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藻類揮發(fā)性有機(jī)化合物研究進(jìn)展

2017-11-29 01:39左照江
水生生物學(xué)報(bào) 2017年6期
關(guān)鍵詞:萜烯微囊硫化物

左照江

(浙江農(nóng)林大學(xué)林業(yè)與生物技術(shù)學(xué)院, 臨安 311300)

藻類揮發(fā)性有機(jī)化合物研究進(jìn)展

左照江

(浙江農(nóng)林大學(xué)林業(yè)與生物技術(shù)學(xué)院, 臨安 311300)

在水域生態(tài)系統(tǒng)中, 藻類釋放的揮發(fā)性有機(jī)化合物(VOCs)種類眾多, 主要有萜烯類、醛類、醇類、酯類、酮類、脂肪族烴、芳香族、硫化物和鹵化物。這些VOCs通過不同的次生代謝途徑形成, 可為多種環(huán)境因素所誘導(dǎo)產(chǎn)生。在逆境脅迫下, VOCs具有提高藻細(xì)胞抗逆性的作用。當(dāng)VOCs釋放到水體中后, 可為同種藻細(xì)胞傳遞脅迫信息, 使感受細(xì)胞做好防御準(zhǔn)備。對(duì)于異種藻細(xì)胞, VOCs通過化感作用抑制其生長(zhǎng), 從而保證VOCs釋放者的競(jìng)爭(zhēng)優(yōu)勢(shì)。此外, VOCs還可驅(qū)避捕食者, 保護(hù)藻細(xì)胞免受傷害。未被感受者利用的VOCs經(jīng)揮發(fā)進(jìn)入大氣層后, 會(huì)參與水域上空二次有機(jī)氣溶膠的形成。通過對(duì)藻類VOCs的種類、形成途徑、誘導(dǎo)釋放因素及其生態(tài)作用進(jìn)行綜述, 以期對(duì)藻類VOCs的研究有所幫助。

揮發(fā)性有機(jī)化合物; 形成途徑; 誘導(dǎo)因素; 生態(tài)作用

揮發(fā)性有機(jī)化合物(Volatile organic compounds, VOCs)是指“在生理溫度下植物體內(nèi)可迅速?gòu)囊簯B(tài)轉(zhuǎn)化為氣態(tài)并釋放到大氣中的低沸點(diǎn)有機(jī)物分子”[1]。在高等植物體內(nèi), VOCs主要通過次生代謝途徑合成, 其種類約為30000多種[2]。在陸地生態(tài)系統(tǒng)中, 這些VOCs是植物與植物以及植物與昆蟲間的信號(hào)物質(zhì), 具有重要的信息傳遞功能[3, 4]。

除高等植物外, 藻類植物也會(huì)產(chǎn)生大量的VOCs, 其種類豐富并含有獨(dú)特的化合物成分, 例如,土味素、2-甲基異莰醇、硫化物和鹵化物等[5, 6]。藻類VOCs可為多種環(huán)境因素所誘導(dǎo)產(chǎn)生[7—10], 其不僅可提高產(chǎn)生者的抗逆性, 還可作為信號(hào)物質(zhì)在水域生態(tài)系統(tǒng)中進(jìn)行信息傳遞, 從而有利于釋放者及其種群在激烈的競(jìng)爭(zhēng)環(huán)境中生存與繁衍[8, 9]。這些VOCs從水體釋放到大氣中后, 還會(huì)參與二次有機(jī)氣溶膠形成進(jìn)而影響大氣環(huán)境[11]。因此, 本文就藻類VOCs種類、形成途徑、誘導(dǎo)產(chǎn)生因素及其在生態(tài)系統(tǒng)中的功能作用進(jìn)行綜述, 以期對(duì)藻類VOCs的深入研究有所裨益。

1 藻類VOCs的種類

在水域生態(tài)系統(tǒng)中, 藻類VOCs組成豐富, 根據(jù)其化合物類型可分為萜烯類、醛類、醇類、酯類、酮類、脂肪族烴、芳香族、硫化物和鹵化物。在不同藻種中, VOCs組成存在較大差異。萊茵衣藻(Chlamydomonas reinhardtii)主要釋放醛類、醇類、酯類、酮類、萜烯類、烷烴類和烯烴類化合物[8, 9]; 銅綠微囊藻(Microcystis aeruginosa)除釋放C15-C20 脂肪族烴、臭樟腦、萜烯類、β-環(huán)檸檬醛和β-紫羅蘭酮等化合物外[12], 還釋放大量的芳香族和硫化物[10]; 紅藻會(huì)釋放大量的鹵化物[5]。藻類VOCs釋放到水體中后會(huì)改變水體品質(zhì), 使其呈現(xiàn)特殊氣味, 例如魚腥藻(Anabeana)、微囊藻和顫藻(Oscillatoria)3 種藍(lán)藻釋放的土味素和2-甲基異莰醇會(huì)使水體呈現(xiàn)泥土氣味[13]。對(duì)于食用藻類而言, 所釋放的VOCs構(gòu)成了其獨(dú)特的風(fēng)味, 例如海帶(Laminaria japonica)的腥味主要來自己醛、辛醛、壬醛、反-2-辛烯醛和1-辛烯-3-醇[14]; 龍須菜(Gracilaria lemaneiformis)的獨(dú)特風(fēng)味主要來自醛、酮等羰基類化合物和烴類化合物[15]; 壇紫菜(Pyropia haitanensis)的風(fēng)味則主要由硫化物和胺類化合物構(gòu)成[16]。

2 藻類VOCs的形成途徑

2.1 萜烯類VOCs

萜烯類VOCs是藻類和高等植物釋放最為廣泛的一類化合物, 主要有異戊二烯(半萜, C5)和以其為基本單位構(gòu)成的單萜(C10)和倍半萜(C15)[17]。根據(jù)文獻(xiàn)報(bào)道, 來自于海洋的異戊二烯釋放量約為1.9—11.6 Tg C/年[18—20], 單萜約為0.013—29.5 Tg C/年[20]。萜烯類VOCs不僅釋放量大, 而且組成豐富,例如, 銅綠微囊藻和水華微囊藻(M. flos-aquae)釋放的單萜主要是α-蒎烯、桉樹腦、檸檬烯和β-環(huán)檸檬醛, 倍半萜主要是雪松烯、長(zhǎng)葉烯和柏木烯[10]。

在真核細(xì)胞中, 單萜主要通過質(zhì)體中的甲基赤蘚醇-4-磷酸途徑(Methylerythritol-4-phosphat pathway, MEP)[21]合成; 倍半萜主要通過細(xì)胞質(zhì)中的甲羥戊酸途徑(Mevalonate pathway, MVA)[22]合成, 其中MEP途徑以三磷酸甘油醛和丙酮酸為底物,MVA途徑以乙酰-CoA為底物。這些底物經(jīng)一系列酶促反應(yīng)后均會(huì)形成二甲基烯丙基焦磷酸(DMAPP), 此化合物是所有萜烯類化合物合成的前體物質(zhì), 其在異戊二烯合成酶(ISPS)直接催化下可形成異戊二烯; 而經(jīng)不同酶催化后再在萜烯合酶(單萜合酶和倍半萜合酶)催化下則可形成單萜和倍半萜(圖1)[23, 24]。Hemmerlin等[25]研究表明, 采用膦胺霉素抑制MEP途徑后, 飼喂的甲羥戊酸可用于合成質(zhì)體醌(MEP途徑合成), 這表明此兩條途徑間存在著物質(zhì)交流。異戊烯基焦磷酸(IPP)和DMAPP是MEP和MVA途徑中2種相同的中間產(chǎn)物, 也是萜烯類VOCs合成的C5前體物質(zhì)。在這2種物質(zhì)中,IPP可能會(huì)通過質(zhì)體膜上的IPP轉(zhuǎn)運(yùn)體進(jìn)行跨膜轉(zhuǎn)運(yùn)從而使兩條途徑間進(jìn)行交流[26], 然而關(guān)于IPP跨膜轉(zhuǎn)運(yùn)尚需大量的研究予以證實(shí)。原核藻類沒有質(zhì)體等細(xì)胞器分化, 其MEP合成途徑也在細(xì)胞質(zhì)中完成, 因此IPP和DMAPP可為2條途徑所共用[27]。

此外, 藻細(xì)胞中β-胡蘿卜素經(jīng)β-胡蘿卜素7,8(7′,8′)-氧化酶氧化降解也會(huì)產(chǎn)生單萜類化合物——β-環(huán)檸檬醛(C10)[28], 然而在高氧濃度下, β-胡蘿卜素則被降解為β-紫羅蘭酮(C13)[29]。

圖1 萜烯類VOCs的合成途徑[23, 24]Fig. 1 The synthetic pathway of terpenoids

2.2 C6綠葉揮發(fā)物

C6綠葉揮發(fā)物(C6 green leaf volatiles, GLVs)是指脂肪酸經(jīng)脂氧合酶途徑(圖2)氧化降解形成的飽和與不飽和C6醇、醛和酯類VOCs[2]。對(duì)于α-亞麻酸而言, 其脂氧合酶途徑的代謝初產(chǎn)物為(Z)-3-己烯醛, 經(jīng)轉(zhuǎn)化后形成(Z)-3-己烯醇、(E)-2-己烯醛和(E)-2-己烯醇等化合物; 而亞油酸的初產(chǎn)物則為己醛, 經(jīng)醇脫氫酶轉(zhuǎn)化后為己醇(圖2)[30]。目前,GLVs的報(bào)道主要是高等植物受到機(jī)械損傷或昆蟲取食時(shí)釋放[31]。然而, 我們?cè)趯?duì)萊茵衣藻進(jìn)行研磨損傷后發(fā)現(xiàn), 其VOCs中出現(xiàn)己醛, 并且脂氧合酶活性較高; 機(jī)械損傷的兩種原核微囊藻(銅綠微囊藻和水華微囊藻)VOCs中則未發(fā)現(xiàn)GLVs, 并且水華微囊藻中的脂氧合酶活性較低(未發(fā)表)。此外, 乙酸和NaCl脅迫也會(huì)誘導(dǎo)萊茵衣藻釋放己醛, 并且其釋放量隨乙酸脅迫時(shí)間延長(zhǎng)而增加[8, 9]。由此推斷,GLVs產(chǎn)生機(jī)制可能是原核藻類進(jìn)化到真核藻類后才形成的。

2.3 脂肪族烴

脂肪族烴是指長(zhǎng)鏈烴類化合物, 在藻類VOCs中大量存在[8, 9, 32], 其來源主要是脂肪酸脫羧反應(yīng)[33]。藻細(xì)胞膜脂過氧化反應(yīng)或在脂酰水解酶作用下可產(chǎn)生游離脂肪酸, 其在脂酰-脂酰載體蛋白還原酶(Acyl-ACP還原酶)催化下形成脂肪醛, 再經(jīng)醛脫羰酶作用形成脂肪族烴[34—36]。例如, 十七烷是藍(lán)藻VOCs的標(biāo)志性化合物之一[12, 32], 由醛脫羰酶催化十八醛脫羰基形成[33]。

2.4 芳香族VOCs

芳香族VOCs是含有苯環(huán)的化合物, 其形成過程主要來自苯丙氨酸代謝。例如, 苯乙醛是苯丙氨酸脫羧后再氧化脫氨形成[37]。對(duì)烯丙基苯酚是苯丙氨酸先經(jīng)苯丙烷途徑形成香豆醇, 再被還原成二氫香豆醇后脫氫形成[38]。對(duì)傘花烴是銅綠微囊藻和水華微囊藻釋放的芳香族化合物[10], 然而其形成途徑并非來自苯丙氨酸代謝, 而是萜品烯經(jīng)O2氧化產(chǎn)生[39]。

2.5 硫化物

硫化物在高等植物中并不多見, 可作為某些植物的特有成分。例如, 芥子油甙為十字花科植物所特有, 其合成來源于氨基酸碳鏈的亞甲基化延長(zhǎng),可被昆蟲取食所誘導(dǎo)釋放[40]。在水域生態(tài)系統(tǒng)中,二甲硫醚(DMS)是大量產(chǎn)生的硫化物, 釋放量約為3×1014g/年[41], 其來源主要是海藻中的滲透調(diào)節(jié)物質(zhì)——硫代甜菜堿(Dimethylsulphoniopropionate,DMSP)降解。當(dāng)藻類死亡后, DMSP被微生物DMSP裂解酶分解或經(jīng)光氧化降解為DMS[42—44]。DMSP裂解酶不僅存在于細(xì)菌中, 也存在于一些藻類中, 例如Symbiodinium microadriaticum[45]、赫氏顆石藻(Emiliania huxleyi(Lohm.) Hay and Mohler)和大洋橋石藻(Gephyrocapsa oceanicaKamptner)[46]。de Souza等[47]從大型綠藻石莼(Ulva curvata)體內(nèi)純化出DMSP裂解酶, 其大小為78 kD, 催化反應(yīng)最適pH為8.0。由此可見, 藻類也可自身分解DMSP產(chǎn)生DMS。Lu等[48]對(duì)太湖水樣中的藻類飼喂甲硫氨酸(Met)后, 其硫化物甲硫醇、DMS、二甲基二硫醚、二甲基三硫化物和二甲基四硫化物釋放量明顯升高, 其中二甲基二硫醚的釋放量最高, 而當(dāng)飼喂葡萄糖后則抑制硫化物產(chǎn)生; 同時(shí)飼喂Met和葡萄糖后, 二甲基四硫化物釋放量最高, 由此可見,Met可能是含硫VOCs形成的前體物質(zhì), 細(xì)胞內(nèi)糖水平增加可能會(huì)改變Met代謝, 使其形成高含硫量VOCs。

圖2 GLVs的形成途徑[30]Fig. 2 The forming pathway of GLVs

2.6 鹵化物

含有鹵素(Cl、Br和I)的VOCs在高等植物中亦不多見, 而在水體藻類中則大量產(chǎn)生, 尤其是在海藻之中, 例如, 柏桉藻科、海頭紅科、根葉藻科等紅藻均會(huì)大量釋放含有鹵素的烴、烯、萜烯、醇、醛、酮、酯等化合物[5]。在藻細(xì)胞中, 依賴于釩或血紅素的鹵化物過氧化物酶利用H2O2中的2個(gè)電子氧化鹵素離子形成次鹵酸(HOI、HOBr)[49, 50],次鹵酸再與有機(jī)物反應(yīng)形成鹵化物[51]。在鹵化物過氧化物酶中, 氯化物過氧化物酶可催化氯、溴和碘離子氧化, 溴化物過氧化物酶可催化溴和碘離子氧化[50]。

3 誘導(dǎo)藻類VOCs產(chǎn)生的環(huán)境因素

藻類VOCs可自發(fā)產(chǎn)生, 也可為多種環(huán)境因素所誘導(dǎo)產(chǎn)生。光照、溫度、營(yíng)養(yǎng)條件、水體pH、鹽度等環(huán)境因子始終作用于藻類生命周期中, 這些環(huán)境因子變化會(huì)導(dǎo)致藻類次生代謝途徑改變, 并誘導(dǎo)新VOCs產(chǎn)生與釋放。

3.1 光照

藻類釋放萜烯類VOCs與光照密切相關(guān), 例如,原綠球藻(Prochlorococcus)的異戊二烯釋放速率隨光照強(qiáng)度增加而增加[52]。Bonsang等[53]研究發(fā)現(xiàn),藍(lán)藻、硅藻和綠藻均能釋放異戊二烯, 并且均依賴于光。微氏海鏈藻(Thalassiosira weissflogii)、偽矮海鏈藻(T. pseudonana)、顆石藻(Pleurochrysis carterae)和紅胞藻(Rhodomonas salina)在不同光強(qiáng)下培養(yǎng)4h時(shí), 異戊二烯釋放量最大, 同時(shí)高光強(qiáng)[420和900 μmol/(m2·s)]時(shí)釋放速率較高; 除異戊二烯外, 這幾種微藻還會(huì)釋放單萜, 并且高光強(qiáng)可促進(jìn)其釋放[54]。光促進(jìn)異戊二烯釋放, 可能與光激活異戊二烯合成酶有關(guān), 此激活過程可能是光激活葉綠體基質(zhì)中的Mg2+, 使其濃度增加或改變基質(zhì)pH所致[55]。

海藻會(huì)釋放大量的鹵化物, 其中熱帶海藻的釋放速率明顯高于溫帶海藻[56], 并且1d之內(nèi)中午的釋放速率最高[57]。巨型紅藻(Solieria chordalis)可釋放CH3I、CH3CH2I、CH2ClI、CH2Br2、CHBrCl2、

3.2 溫度

將中心目珪藻(Pleurosira laevis)和滸苔(Enteromorpha flexuosa)分別培養(yǎng)在17℃和23℃條件下,其中在較高溫度時(shí)中心目珪藻CHCl3和滸苔CHBr3釋放量均明顯增加[7]。在不同溫度(4—23.2℃)條件下, 赫氏顆石藻的生長(zhǎng)速率隨溫度升高而增加,然而其DMSP含量則逐漸降低, 這表明其DMS釋放量隨溫度升高而降低[62]。

Meskhidze等[54]通過研究9種微藻異戊二烯和單萜釋放速率表明, 溫度對(duì)藻類萜烯類VOCs釋放無明顯影響。這與高等植物萜烯類VOCs釋放速率隨溫度升高而顯著增加不同。在高等植物中, 高溫通過提高與萜烯合成相關(guān)的酶活性以及儲(chǔ)藏結(jié)構(gòu)內(nèi)部蒸汽壓而促進(jìn)其合成與釋放, 并且這些萜烯類VOCs具有保護(hù)高等植物抵抗高溫脅迫的作用[63—65]。藻類不具備萜烯類VOCs儲(chǔ)存結(jié)構(gòu), 并且生活的水體溫度相對(duì)較低, 因此可能未進(jìn)化出高溫誘導(dǎo)萜烯釋放的響應(yīng)機(jī)制[52]。

肺衣(Lobaria pulmonaria)是真菌、藍(lán)藻和綠藻的三者共生體, 其中藍(lán)藻主要是念珠藻(Nostoc),綠藻主要是Dyctiochloropsis。在熱擊脅迫下, 其釋放的β-環(huán)檸檬醛、3-羥基環(huán)檸檬醛、紫羅烯、二氫獼猴桃內(nèi)酯、香葉基丙酮和羥基羅蘭酮等β-胡蘿卜素降解產(chǎn)物明顯增加, 其原因可能是高溫誘導(dǎo)ROS大量產(chǎn)生并氧化β-胡蘿卜素形成[66]。

3.3 營(yíng)養(yǎng)條件

隨著經(jīng)濟(jì)社會(huì)發(fā)展, 大量營(yíng)養(yǎng)物質(zhì)排放到水體當(dāng)中, 從而致使水域生態(tài)系統(tǒng)內(nèi)含有多樣且復(fù)雜的氮磷營(yíng)養(yǎng)。對(duì)于氮素而言, 可分為無機(jī)氮(、和)和有機(jī)氮(脲、多肽和氨基酸)[67, 68]。當(dāng)采用NaNO3、NaNO2、NH4Cl、脲、絲氨酸、賴氨酸和精氨酸作為唯一氮源培養(yǎng)水華微囊藻24h時(shí), 其釋放的呋喃類、硫化物、萜烯類、芳香族、脂肪族烴、醛類和酯類VOCs存在明顯差異, 其中NaNO3和脲作為氮源時(shí)VOCs種類最多[32]。與VOCs釋放相似, 不同氨基酸作為氮源培養(yǎng)銅綠微囊藻后, 其藻毒素釋放量不同, 其中丙氨酸、亮氨酸和精氨酸作為氮源時(shí)釋放量較多[69]。除不同氮源外, 不同氮濃度也會(huì)影響藻類VOCs釋放。當(dāng)培養(yǎng)基中氮素營(yíng)養(yǎng)耗盡時(shí), 銅綠微囊藻的醇類和β-環(huán)檸檬醛釋放量明顯增加[70]。以NaNO3作為氮源培養(yǎng)水華微囊藻, 其VOCs釋放量隨氮濃度降低而明顯增加, 并且在無氮時(shí)釋放量最大[32]。不同氮濃度培養(yǎng)銅綠微囊藻的結(jié)果與水華微囊藻相似, 通過轉(zhuǎn)錄組分析表明, 無氮條件促進(jìn)了與MEP和MVA途徑前體物質(zhì)形成以及膜脂水解相關(guān)酶基因的表達(dá),從而促進(jìn)了萜烯類和脂肪族烴類VOCs的產(chǎn)生與釋放(未發(fā)表)。

在水域生態(tài)系統(tǒng)中, 磷易形成不溶性鹽沉淀,是藻類生長(zhǎng)的限制性營(yíng)養(yǎng)元素。除無機(jī)磷外, 多聚磷酸鹽亦是水體中可溶性磷的主要存在形式[71]。分別采用K2HPO4和2種多聚磷酸鹽(焦磷酸鈉與六偏磷酸鈉)作為磷源培養(yǎng)水華微囊藻, 其VOCs釋放量以六偏磷酸鈉作為磷源時(shí)最大; 并且隨著磷(K2HPO4)濃度降低VOCs釋放量逐漸增加(未發(fā)表)。此外, 限制P營(yíng)養(yǎng)還會(huì)促進(jìn)藍(lán)藻Trichormus doliolum藻毒素產(chǎn)生, 與對(duì)照相比提高了2.9倍[72]。

隨著水體富營(yíng)養(yǎng)化加劇, 水體中多樣且復(fù)雜的氮磷營(yíng)養(yǎng)會(huì)改變?cè)孱怴OCs釋放, 并誘導(dǎo)新VOCs產(chǎn)生; 同時(shí)隨著優(yōu)勢(shì)藻類的大量繁殖生長(zhǎng), 水體中的營(yíng)養(yǎng)物質(zhì)會(huì)進(jìn)一步消耗甚至出現(xiàn)匱乏, 從而誘導(dǎo)VOCs的大量產(chǎn)生與釋放。

3.4 酸、堿、鹽脅迫

萊茵衣藻釋放的VOCs種類豐富, 當(dāng)采用乙酸、Na2CO3和NaCl對(duì)藻細(xì)胞進(jìn)行酸、堿、鹽脅迫時(shí),其VOCs釋放量均明顯增加, 并且乙酸和NaCl脅迫還會(huì)誘導(dǎo)GLVs (己醛)釋放, 而Na2CO3脅迫則誘導(dǎo)3, 4-二甲基-己烷和5-甲基-2-庚烯兩種特異性成分釋放[8, 9]。由此可見, 乙酸和NaCl脅迫均能誘導(dǎo)脂氧合酶發(fā)揮作用, 從而產(chǎn)生GLVs, 而Na2CO3脅迫則不能誘導(dǎo)脂氧合酶發(fā)揮作用, 其原因可能是Na2CO3脅迫所造成的堿性環(huán)境會(huì)導(dǎo)致脂氧合酶失活。與NaCl處理萊茵衣藻相似, NaNO3脅迫水華微囊藻和銅綠微囊藻24h后, 2種微囊藻中的萜烯類、硫化物、脂肪族烴、芳香族、醛類和酯類VOCs釋放量均明顯增加, 但是并無GLVs釋放[10], 這可能與微囊藻體內(nèi)脂氧合酶活性較低有關(guān)(未發(fā)表)。此外, 鹽脅迫還會(huì)誘導(dǎo)巨型紅藻和Gymnongongrus antarcticus釋放鹵化物[58, 73], 這可能與鹽脅迫誘導(dǎo)藻細(xì)胞H2O2產(chǎn)生以促進(jìn)鹵素離子氧化有關(guān)[74]。

4 藻類VOCs的生態(tài)作用

4.1 提高藻細(xì)胞抗逆性

在鹵化物形成過程中, 鹵素離子的氧化需要H2O2提供提電子, 在高光強(qiáng)、鹽和H2O2脅迫條件下, 鹵化物釋放量均明顯增加[58, 59, 61]。這表明鹵化物形成有利于降低藻細(xì)胞內(nèi)ROS水平, 從而提高藻細(xì)胞對(duì)逆境引起的次生氧化脅迫的抵抗能力。在熱擊脅迫下, 肺衣的β-胡蘿卜素降解產(chǎn)物(β-環(huán)檸檬醛等)釋放量明顯增加, 這些降解產(chǎn)物可能是β-胡蘿卜素淬滅ROS時(shí)的氧化產(chǎn)物, 因此其產(chǎn)生過程是β-胡蘿卜素抵抗ROS的結(jié)果[66]。在高等植物體內(nèi), 萜烯類VOCs釋放可提高植物抵抗高溫和ROS脅迫的能力, 其原因可能為萜烯類VOCs具有淬滅ROS、穩(wěn)定細(xì)胞膜和光合系統(tǒng)的作用[64, 65, 75, 76]。在藻類中, 萜烯類VOCs是其釋放的一類主要化合物, 可被高光強(qiáng)、酸、堿、鹽等逆境脅迫所誘導(dǎo)產(chǎn)生[8, 9, 54],由此推斷, 此類VOCs可能也具有提高藻細(xì)胞抵抗逆境脅迫的能力。pH 5.0乙酸可誘導(dǎo)萊茵衣藻發(fā)生細(xì)胞程序性死亡(Programmed cell death, PCD),在此過程中, VOCs可能會(huì)淬滅細(xì)胞內(nèi)的ROS, 從而使含氧VOCs釋放量明顯增加[77]。由此可見, 在逆境脅迫下, 藻類VOCs的產(chǎn)生與釋放可提高藻細(xì)胞的抗逆性, 從而有利于藻細(xì)胞在逆境脅迫中生存與繁衍。

4.2 誘導(dǎo)防御

萊茵衣藻在pH 5.0乙酸誘導(dǎo)發(fā)生PCD過程中以及NaCl和Na2CO3脅迫下所釋放的VOCs均可誘導(dǎo)其他正常萊茵衣藻細(xì)胞內(nèi)ROS含量升高、保護(hù)酶活性增強(qiáng), 同時(shí)生長(zhǎng)減緩, 這表明逆境脅迫誘導(dǎo)的VOCs在藻細(xì)胞間進(jìn)行了信息傳遞。VOCs通過誘導(dǎo)萊茵衣藻細(xì)胞ROS產(chǎn)生而誘導(dǎo)其保護(hù)酶活性增強(qiáng), 進(jìn)而使其降低生長(zhǎng)并作好防御準(zhǔn)備[8, 9]。Yordanova等[78]研究發(fā)現(xiàn), 黃峰毒素在誘導(dǎo)萊茵衣藻發(fā)生PCD過程中會(huì)釋放乙烯和NO, 采用已發(fā)生PCD的培養(yǎng)基培養(yǎng)正常細(xì)胞后再采用黃峰毒素處理,則無PCD發(fā)生, 推測(cè)乙烯和NO可能為正常細(xì)胞傳遞了脅迫信息。由此可見, 藻細(xì)胞在脅迫條件下可釋放VOCs為其他同種細(xì)胞傳遞脅迫信息, 從而有利于種群生存與繁衍。這種脅迫信息的傳遞與高等植物中VOCs誘導(dǎo)臨近植株防御昆蟲取食相類似[3, 31]。高等植物是由藻類不斷演化而來, 萊茵衣藻是最原始的真核生物, 因此, 植物間的VOCs信號(hào)傳遞機(jī)制可能是進(jìn)化過程中的一種保留機(jī)制, 從而有利于植物應(yīng)對(duì)復(fù)雜的自然環(huán)境并促進(jìn)種群生存與繁衍。

4.3 對(duì)其他藻類的化感作用

在富營(yíng)養(yǎng)化水體中, 藍(lán)藻大量繁殖后, 其他藻類明顯減少, 生物多樣性降低。在此過程中, 除了藻毒素具有抑制其他藻類和水生生物生長(zhǎng)的作用外[79, 80], VOCs也具有化感抑制作用[32, 81]。例如, 采用缺氮條件模擬藍(lán)藻大量繁殖后水體營(yíng)養(yǎng)元素缺乏, 水華微囊藻在此條件下釋放的VOCs可明顯抑制小球藻(Chlorella vulgaris)生長(zhǎng)[32]。

藍(lán)藻釋放的VOCs種類眾多, 在這些化合物中,β-環(huán)檸檬醛、α-紫羅蘭酮、β-紫羅蘭酮和香葉基丙酮等β-胡蘿卜素降解產(chǎn)物具有抑制蛋白核小球藻(C. pyrenoidosa)生長(zhǎng)的作用, 其抑制濃度為2—5 mg/mL[81]。采用β-環(huán)檸檬醛處理谷皮菱形藻(Nitzschia palea), 在0.1—0.5 mg/L時(shí)便可引起15%—20%的藻細(xì)胞發(fā)生破裂[82]。此外, β-環(huán)檸檬醛處理還會(huì)改變?cè)寮?xì)胞顏色, 使微囊藻細(xì)胞呈現(xiàn)藍(lán)綠色, 席藻(Phormidium)呈現(xiàn)粉色, 其原因?yàn)棣?環(huán)檸檬醛通過引起葉綠素a和β-胡蘿卜素迅速降解而改變?cè)寮?xì)胞顏色[83]。由此可見, β-環(huán)檸檬醛可通過引起光合色素降解和細(xì)胞裂解來發(fā)揮化感作用。

在陸地生態(tài)系統(tǒng)中, 萜烯類VOCs是發(fā)揮化感作用的主要化合物。采用水華微囊藻釋放的兩種主要單萜——桉樹腦和檸檬烯處理小球藻和萊茵衣藻后, 兩種藻細(xì)胞的光合色素均會(huì)發(fā)生降解, 其中檸檬烯處理后葉黃素和單乙烯基葉綠素b降解最為嚴(yán)重; 同時(shí)桉樹腦和檸檬烯還會(huì)抑制光系統(tǒng)II(PSII)電子產(chǎn)生與傳遞, 從而使光合效率降低[32, 84, 85]。Zeraatpisheh和Vatanparast[86]研究表明, 桉樹腦具有抑制蝸牛中樞神經(jīng)元K+通道的作用。這表明桉樹腦除可引起藻細(xì)胞光合色素降解和降低光合效率外, 還可能通過抑制K+離子通道擾亂細(xì)胞內(nèi)離子平衡以發(fā)揮化感作用, 然而, 有關(guān)此方面的作用機(jī)制還有待于進(jìn)一步深入研究。

此外, 在藻類VOCs中, 脂肪酸降解產(chǎn)生的十七烷和十四烷可抑制細(xì)菌和念珠菌(Candida albicans)生長(zhǎng)[87]; 二甲基二硫醚可抑制尖孢鐮刀菌(Fusarium oxysporum)細(xì)胞生長(zhǎng)和萵苣(Lactuca sativa)種子萌發(fā)[88], 這表明脂肪族烴和硫化物對(duì)藻細(xì)胞也可能具有化感抑制作用。

4.4 驅(qū)避捕食者

在自然界中, 藻類不但受到環(huán)境變化影響, 還時(shí)刻受到捕食者威脅。在應(yīng)對(duì)捕食者威脅時(shí), 藻細(xì)胞除了形成聚集體和釋放藻毒素外[89], 還可通過VOCs進(jìn)行驅(qū)避。采用褐藻Dictyopteris membranacea飼喂一種藻溝蝦Ampithoe longimana, 與產(chǎn)生C11硫化物的藻株相比, 藻溝蝦更傾向于取食無硫化物產(chǎn)生的藻株[90]。圓海鏈藻(T. rotula)在受到損傷時(shí)會(huì)通過活化磷脂酶A2水解膜脂以釋放游離脂肪酸, 其再經(jīng)脂氧合酶水解產(chǎn)生2,4-癸二烯醛和2,4,7-癸三烯醛用于防御[91]。當(dāng)采用β-環(huán)檸檬醛和2,4,7-癸三烯醛處理水蚤(Daphnia magna)時(shí), 其泳速迅速增加, 這表明藻類釋放的此兩種化合物可用于驅(qū)避捕食者[92, 93]。然而, 對(duì)卵蘿卜螺(Radix ovata)而言, 其可利用絲藻(Ulothrix fimbriata)釋放的VOCs信息進(jìn)行捕食[94]。

4.5 影響大氣化學(xué)

植物釋放的VOCs進(jìn)入大氣層后, 其不飽和鍵極易發(fā)生光氧化反應(yīng), 進(jìn)而形成低揮發(fā)性氧化物,這些氧化物再經(jīng)氣態(tài)/粒子態(tài)轉(zhuǎn)化凝結(jié)后形成二次有機(jī)氣溶膠[95]。雖然水域環(huán)境中的VOCs釋放量明顯低于陸地生態(tài)系統(tǒng), 但是這些VOCs卻可明顯影響廣闊海洋上空的大氣環(huán)境。萜烯類VOCs含有1到多個(gè)不飽和雙鍵, 與大氣中的O3、·OH、NOx極易發(fā)生氧化反應(yīng)并形成多種氧化產(chǎn)物[96, 97]。例如,當(dāng)大氣中O3濃度為0.625 μmol/L時(shí), 植物釋放的α-萜品烯可在5min內(nèi)進(jìn)行反應(yīng)[98]。月桂烯、萜品油烯、3-蒈烯、α-蒎烯、β-蒎烯與·OH反應(yīng)均可生成酮、醛和甲酸[99]。在與O3反應(yīng)時(shí), α-蒎烯可生成蒎酮酸、蒎醛酸、降蒎酮酸、降蒎醛酸和蒎酸等5種氧化產(chǎn)物[100]。萜烯類VOCs是藻類釋放的一類主要化合物, 其在海洋大氣二次有機(jī)氣溶膠形成中具有重要作用[19]。DMS主要由海藻中DMSP分解形成, 其釋放量約為0.5—1.0×1012摩爾/年[41]。DMS在大氣中被氧化為酸性氣溶膠顆粒, 可明顯影響海洋上空的云層形成與太陽輻射, 這表明藻類釋放的VOCs在海洋上空形成氣溶膠后可能會(huì)影響氣候形成, 進(jìn)而影響人類在海洋上的生產(chǎn)活動(dòng)[11, 101]。

5 結(jié)語

在水域生態(tài)系統(tǒng)中, 藻類時(shí)刻處于多種環(huán)境因子的影響之中, 這些環(huán)境因子變化均會(huì)誘導(dǎo)藻細(xì)胞釋放大量的VOCs。在這些VOCs中, 哪些化合物是藻細(xì)胞應(yīng)對(duì)環(huán)境變化的功能成分, 同時(shí)其通過何種機(jī)制提高藻細(xì)胞的抵抗能力, 尚需大量的VOCs單體和突變體藻株予以證實(shí)。藻類VOCs作為水域生態(tài)系統(tǒng)中的信號(hào)物質(zhì), 其產(chǎn)生、傳遞以及感受者(藻類或水生動(dòng)物)感知與應(yīng)答是一個(gè)復(fù)雜過程, 是水生生物在漫長(zhǎng)進(jìn)化過程中所形成的生存策略。VOCs在空氣中易于傳播, 而水體中藻類VOCs如何進(jìn)行傳播及其傳播的有效距離尚不清楚。藻類釋放的VOCs種類眾多, 這些VOCs中哪些是傳遞信息的化合物亦不明確。VOCs信號(hào)為感受者所感知后,

其信號(hào)作用得以發(fā)揮——誘導(dǎo)防御、抑制藻細(xì)胞生長(zhǎng)以及驅(qū)避捕食者。在此過程中, VOCs信號(hào)所傳遞的具體信息、感受VOCs信號(hào)的受體是何物質(zhì)以及胞內(nèi)信號(hào)如何進(jìn)行傳遞并調(diào)控基因表達(dá), 這些都是闡明藻類VOCs信號(hào)機(jī)制所需研究的問題。藻類VOCs中, 不為感受者所利用的成分經(jīng)水體表面揮發(fā)后進(jìn)入大氣層中, 從而參與二次有機(jī)氣溶膠的形成。海藻VOCs進(jìn)入海洋上空形成二次有機(jī)氣溶膠后對(duì)海洋氣候具有哪些影響, 海洋氣候變化后是否會(huì)反作用于海藻生長(zhǎng)及其VOCs釋放, 從而使兩者間形成相互調(diào)節(jié)機(jī)制, 對(duì)此問題進(jìn)行闡釋將有助于氣候預(yù)測(cè)以及海洋的開發(fā)與利用。

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The REVIEW OF RESEARCH ADVANCES IN ALGAL VOLATILE ORGANIC COMPOUNDS

ZUO Zhao-Jiang
(School of Forestry and Biotechnology, Zhejiang A amp;F University, Lin’an 311300, China)

In aquatic ecosystem, abundance of various volatile organic compounds (VOCs) are released from algae, including terpenoids, aldehydes, alcohols, esters, aliphatic hydrocarbons, aromatic series, sulfides and halogenides. These VOCs are synthesized from different secondary metabolism pathways, which can be induced by lots of environmental factors. Under environmental stresses, VOCs play an important role in improving algal resistance. When VOCs are released to water bodies, they transfer stress information to the congener algal cells, and the acceptors can prepare to defense the coming stress. For heterogeneous algal cells, VOCs can inhibit their growth via allelopathic effects and keep emitter's competitive advantages. Meanwhile, VOCs can also repel predators to protect algal cells. The VOCs which can not be captured by acceptors volatilize into the atmosphere and take part in the formation of secondary organic aerosols over the waters. To aid in helping the study of algal VOCs, this research review summarizes the types, forming pathways, inducing factors and ecological functions of algal VOCs.

Volatile organic compounds; Forming pathway; Inducing factor; Ecological function

Q946

A

1000-3207(2017)06-1369-11

2016-11-18;

2017-04-11

國(guó)家自然科學(xué)青年基金(31300364); 浙江省自然科學(xué)基金(LY17C160004); 浙江農(nóng)林大學(xué)科研發(fā)展基金人才啟動(dòng)項(xiàng)目(2013FR069)資助 [Supported by the National Natural Science Foundation of China (No. 31300364); the Natural Science Foundation of Zhejiang Province (No. LY17C160004); the School Development Fund for Scientific Research Personnel Startup Project (No.2013FR069)]

左照江(1982—), 男, 河北肅寧人; 博士; 副教授; 主要從事化學(xué)生態(tài)研究。E-mail: zuozhaojiang@126.com

10.7541/2017.169

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