[50]研究發(fā)現(xiàn),相比于無污染土壤,重金屬污染土壤中古菌的數(shù)量減少1.3%±0.3%.一般認(rèn)為古菌不會(huì)在植物根際存在[51],但隨著研究的進(jìn)展,科學(xué)家們?cè)谑苤亟饘傥廴就寥赖闹参锔H也發(fā)現(xiàn)了古菌[52].例如,唐黎等[49]通過提取重金屬污染場地中棉花不同生長時(shí)期根際土中微生物的DNA,采用T-RFLP分析微生物群落結(jié)構(gòu),發(fā)現(xiàn)了在棉花根際土中有古菌存在,且古菌的群落結(jié)構(gòu)受季節(jié)變化影響.OLLIVIER等[13]在Pb和As污染土壤中的植物根際發(fā)現(xiàn)了氨氧化細(xì)菌和氨氧化古菌,且細(xì)菌和古菌的數(shù)量不相同,有機(jī)質(zhì)較低的土壤中細(xì)菌數(shù)量多于古菌,有機(jī)質(zhì)較豐富的土壤中古菌的數(shù)量顯著高于細(xì)菌.氨氧化過程是硝化作用的第一步,氨氧化細(xì)菌和古菌通過控制單加氧酶(amoA)的活性來控制氨氧化成亞硝酸鹽的速率.研究[53-54]發(fā)現(xiàn)根際土中泉古菌門的古細(xì)菌含有amoA基因,證實(shí)氨氧化古菌能調(diào)控植物根際的氨氧化過程.
關(guān)于根際古菌在植物修復(fù)重金屬污染土壤過程中的作用仍不清晰,根際古菌群落結(jié)構(gòu)、群落功能、環(huán)境影響因子和植物的關(guān)系等關(guān)鍵問題仍是未知的.在今后的研究中應(yīng)多關(guān)注根際古菌在植物修復(fù)重金屬污染過程中對(duì)根際植株生長的影響、根際古菌自身群落結(jié)構(gòu)的變化以及根際古菌群落與根際其它微生物群落的關(guān)系,進(jìn)一步探索根際古菌在重金屬污染土壤植物修復(fù)過程中的作用.
1.4根際微生物之間相互作用
根際微生物在重金屬污染土壤中相互作用的關(guān)系主要包括消極作用、積極作用(互利共生、偏利共生)、中立作用(既無利又無害)[55].在實(shí)際的重金屬污染修復(fù)中多利用根際微生物的積極作用探討在植物根際起修復(fù)作用的微生物群落輔助植物修復(fù)重金屬污染的過程.
當(dāng)前,根際菌群的研究多于內(nèi)生菌群的研究,因?yàn)楦H微生物產(chǎn)生的根系分泌物對(duì)植物修復(fù)重金屬污染土壤有重要作用.根際分泌的有機(jī)酸、鐵載體、ACC脫氨酶、生長素等與重金屬結(jié)合減少重金屬對(duì)植物的毒害,增加植物的抗氧化防御系統(tǒng),促進(jìn)植物的生長[7].關(guān)于根際微生物種類的研究中,單獨(dú)研究根際細(xì)菌的較多,研究根際細(xì)菌與真菌結(jié)合的較少,但根際真菌在植物修復(fù)重金屬污染過程中能調(diào)節(jié)重金屬在根際的分布情況,促進(jìn)寄主植物對(duì)重金屬元素的吸收、轉(zhuǎn)運(yùn),減少重金屬對(duì)植物的毒害,促進(jìn)植物在重金屬污染土壤中的生長,使植物生物量增加等作用[46-47].今后研究應(yīng)多關(guān)注根際細(xì)菌與真菌結(jié)合的研究,綜合利用細(xì)菌與真菌互利共生的關(guān)系,增加植物根際的重金屬耐性,促進(jìn)根際植物的生長,輔助根際植物修復(fù)重金屬污染土壤,加強(qiáng)對(duì)根際古菌與其它根際微生物群落關(guān)系的研究.
2不同污染場地中根際微生物群落多樣性差異及其對(duì)重金屬修復(fù)效果的影響
根際微生物的活性受土壤溫度、營養(yǎng)狀況、水的可利用性及其它一些能引發(fā)群落耐性的環(huán)境污染因素的影響[56].此外,根際微生物也會(huì)受到土壤污染類型(單一或復(fù)合污染)、是否添加改良劑和改良劑種類以及根際植物種類等的影響[57].如污染土壤的理化性質(zhì)會(huì)影響根際微生物的分離和純化,土壤污染程度及土壤特性會(huì)影響根際微生物群落結(jié)構(gòu)的分布,植物的種類會(huì)影響根際微生物在根際的定殖等[15].THION等[32]研究發(fā)現(xiàn)子囊菌亞門、擔(dān)子菌亞門和接合菌亞門3種AMF,它們?cè)诓煌闹参锓N類、土壤pH、含N量和含水量條件下物種組成和功能都各不相同,且根際土中AMF的多樣性顯著高于非根際土.這表明在植物修復(fù)重金屬污染過程中根際微生物的結(jié)構(gòu)與功能受多種因素的影響.
土壤污染的類型主要分為單一或者復(fù)合重金屬污染以及重金屬與有機(jī)物的復(fù)合污染.研究發(fā)現(xiàn)重金屬種類和濃度會(huì)影響植物修復(fù)重金屬過程中根際微生物的種類、豐富度及其群落結(jié)構(gòu)[58-59].重金屬種類能影響根際微生物的多樣性,在不同種類重金屬污染土壤中優(yōu)勢植物的種類不同,根際微生物的多樣性及優(yōu)勢種也會(huì)有差異.表2顯示,單一重金屬污染條件下,植物種類不同,根際微生物的多樣性和優(yōu)勢種有顯著差異.不同種類重金屬污染條件下,同種植物的根際微生物的多樣性和優(yōu)勢種不相同.幾種相同重金屬污染條件下,不同植物和不同重金屬濃度下的根際微生物的多樣性和優(yōu)勢種也有所差異.不同濃度Pb污染條件,水稻根際微生物的多樣性不相同[60].不同濃度As污染下,荊(Wattle)和發(fā)草(Deschampsiacaespitosa)的根際微生物多樣性和優(yōu)勢種均有所差異[61].相同重金屬元素的復(fù)合污染下,各重金屬濃度不同,植物根際微生物多樣性及優(yōu)勢種類也不同.在同一重金屬污染的土壤中重金屬濃度的差異使得根際微生物的多樣性發(fā)生改變,產(chǎn)生的根際微生物的優(yōu)勢種也不同,可見重金屬濃度對(duì)土壤中根際微生物群落結(jié)構(gòu)具有重要影響.
表2 不同重金屬對(duì)根際微生物多樣性及優(yōu)勢種的影響
受重金屬污染的土壤一般酸性高且貧瘠,常添加改良劑調(diào)節(jié)土壤pH、改善土壤基本理化性質(zhì)[70].其中有機(jī)質(zhì)改良劑(如生物炭)可以改善微生物營養(yǎng),提升微生物活性[71].生物炭能為植物提供更多的營養(yǎng)物質(zhì)促進(jìn)植物生長和吸引有利的細(xì)菌以刺激真菌的生長[72].生物炭能改變根際環(huán)境的物理化學(xué)參數(shù),為細(xì)菌和真菌提供一個(gè)安全有利的生長環(huán)境[33];添加生物炭后能改變土壤中根際微生物呼吸作用的速率,影響土壤中的碳循環(huán),使土壤的理化性質(zhì)發(fā)生改變,增加微生物的生物量,從而影響根際微生物的種類及其在根際的定殖情況[72].礦物類改良劑在植物修復(fù)重金屬污染土壤中對(duì)根際微生物具有重要影響.杜瑞英等[73]研究多金屬污染土壤中微生物群落功能對(duì)麻瘋樹-化學(xué)聯(lián)合修復(fù)的響應(yīng)時(shí)發(fā)現(xiàn),施用改良劑粉煤灰后土壤微生物指標(biāo)顯著高于對(duì)照土壤,說明粉煤灰對(duì)污染土壤環(huán)境的改善具有重要的影響.改良劑的投加能沉淀重金屬或提高土壤對(duì)重金屬的吸附,從而降低有效態(tài)金屬含量.因而施加改良劑有利于提高土壤微生物群落功能多樣性,有利于加強(qiáng)植物修復(fù)重金屬污染效果[74].
植物種類不同,根際微生物的多樣性和優(yōu)勢種有顯著差異[61-62].CLAY等[5]研究表明,重金屬土壤中根際細(xì)菌的群落結(jié)構(gòu)會(huì)因?yàn)橹参锏姆N植而發(fā)生改變,同時(shí)根際細(xì)菌群落的相互作用也會(huì)影響植物的多樣性與修復(fù)效果.例如,在受重金屬污染的連續(xù)區(qū)域中,植物根際土中的細(xì)菌群落比非根際土中的豐富很多,且不同植物的根際細(xì)菌群落結(jié)構(gòu)也有差異,其中優(yōu)勢植物種高羊茅根際中的耐性細(xì)菌的種類與數(shù)量最多.研究者[75]認(rèn)為,該區(qū)域耐性細(xì)菌與高羊茅具有最強(qiáng)的共生關(guān)系,這種共生關(guān)系減少了該區(qū)域的植物多樣性,增強(qiáng)了高羊茅的優(yōu)勢性,使其成為該區(qū)域修復(fù)重金屬的先鋒植物,極大地增強(qiáng)了高羊茅修復(fù)重金屬污染效果.根際微生物群落多樣性越豐富,植物修復(fù)重金屬效果越顯著.
3當(dāng)前研究存在的問題
關(guān)于根際微生物群落多樣性在植物修復(fù)重金屬污染土壤中的大多數(shù)研究只考慮單一細(xì)菌或真菌的作用(表3),其它重金屬的干擾會(huì)影響修復(fù)效果.然而實(shí)際污染場地多以復(fù)合污染為主,此外,細(xì)菌和真菌都是根際微生物重要組成部分,它們對(duì)植物吸收、重金屬積累都具有重要影響,單一地研究細(xì)菌或真菌無法反映出整個(gè)根際微生物群落對(duì)植物修復(fù)體系的影響.當(dāng)前的研究中較少關(guān)注根際細(xì)菌與耐性植物之間的相互選擇性以及重金屬污染場地的植物再生.大多數(shù)研究中只關(guān)注根際細(xì)菌或真菌的群落結(jié)構(gòu)變化,較少關(guān)注整個(gè)根際微生物群落的結(jié)構(gòu)與功能.關(guān)于根際微生物群落結(jié)構(gòu)的研究爭議主要是單一污染與復(fù)合污染在哪種條件下根際微生物群落結(jié)構(gòu)更豐富.THION等[32]研究發(fā)現(xiàn)受寄主紫花苜蓿的調(diào)控下復(fù)合污染場地中具有修復(fù)作用的真菌群落結(jié)構(gòu)比較單一,真菌種類和數(shù)量都比較少.SOLIS-DOMINGUEZ等[28]研究受Pb、Cr復(fù)合污染的鉛鋅尾礦時(shí)發(fā)現(xiàn)菌根叢枝真菌與受單一重金屬污染的菌根叢枝真菌相比,菌落的種類與結(jié)構(gòu)更加豐富.根際微生物群落結(jié)構(gòu)變化與植物修復(fù)重金屬污染的作用效果緊密相關(guān),結(jié)構(gòu)豐富度越高,修復(fù)效果越好.但在具體的修復(fù)應(yīng)用中根際微生物群落結(jié)構(gòu)變化與修復(fù)效果的關(guān)系如何仍有待研究.
4展望
利用植物修復(fù)土壤重金屬污染具有很大的潛力.在進(jìn)行重金屬污染土壤的生物修復(fù)過程中,重視并應(yīng)用植物根際微生物轉(zhuǎn)移、積累重金屬的特性,實(shí)現(xiàn)根際微生物與超富集植物協(xié)同治理具有重要的實(shí)踐價(jià)值.對(duì)于根際微生物群落多樣性在植物修復(fù)重金屬污染土壤的研究可從以下幾方面展開:
(1)理論機(jī)制.通過植物根際微生物菌種篩選、鑒定、繁殖、菌劑生產(chǎn)等方面技術(shù)的應(yīng)用,進(jìn)一步了解根際微生物群落中各菌種的生物和生態(tài)學(xué)特征及其功能機(jī)制,采取多菌聯(lián)用技術(shù),將2種及2種以上具有相同功能的根際菌應(yīng)用于植物修復(fù)重金屬污染過程中,進(jìn)一步加強(qiáng)菌種、植物根系、介質(zhì)載體三者相互的復(fù)合功能研究.同時(shí)還需分析不同植物種類根際菌群的特點(diǎn),根系生長對(duì)接種生物生長繁殖和生物學(xué)特征及功能的影響,根際微生物、內(nèi)生菌、菌根等與宿主的關(guān)系以及根際菌增強(qiáng)植物抗性與促進(jìn)植物吸收重金屬的過程原理等.
(2)適用技術(shù)開發(fā).重金屬污染土壤的主要類型有土壤質(zhì)地污染、理化性質(zhì)上的污染、重金屬種類與污染程度等方面的污染.由于重金屬污染類型的不同,以及受污染場地地理環(huán)境的差異,如氣候變化導(dǎo)致降水量、氣溫等條件的不同,用于場地重金屬修復(fù)的超富集植物和耐性植物的種類多樣,至今我國已發(fā)現(xiàn)400多種重金屬超富集植物,因此,對(duì)于根際菌菌劑產(chǎn)品的研發(fā)要有針對(duì)性,要結(jié)合受污染場地的土壤類型特點(diǎn)、場地環(huán)境影響因素,以及與超富集植物或耐性植物的匹配性篩選出高效根際菌菌種或高效根際菌群,并結(jié)合植物群落的配置特點(diǎn)研發(fā)適用且高效的植物-微生物聯(lián)合修復(fù)技術(shù),使其向技術(shù)化的方向發(fā)展.
表3 根際微生物在植物修復(fù)重金屬污染土壤中的應(yīng)用
(3)利用生物信息學(xué)技術(shù),對(duì)具有耐重金屬特性的根際微生物群落結(jié)構(gòu)進(jìn)行基因測序,分析根際微生物的多樣性及其優(yōu)勢種,進(jìn)一步分析根際微生物群落結(jié)構(gòu)與功能之間的關(guān)系,探討根際微生物在植物根際的定殖機(jī)理.研究重金屬污染場地中具有植物促生功能的根際促生菌群落,闡明其植物促生機(jī)制,將其有效地應(yīng)用到實(shí)際生產(chǎn)中.
(4)技術(shù)產(chǎn)品安全性問題.篩選并進(jìn)行繁殖的抗性根際微生物菌種,一般具有很強(qiáng)的抵抗重金屬毒性的能力,因此在實(shí)際修復(fù)應(yīng)用過程中應(yīng)當(dāng)注重根際微生物菌種的安全性與可控性.重金屬污染土壤治理的目標(biāo)之一是保護(hù)周邊生態(tài)環(huán)境尤其是土壤與水域環(huán)境,在生態(tài)治理過程中應(yīng)注重植物物種、微生物菌種與生態(tài)環(huán)境相互之間的關(guān)系,應(yīng)用時(shí)必須綜合考慮環(huán)境的生態(tài)安全問題.
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Research on Function of Rhizosphere Microbial Diversity in Phytoremediation of Heavy Metal Polluted Soils
DING Qiaobei, CHAO Yuanqing*, WANG Shizhong, CHEN Yanmei, QIU Rongliang*
(School of Environmental Science and Engineering, Sun Yat-Sen University, Guangzhou 510275, China)
Abstract:Rhizosphere microbes play important roles in phytoremediation of heavy metal polluted soils. They can strengthen the growth of plants and promote absorption, transportation and accumulation of heavy metals in the rhizosphere by excreting plant growth hormone, siderophore, 1-aminocyclopropane-1-carboxylate (ACC) deaminase, flavonoids compounds, phenolic acids and other organic substances. These secretions could further produce functional microbial community which can adapt to the stressed rhizosphere environment. The roles of rhizosphere microbes in phytoremediation of heavy metal contaminated soils was reviewed. Based on the systematical summary on the functions of rhizospheric bacteria, fungi and archaea in phytoremediation, the influences by pollution type, ameliorant and rhizosphere plant species to rhizosphere microbial activities were reported. The perspectives of the future research on related issues were also discussed.
Key words:heavy metal contaminated soil; phytoremediation; rhizosphere microbes; microbial community
中圖分類號(hào):Q89;X53
文獻(xiàn)標(biāo)志碼:A
文章編號(hào):1000-5463(2016)02-0001-12
*通訊作者:晁元卿,講師,Email:chaoyuanq@mail.sysu.edu.cn;仇榮亮,教授,Email:eesqrl@mail.sysu.edu.cn.
基金項(xiàng)目:國家自然科學(xué)基金項(xiàng)目(41403060,2012AA06A202)
收稿日期:2015-08-30《華南師范大學(xué)學(xué)報(bào)(自然科學(xué)版)》網(wǎng)址:http://journal.scnu.edu.cn/n