李紅敏,裴海燕,2??,孫炯明,金 巖,徐杭州
(1:山東大學環(huán)境科學與工程學院,濟南250100)(2:山東省環(huán)境科學工程技術(shù)研究中心,濟南250061)
擬柱孢藻(Cylindrosperm opsis raciborskii)及其毒素的研究進展與展望?
李紅敏1,裴海燕1,2??,孫炯明1,金 巖1,徐杭州1
(1:山東大學環(huán)境科學與工程學院,濟南250100)
(2:山東省環(huán)境科學工程技術(shù)研究中心,濟南250061)
隨著水體富營養(yǎng)化程度的加劇,淡水藍藻水華在全球范圍內(nèi)頻繁暴發(fā).擬柱孢藻(Cylindrospermopsis raciborskii)作為一類有害水華藍藻,具有一定的產(chǎn)毒能力和較強的入侵性,日漸受到人們的廣泛關(guān)注.近年來,擬柱孢藻不斷由熱帶、亞熱帶地區(qū)向溫帶地區(qū)擴張,并在眾多水體中出現(xiàn)暴發(fā)現(xiàn)象,若其毒素進入到飲用水中,無疑將對人類健康造成潛在威脅.本文從擬柱孢藻的分布演變、生態(tài)生理特征、毒性、檢測和去除等研究前沿方面進行了綜述,并對相關(guān)研究領(lǐng)域進行展望,為進一步研究擬柱孢藻的生態(tài)毒理、有效控制和去除提供科學依據(jù).
擬柱孢藻;全球分布;生態(tài)生理學;擬柱孢藻毒素;檢測;去除
近年來,由于水體富營養(yǎng)化,藍藻水華在湖泊、水庫及河流等水體中頻繁暴發(fā),不僅對當?shù)厮鷳B(tài)系統(tǒng)平衡造成一定破壞,而且某些產(chǎn)毒藍藻的大規(guī)模擴散,還會導致水體中毒素濃度升高,當被污染水體作為飲用水水源或娛樂水源時,會對人類和動物的健康造成嚴重危害[1-2].在世界范圍內(nèi),可形成水華的藍藻除最常見的微囊藻、魚腥藻和束絲藻之外[2],擬柱孢藻(Cylindrospermopsis raciborskii)作為能產(chǎn)生毒素的另一藍藻種類,由于其毒性、暴發(fā)性和入侵性而日益受到關(guān)注[3-5].擬柱孢藻對氮、磷營養(yǎng)鹽的利用效率高,對溫度和光照的適應性強,在高溫、富營養(yǎng)化的湖泊和水庫中可快速生長,頻繁形成水華[6-7].
擬柱孢藻可產(chǎn)生柱孢藻毒素(cylindrospermopsin,CYN)和麻痹性貝類毒素(paralytic shellfish poisoning,PSP)等有毒物質(zhì)[8-10],其中CYN作為一種肝毒素,能導致肝臟和腎臟受損,并可對DNA和RNA產(chǎn)生損傷[9,11-13].隨著全球氣候變暖以及水體富營養(yǎng)化程度的加劇,擬柱孢藻呈現(xiàn)出顯著的全球擴張趨勢,其種群分布不斷由熱帶、亞熱帶向溫帶地區(qū)擴張,全球諸多地區(qū)的淡水飲用水水源地相繼檢測到擬柱孢藻的存在,且具有成為優(yōu)勢種的潛能.
擬柱孢藻在全球范圍內(nèi)的快速擴張,無疑將對眾多淡水水源地造成嚴重威脅,因此有必要對該藻進行深入研究以更好地了解其生理特征,從而有效控制和消除該藻對人類的潛在危害.目前,人們對其地域擴張、生長條件和產(chǎn)毒特性等進行了初步探索研究,但國內(nèi)學者對其關(guān)注和研究較少.此外,有效檢測和去除擬柱孢藻及其毒素對保證飲用水安全具有十分重要的意義,也相繼受到了各國環(huán)境研究者的關(guān)注.
本文主要從擬柱孢藻的分布、生態(tài)生理特征、產(chǎn)毒種類及毒性、檢測和去除等幾個方面進行概述,并對相關(guān)領(lǐng)域進行展望,為擬柱孢藻的進一步研究提供參考.
在全球范圍內(nèi),擬柱孢藻目前主要分布在熱帶、亞熱帶地區(qū),并逐漸向溫帶地區(qū)擴散[14-15].早在1899-1900年,人們在印度尼西亞爪哇島首次發(fā)現(xiàn)擬柱孢藻[16],1939年在印度和其他熱帶地區(qū)也發(fā)現(xiàn)了它的存在[17],因而人們初步判定其為一株熱帶藻種(表1).此后,諸多報道相繼證明擬柱孢藻在熱帶、亞熱帶和溫帶地區(qū)以及除南極洲以外的所有大洲均有分布[3],其全球分布情況如圖1所示.
在國內(nèi),擬柱孢藻廣泛分布于廣東省的多處水庫和湖泊中[18-23],并在鶴地、高州等水庫及多個蝦池中檢測到其水華的暴發(fā)[7,24].此外,福建[25]、云南[26-27]、湖北[28-29]、臺灣[30]、太湖流域[31]以及山東[29]等地也均檢測到擬柱孢藻的存在.
1979年澳大利亞昆士蘭發(fā)生的“帕門島神秘疾病”事件使人們認識到擬柱孢藻存在產(chǎn)毒藻種[9].由于接觸了擬柱孢藻污染的水體,當?shù)?48名居民表現(xiàn)出中毒現(xiàn)象,并伴有腸胃炎的癥狀[8],隨后研究人員發(fā)現(xiàn)從這些水體中分離出來的擬柱孢藻具有高毒性,并將其產(chǎn)生的化合物命名為柱孢藻毒素[32].截至目前,產(chǎn)毒擬柱孢藻主要分布于澳大利亞[5,9]、新西蘭[33]、亞洲的東部和東南部地區(qū)[23-34]以及沙特阿拉伯地區(qū)[35].
目前,研究人員針對擬柱孢藻的分布和擴張?zhí)岢龅募僬f主要有兩種[36]:其一是非洲中心說,即認為擬柱孢藻起源于非洲的熱帶湖泊,隨后擴散到赤道附近的印度尼西亞和中美洲等地區(qū);其二是澳大利亞中心說,即由澳大利亞散布到其他熱帶、亞熱帶及溫帶地區(qū).以上推斷都是在流行病學、水文數(shù)據(jù)和物種生理特征的分析基礎之上建立起來的.目前,眾多學者正通過對不同區(qū)域的藻種進行基因數(shù)據(jù)分析來推斷其相似性以進一步驗證假說的科學性[4,14,37-38].此外,人們對于擬柱孢藻擴張到溫帶地區(qū)的具體原因,尚未達成共識.根據(jù)目前文獻介紹[3,5,14],推斷擬柱孢藻不斷向溫帶地區(qū)擴張的原因主要為:水體富營養(yǎng)化程度的加劇及全球氣候變暖;擬柱孢藻對溫度及氮磷等外部條件的適應性強;不同區(qū)域之間的船艙運輸、鳥類傳播等.
圖1 全球擬柱孢藻分布(修改自參考文獻[3])(?擬柱孢藻,*擬柱孢藻可產(chǎn)CYN及CYN類似物,?擬柱孢藻可產(chǎn)PSP)Fig.1 Global distribution of Cylindrospermopsis raciborskii
表1 擬柱孢藻出現(xiàn)地區(qū)匯總Tab.1 A region summary of Cylindrospermopsis raciborskii occurrence
2.1 形態(tài)特征
擬柱孢藻為藍藻類原核生物,屬于念珠藻目念珠藻科擬柱孢藻屬,以拉式擬柱孢藻(Cylindrospermopsis raciborskii)為模式種[86].
擬柱孢藻是一種絲狀藻,整條藻絲粗細均勻,但不同地區(qū)形態(tài)大小各具差異.本實驗室拍攝到擬柱孢藻(分離自山東省某水庫)形態(tài)如圖2所示.藻絲通常寬2~5μm,長度變化范圍大(圖2),常介于10~1000μm之間[87-88],大多數(shù)為直線形,某些地區(qū)的藻絲呈卷曲形[71,89];擬柱孢藻的單個細胞長度約為3~10μm,由于其很少出現(xiàn)細胞收縮,單個細胞往往較難區(qū)分[88];在其生長旺盛期,可觀測到偽空胞,為其在水體中提供浮力,以獲得適宜的生長條件;藻絲末端有時可見異形胞(圖2A、D),數(shù)目少而不定,具有固氮功能.野生擬柱孢藻常有異形胞,而實驗室人工培養(yǎng)的擬柱孢藻少見異形胞且藻體較大.Plominsky等[90]實驗研究發(fā)現(xiàn),在缺乏無機氮源的條件下,擬柱孢藻可產(chǎn)生異形胞進行固氮作用,當繼續(xù)供給無機氮源時,約5 d后異形胞又會自行消失.此外,當出現(xiàn)環(huán)境脅迫時,擬柱孢藻可形成厚壁孢子(圖2),且孢子一般位于藻絲中部和尾部之間,每個個體的孢子數(shù)量一般為1~4個[89].
2.2 暴發(fā)繁殖特點
隨著全球氣候變暖程度的加深,擬柱孢藻的生長范圍逐漸擴大[5,36].擬柱孢藻可在熱帶和亞熱帶水體中成為優(yōu)勢藻種,在夏季容易形成水華,當溫帶地區(qū)氣溫較高時(夏季)可在水體中占優(yōu)勢[23,91-92].擬柱孢藻適合在溫度為20~35℃條件下生長,最適生長溫度約30℃[93],適宜高溫生長的特點表明它是最可能從全球氣候變暖中受益的藍藻之一.擬柱孢藻水華既可在分層的、較深的水庫(>15m)中暴發(fā)[92],也可在淺水中暴發(fā)[81,91].由于擬柱孢藻含有偽空胞,在水體中不易下沉,可在水體中自由浮動,所以其生長對光照要求不高,多項研究表明,擬柱孢藻的暴發(fā)與水體是否分層無關(guān)[94-95].
圖2 擬柱孢藻常見形態(tài)(A:具有末端異形胞;B:具有一個靠近末端的厚壁孢子(無異形胞);C:一條具有一個靠近藻絲中部的厚壁孢子(無異形胞),另一條既無厚壁孢子也無異形胞;D:具有一個末端異形胞和三個靠近藻絲末端的厚壁孢子(不靠近異形胞))Fig.2 Common morphology of Cylindrospermopsis raciborskii
2.3 生長影響因素
2.3.1 營養(yǎng)鹽的影響 擬柱孢藻呈現(xiàn)多樣化的營養(yǎng)策略,對氮、磷營養(yǎng)鹽的利用效率高.擬柱孢藻能夠適應不同的氮源,在低氮條件下可產(chǎn)生異形胞來固定空氣中的氮氣,且能有效利用有機氮[96-97].實驗研究發(fā)現(xiàn),當供給氮源為時藻絲生長速率最快,其次為最后為尿素氮[98-99].在產(chǎn)毒方面,不同藻株產(chǎn)毒情況受氮濃度影響不同.例如,對于產(chǎn)生CYN的擬柱孢藻藻株(Woloszynska),有研究表明,當?shù)磪T乏時會促進CYN的生成,相反地,當提供充足氮源(尤其是)時,CYN濃度降低[100];而對于產(chǎn)生蛤蚌毒素的擬柱孢藻藻株(分離自巴西),Brentano等[101]發(fā)現(xiàn)隨著溶解性無機氮的增加,其產(chǎn)生的蛤蚌毒素濃度增大.為此,擬柱孢藻的具體產(chǎn)毒機理與氮源之間的關(guān)系仍需進一步探究.
擬柱孢藻對磷有很強的吸收和儲存能力,在磷營養(yǎng)不足的環(huán)境下容易成為優(yōu)勢藻種,當湖泊或水庫中磷濃度較低時,擬柱孢藻更易形成水華[97].擬柱孢藻相比微囊藻、束絲藻具有更高的胞外磷酸酶活性,在缺乏無機磷源的環(huán)境下可以充分利用有機磷源[94,97].最近,Willis等[102]在增加磷濃度的條件下發(fā)現(xiàn)擬柱孢藻生長速率下降,猜測磷會被優(yōu)先儲存而非用于其生長.另有研究發(fā)現(xiàn),在磷限制的情況下,CYN產(chǎn)生率與其指數(shù)生長期的生長率呈正相關(guān)[103].
此外,戴景峻等[104]發(fā)現(xiàn)氮對擬柱孢藻N8的生長限制作用比磷更為顯著,低氮顯著限制擬柱孢藻N8的生長,這種抑制不因磷濃度的升高而解除,相反,氮濃度的升高可延長擬柱孢藻在磷限制性條件下的生長時間.而有研究表明,低的氮磷比能誘導湖泊中固氮藍藻的形成[105],因此,不同氮磷比對擬柱孢藻的影響情況有待進一步研究.
近年來,關(guān)于氮、磷營養(yǎng)元素對擬柱孢藻影響的研究逐漸增多,這對了解擬柱孢藻的營養(yǎng)利用策略和預測擬柱孢藻在不同營養(yǎng)條件下暴發(fā)的可能性具有重要意義.目前,關(guān)于氮、磷對其生長的影響機理研究尚不充分.尤其關(guān)于N、P等營養(yǎng)元素對擬柱孢藻產(chǎn)毒的影響研究較少,例如,關(guān)于氮磷對其產(chǎn)毒的具體影響機制尚不明確,仍需要對不同區(qū)域藻種做一步研究.
2.3.2 溫度及光照的影響 擬柱孢藻對溫度和光照的適應性較強,能夠適應環(huán)境中波動的溫度和光照條件,但溫度和光照等環(huán)境因子對擬柱孢藻的形態(tài)變化和產(chǎn)毒也有一定影響[64,87,89,106].研究表明,即使在溫度低至16℃時,擬柱孢藻仍可保持一定的生物量[107],Bonilla等[108]發(fā)現(xiàn)擬柱孢藻甚至可以在11℃的條件下生長,而最適生長溫度可達到35℃[112].溫度能夠影響厚壁孢子的萌發(fā)和細胞分裂,Briand等[83]發(fā)現(xiàn)足夠的溫度能夠促進藻體孢子萌發(fā).其次,溫度對藻絲長度也有一定影響,有研究發(fā)現(xiàn),低溫和高溫處理下擬柱孢藻藻絲長度小于正常溫度下藻絲長度,猜測可能是由于溫度過高和過低會對藻細胞分裂分化活動受到影響,在形態(tài)上表現(xiàn)為藻絲變短[110],其具體原因仍需探討.此外,溫度也會影響CYN產(chǎn)量,有研究發(fā)現(xiàn),擬柱孢藻最適生長溫度和最大產(chǎn)毒量并不成正相關(guān),例如Saker等[64]研究表明,在20℃時細胞毒性最高,而擬柱孢藻在25~30℃時有最大生長率,但該溫度下藻毒素濃度較低.
Wojciechowski等[87]以生長速率和藻絲長度為指標評估了擬柱孢藻在不同光照條件下的生態(tài)生理學反應,實驗結(jié)果表明,擬柱孢藻在低光照強度下的生長速率明顯低于較高光照強度下的生長速率,而在低光照強度下藻絲的平均長度卻明顯大于高光照強度下的藻絲長度.在產(chǎn)毒方面,光照是否影響其產(chǎn)毒能力也存在爭議.有研究表明光照強度、光照周期及光譜成分影響擬柱孢藻產(chǎn)毒能力,擬柱孢藻細胞內(nèi)蛤蚌毒素(STX)濃度與光照強度呈正相關(guān),黑暗條件下產(chǎn)生的毒素濃度較低,較高光強條件下細胞內(nèi)STX濃度明顯增高[111].而Pierangelini等[106]發(fā)現(xiàn)擬柱孢藻在不同光照條件下,平均胞內(nèi)CYN濃度不變,表明CYN的產(chǎn)生不受光照條件影響.因此,為更好地理解擬柱孢藻在亞熱帶地區(qū)季節(jié)性暴發(fā)的成因及危害,后續(xù)研究有必要針對更多不同地區(qū)的藻株,進一步探究光照和溫度條件對擬柱孢藻的生長、形態(tài)變化和產(chǎn)毒等的影響及其相關(guān)機理.
2.3.3 鹽度的影響 總體來說,擬柱孢藻適合在低鹽度的條件下生長,淡水環(huán)境最適宜其生長[36],而高鹽條件對擬柱孢藻的生長會產(chǎn)生抑制作用[112].但當水體中溶解性礦物質(zhì)濃度較高時,擬柱孢藻也可在微咸水中生長[91,113].Moisander等[114]研究發(fā)現(xiàn),當NaCl濃度為2~6 g/L時,擬柱孢藻的生長受到限制,而高于10 g/L時已不能通過固定二氧化碳進行光合作用.
2.3.4 pH的影響 擬柱孢藻能夠在pH較高的水體中生存.雖然pH的升高會使CO2濃度降低,但擬柱孢藻可以有效利用其他的碳源如進行能源補充[115].在高pH值和低CO2環(huán)境下,擬柱孢藻是良好的競爭者,可成為優(yōu)勢藻種[116].據(jù)報道,擬柱孢藻大多在較高pH(7.0~9.6)的湖泊中出現(xiàn)[36,91].
3.1 產(chǎn)毒
目前,已知擬柱孢藻可產(chǎn)生的代謝毒素主要是CYN和麻痹性貝類毒素(PSP).不同地區(qū)的擬柱孢藻產(chǎn)毒情況不同,有的能產(chǎn)生柱孢藻毒素或麻痹性貝類毒素,有的則不能.澳大利亞、新西蘭、亞洲等產(chǎn)毒藻種以產(chǎn)生柱孢藻毒素為主[9,32-34].1999年,人們第一次從一株巴西擬柱孢藻中鑒定出麻痹性貝類毒素[10],其中最典型的是蛤蚌毒素(saxitoxin,STX),隨后又陸續(xù)從其他巴西擬柱孢藻藻種中鑒定出新蛤蚌毒素(neosaxitoxin,NSTX)和其他蛤蚌毒素類似物[117-118].截至目前,諸多學者尚未發(fā)現(xiàn)歐洲和北美地區(qū)藻種可產(chǎn)生CYN.但先前很多研究發(fā)現(xiàn),某些不產(chǎn)CYN的歐洲和亞洲藻種也能產(chǎn)生一些對小鼠有生物毒性的活性代謝物[119],而這些代謝物的種類迄今為止尚不明確.
3.1.1 柱孢藻毒素 CYN是一種肝毒素,1992年Ohtani等[32]首次提出CYN的化學結(jié)構(gòu).CYN(C15H21N5O7S,M=415)是一種易溶于水的多肽生物堿,具有肝毒性、細胞毒性和基因毒性[120].在水體呈電中性的條件下,CYN是一種帶有正電荷和負電荷的兩性離子[121].研究表明,這種兩性離子可以通過陽離子交換和表面絡合作用吸附到土壤及土壤礦物上[122].由于CYN存在一個三環(huán)硫酸胍兩性離子組和尿嘧啶,它可在不同的熱、光和pH值條件下穩(wěn)定存在[123].換言之,CYN是一種耐高溫、難降解的藍藻毒素.
目前,全球范圍內(nèi)許多飲用水水源地中均發(fā)現(xiàn)了CYN的存在.值得關(guān)注的是,CYN涉及至少兩種使人類中毒的疾病[124],如腸胃不適、嘔吐腹瀉、皮膚和眼睛刺激等.CYN可通過干擾不同的代謝途徑,誘導一系列氧化應激、遺傳變異、免疫抑制和肝細胞功能異常等反應[120],且對腎臟、胸腺和心臟也存在一定程度的毒性損害.目前研究指出,CYN的3個主要官能團中,只有尿嘧啶容易發(fā)生氧化反應[125].例如,CYN的尿嘧啶側(cè)鏈可抑制蛋白質(zhì)的翻譯,并可與DNA結(jié)合引起鏈斷裂,能夠促進腫瘤的形成以及導致染色體缺失等[11,13],但其具體的致毒機理仍在探索中.
有學者提出,某些藻種合成CYN是為了更好地適應生長環(huán)境并實現(xiàn)有效的繁殖擴張,即CYN是其長期進化適應環(huán)境的產(chǎn)物[91,128].另有研究表明,柱孢藻毒素的產(chǎn)生與cyr基因簇的存在有關(guān)[129],其具體轉(zhuǎn)化途徑仍在探索中.產(chǎn)生柱孢藻毒素的藻種主要產(chǎn)生3種柱孢藻毒素(CYNs)即cylindrospermopsin(CYN)[10]、7-deoxy-cylindrospermopsin[128]和7-epi-cylindrospermopsin[129],結(jié)構(gòu)如圖3a~c.日前,Wimmer等[130]從一株泰國擬柱孢藻中分離出兩種新的柱孢藻毒素衍生物:7-deoxy-desulfo-cylindrospermopsin和7-deoxy-desulfo-12-acetylcylindrospermopsin,其結(jié)構(gòu)如圖3d~e.其中CYN、deoxy-CYN占總柱孢藻毒素的95%以上,epi-CYN則微量存在,通常占比小于5%.CYN和deoxy-CYN的比例也會隨藻體所處生長周期的不同發(fā)生變化[131].此外,同一地區(qū)的藻種產(chǎn)毒情況也有差異,Willis等[132]將從同一水庫分離的24株擬柱孢藻分別同時培養(yǎng),發(fā)現(xiàn)其生長過程中的平均胞內(nèi)CYN含量及CYN與deoxy-CYN的比值各具差異.
圖3 柱孢藻毒素及其衍生物結(jié)構(gòu)[130]((a)cylindrospermopsin;(b)7-epi-cylindrospermopsin;(c)7-deoxy-cylindrospermopsin;(d)7-deoxy-desulfo-cylindrospermopsin;(e)7-deoxy-desulfo-12-acetylcylindrospermopsin)Fig.3 Themolecular structures of cylindrospermopsin and its analogs
目前,世界衛(wèi)生組織規(guī)定的CYN安全濃度限值為1μg/L.我國沒有明確規(guī)定CYN的安全濃度限值,各國提出的安全濃度參考值不等,為0.1~15μg/L[133],如德國0.1μg/L,澳大利亞、新西蘭1μg/L,巴西15 μg/L.然而,目前對于CYN產(chǎn)生生物毒性作用的機理并不明確,因此有必要開展更多研究以進一步明確CYN的致毒機理,為更有效地對水域進行風險評價和保護近水域人群健康狀況提供理論支持.
目前,可以產(chǎn)生CYN的藻種主要是擬柱孢藻,其次還有魚腥藻、束絲藻、尖頭藻、鞘絲藻、梅崎藻等[134].此外,所有產(chǎn)生CYN的藻種均生活在淡水或者微咸水中,目前尚未有報道表示海水藍藻能產(chǎn)生CYN,對此結(jié)果也需要展開進一步調(diào)查和研究方可確認.
圖4 蛤蚌毒素結(jié)構(gòu)[133]Fig.4 Themolecular structure of saxitoxin
3.1.2 蛤蚌毒素 蛤蚌毒素(STX)是一種三環(huán)化合物(C10H17N7O4,M=299),結(jié)構(gòu)如圖4.它易溶于水,在自然淡水中能夠保持完整性達90 d[135],但在高溫下易降解,可降解成毒性更高的變體[136].目前認為,產(chǎn)生STX的藍藻主要是魚腥藻和束絲藻,部分擬柱孢藻及鞘絲藻也可合成該毒素[137].
STX是一種鈉離子通道阻隔劑,同時也能夠干擾鈣離子和鉀離子通道,影響神經(jīng)細胞的軸突傳導,是一種神經(jīng)毒素,并可導致心輸出量降低[138].自20世紀以來,STX已成為多宗中毒事件的元兇,不慎接觸能導致機體麻木、癱瘓甚至死亡[139],但到目前為止,通過飲用水途徑中毒尚未有記錄.目前,尚沒有官方準則對飲用水中STX限值作出規(guī)定,澳大利亞采用3μg/L作為STX的安全濃度限值[140].研究表明,環(huán)境因素會對含擬柱孢藻水體中STX濃度產(chǎn)生一定影響,有學者提出,STX的產(chǎn)生可能與水的硬度和鹽度有關(guān)[141].最近,Brentano等[101]發(fā)現(xiàn)在擬柱孢藻產(chǎn)生STX的過程中,水體電導率和溶解性無機氮濃度對STX濃度影響最大,并猜測可能是由于擬柱孢藻在離子應力條件下,通過分泌STX改變細胞滲透性來調(diào)節(jié)細胞的自我平衡.此外,無機氮源的增加有利于擬柱孢藻的生長,導致分泌更多的STX.
3.2 檢測
CYN對人體有嚴重危害性且能夠在水體中持久存在,因此,定性定量地對CYN進行有效檢測對保證水質(zhì)安全以及人類健康至關(guān)重要.目前關(guān)于CYN的檢測,常用以下幾類方法:
3.2.1 物理化學分析 高效液相色譜(HPLC)法是一種比較精確的純化和測定毒素的方法,其分析速度快,色譜柱可重復多次使用,目前已廣泛用于CYN的測定[142-143].例如,高效液相色譜-紫外分光光度法(HPLCUV)可通過液相色譜分離CYN后,利用紫外分光光度法定量測定,研究表明,CYN在262 nm處有最大吸收波長[32,142].在此基礎上,利用HPLC-PDA(光二極管陣列)法能得到CYN的明顯吸收峰,干擾較少,可比較精確地測定CYN濃度.Welker等[144]評估了利用HPLC-PDA法檢測環(huán)境樣品中CYN的可行性,通過對比幾種不同的洗脫液,發(fā)現(xiàn)只有以三氟乙酸和甲醇作為洗脫液時,CYN才能被截留在C18色譜柱上,且在20 min內(nèi),用不同梯度洗脫液(5%三氟乙酸+0~50%甲酸)洗脫時,CYN相對應的峰高、峰面積和保留時間具有良好的重現(xiàn)性,且響應值(峰面積)與CYN濃度之間也呈現(xiàn)很好的線性關(guān)系.然而,用該方法檢測環(huán)境樣品中的CYN也存在局限性.例如,用純水萃取CYN時,樣品中存在基體干擾,可能會覆蓋CYN色譜,建議在純化過程中進一步改進.隨后,Metcalf等[145]創(chuàng)建了一種固相提取技術(shù)(SPE),即以C18和聚石墨化碳墨盒系列(混合模式)為填料來提取檢測湖水中CYN,檢測限可達1μg/L,并且填料能夠?qū)崿F(xiàn)100%再生.根據(jù)以上研究,Meriluoto和Codd等[146]提出了一種以C18-聚石墨為材料提取CYN,利用HPLC/PDA法定量測定CYN濃度的標準操作流程.此后,W?rmer等[147]又對前面的固相提取技術(shù)(SPE)進行了改進,在對樣品進行酸化并加入NaCl后,僅利用石墨化碳墨盒提取CYN(二氯甲烷作為洗脫液).
另外,高效液相色譜-串聯(lián)質(zhì)譜(HPLC-MS/MS)同樣適用于檢測水體中低濃度的CYN,分析速度快,靈敏度高,已被大量采用.Eaglesham等[148]用此方法測定低濃度CYN(在1~634μg/L范圍內(nèi))時,檢測到m/z 416離子([M+H]+)到m/z 194碎片離子的轉(zhuǎn)換,且結(jié)果表現(xiàn)出較好的線性關(guān)系(r2=1.000)和較高的準確度.Stirling和Quillia等[149]利用LC/MS和LC/MS/MS等方法選用大氣壓電離源和離子噴霧接口來檢測水樣中的CYN時,在MS/MS方法中檢測到兩種新的離子轉(zhuǎn)換:m/z 416離子([M+H]+)到m/z 176碎片離子的轉(zhuǎn)換以及m/z 433離子([M+NH4]+)到m/z 194碎片離子的轉(zhuǎn)換.Guzmán-Guillén等[150]以石墨化碳墨盒作為固相萃取材料,利用HPLC-MS/MS定量檢測CYN濃度,檢出限為0.5μg/L,此外,Bogialli等[151]利用LC-MS/MS分析過濾湖水中CYN濃度,得到CYN檢出限為0.3μg/L.而Kikuchi等[152]提出了一種新的LC/ESI-MS(液相色譜/電噴霧離子化-質(zhì)譜)測定CYN的方法,以2-[4-(2-羥乙基)-1-哌嗪基]乙磺酸為內(nèi)標物,在pH為10.5的碳酸緩沖液中提取CYN,再通過陰離子交換柱分離,實驗表明該方法能有效分離和濃縮環(huán)境中低濃度的CYN且無干擾峰.Graham等[153]用反相高效液相梯度洗脫法進一步提高檢測靈敏度,并用單點較準來提高定量精確度,得到CYN和7-deoxy-CYN的檢出限可達0.01μg/L.此外,黎志軒等[154]成功利用超高效液相色譜-串聯(lián)質(zhì)譜(UPLC-MS/MS)法對水中低濃度CYN進行檢測,水樣經(jīng)1~3次凍融后,用Waters超高效液相色譜-串聯(lián)質(zhì)譜(UPLC-MS/MS)直接測定水中的柱胞藻毒素,并以416.1>194 m/z特征離子進行定量,得出該方法檢出限為0.5μg/L.
3.2.2 免疫學分析 免疫分析法是一種定量測定方法,靈敏度高.酶聯(lián)免疫法(ELISA)已用于檢測柱孢藻毒素[155-156]及蛤蚌毒素[157-158]等,它可以通過識別并與特異性抗體結(jié)合來檢測藻毒素,檢測濃度范圍為0.05~2.00μg/L[121],目前,市面上可買到的主要是96微孔板Abraxis和Beacon柱胞藻毒素試劑盒,它們檢測精度高,簡單方便且易操作.Bláhová等[155]比較了ELISA和LC-MS兩種方法對地表水中CYN的檢測,結(jié)果表明兩者具有很好的一致性,但用ELISA檢測到的濃度要比LC-MS高,主要是由于免疫分析法不能辨別毒素變異體(如deoxy-CYN和7-epi-CYN等),同時交叉反應(即使是極少的)以及樣本中其他化合物的存在也可能導致檢測出的毒素濃度偏高.隨后,Mohamed[159]、Zamyadi[160]和Graham[153]等也報道了利用ELISA對CYN的檢測,目前,ELISA已越來越多地應用于CYN的檢測研究中.
3.2.3 生物試驗分析 人們最早通過生物學反應檢測水體中藻毒素,而小鼠體內(nèi)試驗是最常見的生物體內(nèi)試驗分析方法,主要根據(jù)不同的臨床癥狀及半致死量揭示水體中CYN的毒性及存在情況[32,161].除小鼠外,還有學者利用昆蟲[162]、無脊椎動物(蝦、水蚤、蝸牛)[163-165]、脊椎動物[166]以及植物(薺菜籽苗)[167]等進行了CYN的毒性評價研究.其中,Metcalf等[164]用鹽水蝦做毒性試驗,發(fā)現(xiàn)鹽水蝦死亡率與CYN濃度之間呈明顯的劑量效應關(guān)系,在24~72 h之間LC50(半致死濃度)值在0.71~8.1 mg/ml之間.此類體內(nèi)試驗方法屬于半定量檢測方法,靈敏度較差,所得到的毒性結(jié)果與試驗生物品系有關(guān),且易受其他因素干擾.
另外,細胞毒性檢測分析技術(shù)也已用于CYN檢測,該技術(shù)是對傳統(tǒng)生物體內(nèi)試驗分析方法的改進,可對毒素進行精確定量.研究表明,CYN可對多種細胞產(chǎn)生細胞毒性,Runnegar等[168]第一次用小鼠肝細胞來檢測CYN毒性,發(fā)現(xiàn)CYN(3.3~5.0μmol/L)可引起小鼠肝細胞嚴重死亡.此后Neuman等[169]研究了HepG2(肝)、BE-2(骨髓)、Caco-2(結(jié)腸)和MNA(腦)4種細胞系對CYN及deoxyCYN的毒性反應,發(fā)現(xiàn)在一定濃度CYN刺激下,四種細胞系均可觀察到明顯的細胞形態(tài)學變化,而Caco-2細胞系對CYN和deoxyCYN最為敏感,在48 h處理下,0.25μg/ml CYN即可對Caco-2細胞引起刺激反應.此外,F(xiàn)roscio等[170-171]通過一種快速無細胞蛋白質(zhì)合成抑制試驗來檢測CYN,利用兔網(wǎng)織紅細胞裂解液系統(tǒng)對CYN的檢出范圍為200~1200 μg/L,其結(jié)果與LC-MS/MS和HPLC-PDA檢測結(jié)果有很好的一致性.
當作為飲用水水源地的湖泊或水庫發(fā)生擬柱孢藻暴發(fā)時,無疑將對飲用水安全造成嚴重威脅,因此,對含藻水源水進行安全處理也成為當務之急.目前,水處理流程通常分為兩類:一類是對水中污染物的截留(混凝、吸附、過濾等),另一類是對水中污染物的降解(氯氧化、紫外線輻射、臭氧氧化等).一方面,在含藻水處理過程中,應盡量實現(xiàn)藻細胞的完整無破損去除,減少胞內(nèi)毒素的釋放[133,157].另一方面,對于原水中已經(jīng)存在的藻毒素,也應該在后續(xù)處理中一并去除,保證飲用水安全.近年來,由于全球很多水源地均發(fā)現(xiàn)擬柱孢藻存在,針對擬柱孢藻及其毒素的去除也成為人們研究的熱點,但目前國內(nèi)外研究尚不完善.以下主要介紹近年來擬柱孢藻及其毒素的去除研究現(xiàn)狀和展望.
4.1 擬柱孢藻去除研究
4.1.1 混凝 通常飲用水處理的第一步是混凝/絮凝-沉淀,目的是去除水體中的膠體物質(zhì)并降低濁度.對于藻濃度較高的水源水,一般水廠加入鐵或鋁鹽,中和負電荷藻細胞并阻止粒子之間的靜電斥力,使藻細胞凝聚,形成更大的顆粒(絮體),進而被沉降去除.該方法可有效去除藻細胞,但對水體中胞外藻毒素的去除效果不明顯.
Ho等[157]用Al2(SO4)3·18H2O作為混凝劑,研究了混凝、過濾、反沖洗和含藻底藻泥堆置過程中擬柱孢藻的細胞完整性以及毒素的釋放(降解)情況.研究表明混凝能有效去除擬柱孢藻,但隨著混凝劑劑量的增多,胞外CYN占總CYN的比例增大,說明投加混凝劑后胞內(nèi)毒素部分釋放到胞外,而總CYN濃度降低,表明混凝劑對CYN有一定的吸附效果.同時,李紹秀等[172]探討了二氧化氯氧化與混凝工藝結(jié)合去除擬柱孢藻的最佳工藝條件,在ClO2投加量0.5mg/L,聚合氯化鋁15mg/L,ClO2與混凝劑一起投加的條件下,除藻率為98.9%.但是,利用ClO2殺滅擬柱孢藻的同時會產(chǎn)生甲苯等有毒副產(chǎn)物[173],同時ClO2能夠破壞擬柱孢藻細胞,導致胞內(nèi)毒素釋放[156].目前,關(guān)于混凝去除水體中擬柱孢藻的研究較少,需針對不同混凝劑的混凝特性、去除效果、毒素釋放情況和應用注意事項等進行深入研究,并結(jié)合其他工藝探索更高效的混凝去除方法.
4.1.2 砂濾 實踐證明,慢速砂濾能夠同時去除藍藻及藍藻毒素.CYN可以通過陽離子交換和表面絡合作用吸附到土壤及土壤礦物上[122],Maghsoudi等[174]發(fā)現(xiàn)2 h內(nèi)72.6%的CYN可吸附到天然沉積物上.此外,Klitzke[175]等研究了低溫、厭氧及存在溶解性有機碳(DOC)的條件下,CYN在砂質(zhì)沉積物上的降解情況,表明低溫厭氧不利于CYN的降解,而DOC的存在則可縮短CYN在天然沉積物上降解的延滯期.由此看出,CYN在砂質(zhì)沉積物上的降解易受環(huán)境條件的制約.
另外,生物砂濾是一種用特定降解菌與砂濾結(jié)合以提高藻細胞及藻毒素去除效率的方法.Mouchet和Bonnély[176]等研究表明,用此方法可去除99%的藻細胞.由此可見,砂濾與其他處理工藝結(jié)合可能會大大提高去除效率.該方法無污染、成本低,可初步探索將其用于擬柱孢藻及CYN的去除.
4.1.3 膜處理(微濾、超濾、納濾、反滲透) 膜過濾法根據(jù)膜的孔隙大小可分為:微濾(0.1~10μm)、超濾(1~100 nm)、納濾(約1 nm)和反滲透(0.1 nm).該項技術(shù)對水源水中藻細胞及藻毒素的去除方面有較好的應用前景,已證明納濾能有效去除CYN[177-178],例如Dixon等[177]研究納濾法去除CYN,發(fā)現(xiàn)CYN去除效率高達90%~100%.但是納濾和反滲透過程復雜并且成本昂貴,且濾膜堵塞和細胞破裂的問題廣泛存在于現(xiàn)有的大部分過濾技術(shù)中,因此,該方法并沒有普及應用于對藻細胞的去除.
4.2 柱孢藻毒素去除研究
4.2.1 傳統(tǒng)方法 1)氯氧化.氯氧化法是飲用水消毒工藝中最常用的方法.它可在水中持久存在,防止病原體等污染物對飲用水的污染.各種研究表明,CYN在氯氧化過程中被迅速降解轉(zhuǎn)化,去除效率高,但會伴有消毒副產(chǎn)物生成[160].CYN的氯氧化降解速率受pH、溫度等影響.Rodríguez等[179]發(fā)現(xiàn)CYN在較高pH下,更易失活,在pH為7時有最大降解速率,具有明顯的二級速率常數(shù);當溫度從10℃升到30℃時,其降解速率升高2倍.Merel等[142]研究了氯氧化CYN的特性并就消毒副產(chǎn)物的成分進行分析,發(fā)現(xiàn)在20℃、pH為7的條件下,氯氧化可在2 min內(nèi)降解98%以上的CYN;即使裂解尿嘧啶時伴隨著3種消毒副產(chǎn)物形成,但副產(chǎn)物的毒性與CYN相比有所降低[125,142].因此,在利用氯氧化降解CYN時,需充分考慮反應條件對其降解特性的影響,除溫度、pH外,其他因素是否會對其降解產(chǎn)生影響有待研究.此外,人們關(guān)于氯氧化CYN的分解途徑及消毒副產(chǎn)物種類及毒性的研究尚不充分,有待進一步完善.
2)氯胺和二氧化氯.用二氧化氯和氯胺去除藍藻毒素的研究并不廣泛,且此方法對CYN去除效果不明顯.研究顯示,相同條件下,用二氧化氯降解CYN時,CYN的半衰期為14.4 h,然而用氯氧化只需要1.7 min[179].同樣,氯胺作為弱氧化劑,與CYN反應緩慢,反應動力學常數(shù)較氯氧化低2400倍[180].Cheng等[1]用氯胺和二氧化氯以標準水處理劑量去除CYN時,沒有發(fā)現(xiàn)顯著的去除效果.在殺藻方面,朱璐瑤[156]和李紹秀等[173]研究發(fā)現(xiàn)ClO2殺滅擬柱孢藻過程中會造成CYN釋放,且伴有有機副產(chǎn)物生成,其產(chǎn)生的有機副產(chǎn)物種類與ClO2投加量有關(guān):ClO2投加量越大,生成有機物的種類越少.綜上所述,氯胺和二氧化氯不適合用于去除柱孢藻毒素;在殺藻應用中,應注意ClO2用量及反應時間等因素.
3)高錳酸鹽.高錳酸鹽是一種強氧化劑,主要用于對水體進行預處理,以減少氯的使用和消毒副產(chǎn)物的形成.作為強氧化劑,高錳酸鹽不產(chǎn)生有毒副產(chǎn)物[181].與其他氧化劑相比,高錳酸鹽相對成本較低且易于管理,pH值適用范圍寬[182],但其氧化過程周期長且會使水體呈現(xiàn)粉色[183].研究表明,高錳酸鹽對CYN的去除效果不明顯,Cheng等[1]使用劑量為360 mg·min/L的KMnO4氧化CYN時,沒有發(fā)現(xiàn)去除效果.而KMnO4在微生物控制方面有更好的效果,有研究表明一定劑量的KMnO4能夠使擬柱孢藻失活,且不會造成毒素釋放[1].因此,它可以作為混凝除藻的助劑,并起到一部分預氧化(替代臭氧、二氧化氯等)作用以處理水中其他物質(zhì),達到強化混凝的效果,一般不建議單獨作為主要氧化劑使用.
4)活性炭吸附.活性炭吸附法能有效去除飲用水中的藍藻及藍藻毒素.使用活性炭吸附不僅不會造成藻細胞破損及其胞內(nèi)毒素的釋放,還可以有效去除細胞外CYN[184],但處理成本相對較高.Ho等[185]研究表明,CYN去除效果與活性炭粒徑有關(guān),粒徑較小的去除效果較好,但接觸時間分別為30、45和60 min時去除效果沒有顯著差異.目前,需要更加系統(tǒng)的研究來確定可有效吸附CYN的活性炭類型、劑量和接觸時間等最佳條件,以提高該方法的處理效率.與此同時,活性炭也可以通過在表面或內(nèi)孔固定和培養(yǎng)特定的微生物,隨后通過生物降解去除藻毒素,可有效緩解毒素解吸困難的問題,藻毒素的清除機理也從吸附轉(zhuǎn)為生物降解[186].雖然活性炭能夠有效地去除藻毒素,但完全吸附去除需要大量不同的吸附劑,因此該方法需要與其他處理工藝結(jié)合而不適合作為一種單獨的處理手段.
5)生物降解.生物降解藍藻毒素處理費用低、無二次污染.不少研究學者已發(fā)現(xiàn)CYN可被生物降解,且降解情況受溫度、CYN初始濃度以及其他條件的影響.例如,Senogles等[186]和Smith等[187]證明CYN能夠在自然水體中進行降解,并表明CYN的生物降解速率與CYN初始濃度之間近似呈線性關(guān)系,而溫度和含銅滅藻劑的存在均能夠影響CYN的降解[187].Klitzke等[188]只在沉積物中觀察到CYN的生物降解,并且發(fā)現(xiàn)對沉積物進行預處理能夠增強CYN的生物降解效果.另外,Maghsoudi等[189]分別研究了湖水原水、過濾后湖水、添加藻青蛋白的濾后湖水、沉淀池污泥和濾后污泥等介質(zhì)中CYN的生物降解程度,結(jié)果表明,CYN僅在沉淀池污泥中出現(xiàn)生物降解,12 d后CYN濃度由2.5μg/L降到1.0μg/L以下.由此可見,生物降解受環(huán)境限制較大,降解周期長,且在許多地區(qū)水庫及底泥中并沒有觀察到CYN的生物降解,若能分離出CYN特定降解菌株,無疑將會大大提高CYN生物降解效率.
Mohamed等[159]首次從藍藻水華中分離出一株CYN降解菌Bacillus(AMRI-03),該菌株在CYN存在的條件下生長良好且沒有出現(xiàn)生長滯后期,其生長量隨初始CYN濃度的增加而增大;同時,也發(fā)現(xiàn)CYN降解速度與CYN初始濃度有關(guān),當CYN最高濃度為300μg/L時,6 d即可降解完全,而濃度較低時完全降解需要7~8 d.日前,Dziga等[190]從印度某湖泊分離出一株氣單胞菌,并證明該菌株可有效降解CYN,生物降解效果取決于pH和溫度.此外,何雅孜等[191]分離篩選的一株溶藻細菌L7能夠顯著抑制某富營養(yǎng)化水體中擬柱孢藻(優(yōu)勢種)的生長.因此,發(fā)現(xiàn)、分離更多CYN降解菌和擬柱孢藻溶藻菌對高效降解CYN及抑制擬柱孢藻的生長有重要意義.
4.2.2 高級氧化法 羥基自由基(OH·)具有強氧化性,與底物反應迅速,幾乎可以氧化所有的有機化合物.高級氧化法采用OH·作為主要氧化劑可高效去除飲用水中多種毒素及其他污染物,已經(jīng)受到諸多研究人員的廣泛關(guān)注.
1)光催化氧化.紫外輻射可以生成高活性的OH·,已成為一項具備廣泛應用前景的飲用水消毒手段,使用波長在240~280 nm處的紫外光誘導水體中微生物失活以優(yōu)化水質(zhì).研究表明,光催化氧化可以有效去除CYN,其中對降解起主要作用的是OH·[192-195].目前研究較多的集中在TiO2光催化氧化.實驗表明,利用TiO2光催化氧化可提高CYN的降解效率,例如Senogles等[196]發(fā)現(xiàn),單獨紫外輻射時,降解CYN的半衰期為14min,而用TiO2光催化氧化可縮短至2min.此外,光催化的處理效果取決于照射燈的類型和設計、光輻照強度、毒素的初始濃度以及水的濁度、pH等.Chen等[194]研究表明隨著光照強度的增強,CYN降解速率升高,而CYN初始濃度和pH的升高則使降解速率降低.總體來說,光催化氧化是一種有效去除CYN的方法,但今后應加強對其潛在的消毒副產(chǎn)物的研究.
表2 擬柱孢藻及其毒素的去除方法比較Tab.2 The comparison of removalmethods on C.raciborskii and CYN
其次,UV或可見光輔助芬頓氧化(即光-芬頓氧化)可提高藻毒素的降解速率,且可以克服芬頓試劑只在酸性環(huán)境下能有效降解藻毒素的局限[197].目前,光-芬頓氧化法去除CYN的研究鮮有報道,有必要探索這種高級氧化法對CYN的去除效果、降解條件和反應途徑等以提高對該方法去除CYN的認識.
2)臭氧氧化.臭氧是一種在水處理過程中應用最普遍的氧化劑之一,可有效去除CYN[1,180,198],且對CYN的去除效率要高于氯氧化和高錳酸鹽氧化[180],但尚未考慮其副產(chǎn)物和殘余毒性.臭氧可用于混凝前或混凝后,但在混凝前投加臭氧可能會造成細胞膜破裂,引起胞內(nèi)毒素的釋放[199],因此不建議在混凝前投加.由于臭氧不能在水中持久存在,又經(jīng)常被用于除去水中微量有機物,因此,臭氧可以和H2O2或Fe(II)聯(lián)用,產(chǎn)生更多OH·,提高對微量藻毒素的降解能力.目前,利用臭氧和O3/H2O2或O3/Fe(II)聯(lián)用去除藻毒素并沒有得到廣泛研究.初步研究顯示,O3/H2O2或O3/Fe(II)聯(lián)用與單獨O3相比,都可強化微囊藻毒素MCs和類毒素ANTX-a的降解[199-200].因此可以進行進一步研究O3/H2O2或O3/Fe(II)聯(lián)用是否可以增強CYN的降解.
近20年來,擬柱孢藻在全球分布范圍不斷擴大,逐漸由熱帶、亞熱帶地區(qū)向溫帶地區(qū)擴散.由于其對光照和溫度的適應范圍廣,并可有效利用環(huán)境中不同形式的氮磷,在許多淡水水生系統(tǒng)中,擬柱孢藻都是良好的競爭者,在非最適生長環(huán)境中,仍可持續(xù)生長,甚至成為優(yōu)勢藻種.在全球溫室效應加劇的背景下,其良好的環(huán)境適應性有助于其生長區(qū)域的急速擴張.
擬柱孢藻可產(chǎn)生柱孢藻毒素和蛤蚌毒素等對人體健康有害的毒素.因此,在研究過程中,除了分離、培養(yǎng)擬柱孢藻并探索其生長條件和營養(yǎng)特性外,還需要調(diào)查其毒素在天然水體中的分布情況并分析該毒素的代謝機理,以進一步控制該毒素對人類及環(huán)境的危害.就此而言,定性定量檢測其代謝毒素也顯得十分重要.在現(xiàn)有檢測技術(shù)中,LC-MS/MS可定量測定CYN及其類似物;酶聯(lián)免疫檢測(ELISA)操作簡單,檢測精度高,也具有較好的應用前景.
其次,擬柱孢藻及柱孢藻毒素在水源水中的存在嚴重威脅人類的飲水安全,在被該藻污染的水源水進入供水系統(tǒng)之前,需完整去除藻細胞,并最大程度實現(xiàn)藻毒素的降解.目前,臭氧、光催化氧化等高級氧化技術(shù)可高效降解柱孢藻毒素.
近年來,對擬柱孢藻的研究日益增多,但仍有許多方面尚未探索,今后有必要在以下方面開展研究:
1)在擬柱孢藻的分布和擴張方面,隨著全球氣候變暖程度的加深,擬柱孢藻可能繼續(xù)向其他地區(qū)擴張,有必要對不同地區(qū)水域進行跟蹤檢測報道,對該藻在湖泊或水庫中的出現(xiàn)應引起重視.此外,人們正通過對不同區(qū)域藻種進行基因數(shù)據(jù)分析,推斷其親緣相似性,但其擴張的具體原因還需進一步深入探究.
2)擬柱孢藻的形態(tài)差異較大,營養(yǎng)元素和環(huán)境因子對其形態(tài)變化及產(chǎn)毒過程有重要影響.因此,一方面,需進一步了解營養(yǎng)元素的變化對其生長和產(chǎn)毒過程的影響.例如,除氮磷外,有必要針對其他元素(如Fe、S元素)對擬柱孢藻生長和產(chǎn)毒過程的影響以及相關(guān)機理展開研究.另一方面,溫度、光照、鹽度、pH等環(huán)境因子對擬柱孢藻生態(tài)生理特征的影響仍在探索中,可針對不同緯度地區(qū)的藻種,進一步探究溫度、光照等環(huán)境因子對其生長的影響及其相關(guān)機理.
3)在擬柱孢藻的產(chǎn)毒過程及其代謝物毒性方面,需進一步研究CYN、STX的產(chǎn)生與其相關(guān)基因表達的關(guān)系以及具體的產(chǎn)毒途徑,此外,CYN、STX對生物體的致毒機理也需進一步探究.目前,對某些擬柱孢藻有毒代謝物尚未做出具體鑒定,后續(xù)應加大對此類代謝物的鑒定和致毒機理的研究,為進一步預測和評價其毒性以及對人類健康的影響提供依據(jù),并制定相關(guān)的有毒代謝物安全濃度限值.
4)自CYN發(fā)現(xiàn)以來,各國學者研究和發(fā)展了諸多此類毒素的測定方法,由于柱胞藻毒素存在異構(gòu)體,在實際應用中,使得一些方法存在一定的局限性(如ELISA),或某些方法存在操作復雜,靈敏度不夠等缺點(如LC-UV),因此,研究并開發(fā)更多精確簡便的測定方法對CYN的檢測以及擬柱孢藻的后續(xù)研究有重要意義.
5)在擬柱孢藻及其毒素的去除方面,關(guān)于混凝法仍需進一步研究不同混凝劑的混凝特性、去除效果、毒素釋放情況和應用注意事項等.生物降解法有較廣闊的應用前景,發(fā)現(xiàn)并分離更多擬柱孢藻溶藻菌及CYN降解菌并探索適宜的生物降解條件對擬柱孢藻及其毒素的去除也具有重要意義.其次,為高效去除CYN并控制消毒副產(chǎn)物的生成,需進一步探究高級氧化法對CYN的降解條件、反應途徑及副產(chǎn)物形成情況,并可通過對多種處理方法的有效結(jié)合達到高效除藻和減少消毒副產(chǎn)物的目的.例如,光-芬頓氧化法、O3/H2O2或O3/Fe(II)聯(lián)用法等是否可以增強CYN的降解需進一步探索.此外,更多兼具良好經(jīng)濟實用性和實踐性的先進去除工藝也需要大量開發(fā)和研究.
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Progress and prospect in the study of Cylindrosperm opsis raciborskii and its toxins
LIHongm in1,PEIHaiyan1,2??,SUN Jiongm ing1,JIN Yan1&XU Hangzhou1
(1:School of Environmental Science and Engineering,Shandong University,Jinan 250100,P.R.China)
(2:Shandong Provincial Engineering Center on Environmental Science and Technology,Jinan 250061,P.R.China)
With the aggravation ofwater eutrophication,harmful cyanobacterial bloom in fresh water becomesmore and more serious on a global scale.Cylindrospermopsis raciborskii,a noxious cyanobacterial species that can form blooms in fresh water,has gradually
widespread attention for its potential ability of toxin production and strong invasiveness.In recent years,Cylindrospermopsis raciborskii has spread from the tropics and subtropics to temperate zoneswith associated blooms inmany water bodies. If the toxins come into drinkingwater,itwill undoubtedly pose a potential threat to human health.In this review,recent literatures concerning its geographic distribution,ecophysiology,toxicity,detection and removal processes are summarized.Meanwhile,we discussed the future research directions in related field,whichwill provide some scientific information for further studieson the ecological toxicology,effective controls and removal of Cylindrospermopsis raciborskii.
Cylindrospermopsis raciborskii;global distribution;ecophysiology;cylindrospermopsin;detection;removal
DOI 10.18307/2017.0401
?2017 by Journal of Lake Sciences
?國家自然科學基金項目(51478251,51322811)資助.2016-10-09收稿;2016-12-12收修改稿.李紅敏(1992~),女,碩士研究生;E-mail:hongmin1059@163.com.
??通信作者;E-mail:haiyanhup@126.com.