胡恒宇,顧貴洲,張 強(qiáng),趙東風(fēng),劉春爽
(中國石油大學(xué)(華東)化學(xué)工程學(xué)院,山東 青島266580)
對(duì)老油田進(jìn)行深度開發(fā)、提高原油采收率已成為當(dāng)前老油田開發(fā)的中心任務(wù)。微生物在地層中就地產(chǎn)氣,是提高原油采收率的重要途徑。微生物在油藏中代謝產(chǎn)生氣體、生物表面活性物質(zhì)、有機(jī)酸[1]等,能提高能源利用效率,殘余油微生物氣化技術(shù)是延長油藏開發(fā)壽命的新技術(shù),在厭氧環(huán)境下,利用產(chǎn)甲烷微生物菌群將石油烴轉(zhuǎn)化為甲烷氣,然后直接開采或就地儲(chǔ)存起來。烷烴厭氧微生物降解產(chǎn)甲烷過程是多種菌參與、多步驟的反應(yīng),影響因素非常多[2-6],主要因素之一就是烴的化學(xué)性質(zhì),特別是溶解性[7]。加快原油厭氧降解速度的可行方法之一就是加強(qiáng)厭氧微生物對(duì)石油烴的攝取能力,而微生物大多生活在水相中[8-9]。在微生物強(qiáng)化石油開采技術(shù)中,營養(yǎng)物質(zhì)和外源微生物的注入不僅可以提高原油采收率,而且微生物還能利用一些廉價(jià)的可再生資源生產(chǎn)大量副產(chǎn)品[10-11]。
根據(jù)微生物的特性,可將油藏中的微生物劃分為若干個(gè)種群,大致包括:烴氧化菌、發(fā)酵菌、硝酸鹽還原菌、鐵還原菌、硫酸鹽還原菌和產(chǎn)甲烷古菌。作者在此對(duì)其中的硫酸鹽還原菌、鐵還原菌、硝酸鹽還原菌降解殘余油產(chǎn)甲烷氣的研究進(jìn)展進(jìn)行綜述。
通過篩選培養(yǎng)的方法,人們?cè)谟筒貎?nèi)部分離得到了多種不同功能的厭氧微生物,主要包括硝酸鹽還原菌、鐵還原菌、硫酸鹽還原菌、發(fā)酵菌和產(chǎn)甲烷古菌[12]等。20世紀(jì)80年代后期,Vogel等證明了烴的厭氧降解過程是可以發(fā)生的[13],Lovley等[14]分離得到了第1株以三價(jià)鐵為電子受體的高效降解甲苯的菌株GS-15,Aeckersberg等[15]分離得到第1株以硫酸鹽為電子受體的能利用長鏈飽和烴的還原菌Hxd3。大量研究[16-21]證實(shí),厭氧的電子受體一般包括硫酸鹽、硝酸鹽、三價(jià)鐵、錳離子、二氧化碳等。
硫酸鹽還原菌是一類以硫酸鹽為電子受體的厭氧微生物,廣泛存在于土壤、海水、河水、地下管道及油藏等缺氧環(huán)境中。研究發(fā)現(xiàn),硫酸鹽還原菌在富含石油烴的環(huán)境(如烷烴、苯及苯系物的環(huán)境)中也能生長,并從這些環(huán)境中分離出了多種硫酸鹽還原菌[22]。分離自海洋沉積物的硫酸鹽還原菌株Tol2可以降解甲苯,但是甲苯不能超過一定濃度,否則對(duì)菌株有毒性,同時(shí)有2種脫氫酶參加了降解反應(yīng)[23]。
Beller等[24]從石油污染土壤中分離得到了菌株prtol1,它是一種硫酸鹽還原菌,以甲苯為唯一電子供體和碳源,降解甲苯為二氧化碳,其中15%轉(zhuǎn)變?yōu)榉菗]發(fā)代謝物。Harms等[25]在烷基苯培養(yǎng)實(shí)驗(yàn)中分離得到2株培養(yǎng)菌oXyS1和mXyS1,它們以硫酸鹽為電子受體,分別能降解鄰二甲苯和間二甲苯,并且都有降解苯甲酸和苯甲酸甲酯的能力,16SrRNA序列分析表明:oXyS1與已知菌株Desulfobacterium cetonicum和Desulfosarcina variabilis相似值分別達(dá)到98.4%和98.7%,而 mXyS1與 Desulfococcus multivorans相似值為86.9%。Kniemeyer等[26]在芳香烴的厭氧降解實(shí)驗(yàn)中得到1株能夠降解乙苯的以硫酸鹽為電子受體的菌株,該菌株能在含有乙苯的培養(yǎng)基上生存,通過富集培養(yǎng)來自不同地區(qū)的海洋樣本,發(fā)現(xiàn)乙苯的降解依賴于硫酸鹽的還原,通過氣相色譜-質(zhì)譜可以檢測(cè)到降解產(chǎn)物。Morasch等[27]從煤氣廠廢水中分離培養(yǎng)1株嚴(yán)格厭氧菌,能降解甲苯、鄰二甲苯、間二甲苯,并且能將它們完全氧化成二氧化碳,并從細(xì)胞提取液中發(fā)現(xiàn)了芐基琥珀酸和甲基富馬酸。Ommedal等[28]從油藏中發(fā)現(xiàn)了1株硫酸鹽還原菌,能夠降解甲苯,為革蘭氏陰性,最適生長溫度34~38℃,最佳鹽度1.5%,pH 值7.2~7.5,細(xì)胞形態(tài)呈桿狀。Aeckersberg等[15]從含十六烷的富集培養(yǎng)基中得到1株厭氧菌HXD3,能利用硫酸鹽為電子受體并將其還原為硫化氫,可以降解C12~C20的長鏈烷烴;以十六烷為電子供體,在脫氫酶的作用下氧化降解,不過厭氧烷烴的氧化第一步是未知的。
Galushko等[29]從海洋沉積物中富集培養(yǎng)硫酸鹽還原菌,在萘存在的富含硫酸鹽的沉積物中,以萘為唯一碳源和電子供體,硫酸鹽被還原成硫化物,萘被完全氧化為降解產(chǎn)物。有學(xué)者從海洋沉積物中分離得到2株菌HXD3和pnd3,都是以硫酸鹽為電子受體的嚴(yán)格厭氧菌,對(duì)C12~C20烷烴有明顯的降解作用,2種菌株共享90%的16SrRNA序列,以降解十六烷為例,降解初期細(xì)胞中主要含有脂肪酸[30]。有學(xué)者從石油污染的河口沉積物中分離得到降解烷烴的硫酸鹽還原菌,此菌種為革蘭氏陰性桿菌,可以降解C13~C18長鏈烷烴,而且以脂肪酸、甲酸鹽為電子供體,硫酸鹽、亞硫酸鹽為電子受體[31]。Cravo-Laureau等[32]研究了正構(gòu)烷烴的代謝情況,提出了正構(gòu)烷烴代謝的可能途徑,再一次證明了正構(gòu)烷烴降解為脂肪酸的路徑,尤其是通過氣相色譜-質(zhì)譜檢測(cè)到產(chǎn)生的琥珀酸可說明正構(gòu)烷烴的厭氧代謝方式。有學(xué)者從含油廢水的儲(chǔ)存設(shè)施和油田生產(chǎn)水中分離得到1株新的菌株Desulfoglaeba alkanexedens,能夠降解C6~C12烷烴,革蘭氏陰性,短桿狀,常成對(duì)出現(xiàn),最適生長溫度37℃、pH值7.2[33]。從海洋滲漏區(qū)的提取物中富集培養(yǎng)得到能夠降解短鏈烴的硫酸鹽還原菌[34]。對(duì)間二甲苯的降解表明,隨著培養(yǎng)時(shí)間的延長,會(huì)有各種優(yōu)勢(shì)菌群占有相應(yīng)的地位,通過PCR-DGGE分析不同時(shí)期的優(yōu)勢(shì)序列可以確定優(yōu)勢(shì)菌群,采用這種方法確定了1株高效烷烴降解菌,該菌以硫酸鹽為電子受體,能降解C6與C10的烷烴[35]。以石油烴為碳源和能源的厭氧生物降解過程,是以硝酸鹽和硫酸鹽為還原劑,通過延胡索酸羧化和甘氨酰自由基的烷基取代,烷烴被羧化,最終完成降解[36]。
有學(xué)者分離得到可以降解甲苯和苯酚的厭氧細(xì)菌,該菌以三價(jià)鐵作為電子受體,最后生成二價(jià)鐵和二氧化碳,使溶液的pH值降低[14]。2001年,又有學(xué)者從受石油污染的水層中分離得到嚴(yán)格厭氧菌株,為革蘭氏陰性菌,分別屬于3個(gè)種屬,能夠降解芳香族化合物[37]。鐵還原菌的降解烷烴種類不一樣,分離的方法也多種多樣,如表1所示。
表1 厭氧降解烴的鐵還原菌Tab.1 The iron-reducing bacteria capable of anaerobic degradation of hydrocarbons
鐵化合物廣泛存在于油藏中,主要有三價(jià)鐵和二價(jià)鐵兩種形態(tài),在烷烴降解反應(yīng)時(shí),涉及到氧化還原反應(yīng),可以作電子受體。鐵還原菌種類繁多,有嗜溫和嗜熱厭氧菌,有以有機(jī)酸、乙酸鹽作為電子受體的希瓦氏菌,也有以氫氣作為電子供體、乙酸為碳源的菌群,且代謝途徑多樣化[39,41-42]。
從廢水中分離得到1種在厭氧條件下純培養(yǎng)能夠降解甲苯的細(xì)菌,初步鑒定為假單胞菌屬。該菌以硝酸鹽作為甲苯降解的電子受體,也能夠降解間二甲苯、苯甲酸、苯甲醛[43]。在厭氧條件下,有學(xué)者以甲苯作為唯一碳源分離出能夠降解甲苯的菌株,超過50%的甲苯碳被氧化成二氧化碳的碳,29%被吸收,甲苯的降解速率為1.8mmol·min-1·L-1,而且具有不同于其它細(xì)菌降解甲苯的途徑[44]。有學(xué)者用多種芳香族化合物培養(yǎng)基分離出幾種假單胞菌屬,并且在厭氧條件下,以硝酸鹽為電子受體,進(jìn)行了降解甲苯的實(shí)驗(yàn),得到7株能夠降解甲苯的菌株。結(jié)果發(fā)現(xiàn),超過50%的14C標(biāo)記的甲苯碳轉(zhuǎn)變?yōu)槎趸嫉奶?,厭氧降解甲苯是通過甲基側(cè)鏈氧化為苯甲酸實(shí)現(xiàn)的,而且高達(dá)37%的碳被吸收進(jìn)細(xì)胞質(zhì)。而對(duì)甲酚、對(duì)羥基苯甲醇、苯甲醛或?qū)αu基苯甲酸的降解更慢或有不同的滯后時(shí)間[45]。
反硝化也稱脫氮作用,是指反硝化細(xì)菌在缺氧條件下,還原硝酸鹽并釋放出分子態(tài)氮(N2)或一氧化二氮(N2O)的過程。能進(jìn)行反硝化作用的只有少數(shù)細(xì)菌,這個(gè)生物群稱為反硝化菌。大部分反硝化細(xì)菌是異養(yǎng)菌,例如脫氮小球菌、反硝化假單胞菌等,它們以有機(jī)物為氮源和能源,進(jìn)行無氧呼吸。通過16S rRNA基因序列分析、細(xì)胞脂肪酸的氣相色譜分析、新的系統(tǒng)發(fā)育和分類等手段得到8種甲苯降解反硝化菌株。構(gòu)建的系統(tǒng)進(jìn)化樹反映了各個(gè)菌株的種屬關(guān)系。利用距離矩陣、最大簡約法和最大似然方法,證明8種反硝化菌株形成了一個(gè)系統(tǒng)發(fā)育的相干集群。通過降解產(chǎn)生的脂肪酸和細(xì)胞形態(tài),也有類似的生理和營養(yǎng)特性[46]。
在相對(duì)較低的甲苯濃度(5×10-6)下,分離得到了10株細(xì)菌,其甲苯降解活性為陽性,對(duì)芳香族化合物有較強(qiáng)的降解能力。通過PCR指紋分析,表明有8個(gè)不同的菌株,所有菌株均是能動(dòng)的革蘭氏陰性桿菌,脫氮產(chǎn)生N2,不水解淀粉,有血紅素亞硝酸鹽還原酶,但只有4株菌有雜交的固氮酶結(jié)構(gòu)基因。16SrRNA基因分析表明這些菌株屬于Azoarcus屬[47]。對(duì)淡水泥漿樣品進(jìn)行了厭氧降解,從中提取出2個(gè)新的反硝化菌EbN1和pbn1,在硝酸鹽存在下可以降解烷基苯與側(cè)鏈長于甲苯的芳烴類,同時(shí)能降解乙苯和丙苯。在甲苯和間二甲苯的泥漿中分離得到2個(gè)反硝化菌ton1和mxyn1,16SrRNA序列分析表明,新菌株與Thauera selenatis有密切的關(guān)系,表現(xiàn)出降解烷基苯的特定能力[48]。通過差異蛋白質(zhì)組學(xué)的方法,找到各種降解途徑表達(dá)的蛋白,從而分離得到了能夠利用芳香族化合物的EbN1。對(duì)354種不同的蛋白質(zhì)進(jìn)行鑒定,其中199個(gè)表現(xiàn)出顯著改變的豐度。這些調(diào)節(jié)蛋白主要體現(xiàn)為不同降解途徑的酶,并揭示了不同階段的生長條件[49]。在反硝化細(xì)菌厭氧降解乙苯的初始反應(yīng)中提取到了EB1,以其在反硝化條件下降解乙苯,69%的14C標(biāo)記的乙苯轉(zhuǎn)換為14CO2。進(jìn)一步研究發(fā)現(xiàn),厭氧條件下菌株EB1細(xì)胞代謝乙苯,生成1-苯基乙醇、苯乙酮和1個(gè)未知的化合物,通過生長實(shí)驗(yàn)和光譜分析,提出該未知的化合物是苯甲酰乙酸乙酯。H218O標(biāo)記表明厭氧降解乙苯的第1個(gè)產(chǎn)品是1-苯基乙醇,羥基來源于水[50]。
在生物治理柴油污染含水層時(shí),通過注射原位氧化劑(O2和NO-3)和添加營養(yǎng)物,可以刺激微生物活性。從含水層樣品中分離的細(xì)菌菌株能夠?qū)x定的碳?xì)浠衔镞M(jìn)行厭氧反硝化降解,而且分離的菌株能夠生長,菌株T2、T4、T6和T10生長在含甲苯的培養(yǎng)基上,菌株 M3、M7、M9、M11、M12生長在含甲苯和間二甲苯的培養(yǎng)基上,其中菌株T2、T4、T6和T10是球菌,菌株 M3、M7、M9、M11、M12是棒狀桿菌。形態(tài)和生理的差異也反映在23SrRNA的結(jié)構(gòu)域Ⅲ和16SrRNA序列上,16SrRNA序列分析顯示菌株T3和M3屬于Azoarcus屬。表明降解石油烴的Azoarcus屬是土著微生物的重要成員,其中約80%屬于β-變形菌、10%~16%屬于γ-變形菌[51]。從加利福尼亞州含水層中分離得到的菌株通過三氯乙烯(TCE)的馴化,能夠以共代謝方式降解苯酚或甲苯。分別測(cè)量這些降解菌的密度,發(fā)現(xiàn)它們可能對(duì)這些基質(zhì)相當(dāng)敏感?;蛐蛄需b定表明,其中60%為苯酚和甲苯降解菌株。通過甲苯鄰單加氧酶(湯姆)探針進(jìn)行雜交分析探討可能的降解途徑,并鑒定這些菌株為革蘭氏陽性菌[52]。
研究表明,厭氧條件下,硝酸鹽還原菌能夠降解烷基甲苯和二甲苯的3種異構(gòu)體,對(duì)異丙基甲苯(甲基)的降解富集培養(yǎng)最快(4周),對(duì)天然芳香烴和二甲苯的降解時(shí)間為6周,而對(duì)其它烷基苯的降解效果不明顯。厭氧烷基苯的降解至少需要2個(gè)酶系統(tǒng)的存在,其中1個(gè)代謝芳香族和乙基甲苯,另1個(gè)代謝甲苯[53]。
有學(xué)者富集了Dechloromonas菌株(RCB和JJ),可以完全代謝各種單芳族化合物(包括苯),其中CO2可以作為替代硝酸鹽的電子受體,用于污染環(huán)境的防治[54]。厭氧降解烴的主要硝酸鹽還原菌見表2。
隨著世界人口的增長,化石能源需求的日益增加以及全球石油資源的減少使人們重新認(rèn)識(shí)到石油產(chǎn)甲烷的重要作用。為更好地利用廢棄油藏中的殘余油,微生物強(qiáng)化石油開采技術(shù)得到大范圍應(yīng)用。
2004年底,俄克拉荷馬日?qǐng)?bào)報(bào)道了俄克拉荷馬大學(xué)Suflita教授的研究工作,即通過微生物的作用,將油藏中殘余油轉(zhuǎn)化為天然氣,可使老油田起死回生[64]。專門從事生物氣研究的美國LUCA公司研究Monument Butte油田時(shí)發(fā)現(xiàn),該油藏的油和水樣在一起培養(yǎng)時(shí),由于微生物作用,在最開始的60d內(nèi)有大量的甲烷產(chǎn)生,297d時(shí)甲烷產(chǎn)生量達(dá)到最大。油藏環(huán)境中的硝酸鹽、四價(jià)錳、三價(jià)鐵、硫酸鹽等電子受體耗盡是甲烷產(chǎn)生的基礎(chǔ),而甲烷正是油藏中石油烴降解的終端產(chǎn)物之一。
產(chǎn)甲烷古菌通過與其它細(xì)菌形成一種特殊的互營關(guān)系,持續(xù)降解石油烴并接受末端電子產(chǎn)生甲烷,地質(zhì)學(xué)證據(jù)表明,數(shù)千年來在地球內(nèi)部(提供厭氧甲烷條件)油藏自發(fā)進(jìn)行著這種產(chǎn)甲烷進(jìn)程。處于厭氧生物鏈最末端的產(chǎn)甲烷古菌在微生物采油、生物修復(fù)以及生物圈碳元素循環(huán)中起著重要作用。通過加入更有潛力的產(chǎn)甲烷微生物來降解石油烴,取得了很好的效果。
表2 厭氧降解烴的硝酸鹽還原菌Tab.2 The nitrate-reducing bacteria capable of anaerobic degradation of hydrocarbons
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