周偉華, 廖健祖, 3, 郭亞娟, 3, 袁翔城, 黃 暉, 劉 勝, 李 濤

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溶解有機(jī)物的光降解及其對(duì)浮游細(xì)菌和浮游植物的影響

周偉華1, 2, 廖健祖1, 2, 3, 郭亞娟1, 2, 3, 袁翔城1, 2, 黃 暉1, 2, 劉 勝1, 2, 李 濤1, 2

(1. 中國(guó)科學(xué)院南海海洋研究所熱帶海洋生物資源與生態(tài)重點(diǎn)實(shí)驗(yàn)室, 廣東廣州510301; 2. 中國(guó)科學(xué)院海南熱帶海洋生物實(shí)驗(yàn)站, 海南三亞572000; 3. 中國(guó)科學(xué)院大學(xué), 北京100049)

作為海洋中最大的動(dòng)態(tài)有機(jī)碳儲(chǔ)庫(kù), 溶解有機(jī)物的光降解(主要是紫外波段)對(duì)生源要素的生物地球化學(xué)循環(huán)以及海洋生態(tài)系統(tǒng)的結(jié)構(gòu)和功能具有重要的影響。本文探討了影響溶解有機(jī)物光降解的環(huán)境因素、其光化學(xué)過(guò)程和產(chǎn)物, 并重點(diǎn)闡述了溶解有機(jī)物的光降解對(duì)浮游細(xì)菌和浮游植物的影響。溶解有機(jī)物的來(lái)源和成分復(fù)雜, 其光降解在不同海區(qū)有不同的生態(tài)效應(yīng), 為了能更準(zhǔn)確地把握其生態(tài)效應(yīng), 需要更全面和深入的研究。

溶解有機(jī)物; 光降解; 紫外線(xiàn); 浮游細(xì)菌; 浮游植物

溶解有機(jī)物(Dissolved organic matter, DOM)是自然界中普遍存在的一類(lèi)復(fù)雜的混合物。目前, 大多數(shù)的海洋DOM分離方法使用孔徑為0.7 μm的玻璃纖維膜進(jìn)行過(guò)濾, 它包括通過(guò)濾膜而且之后用于實(shí)驗(yàn)分析過(guò)程中不因蒸發(fā)丟失的有機(jī)物質(zhì)部分, 以及在過(guò)濾過(guò)程中沒(méi)有被截留的膠體顆粒[1]。

作為海洋中最大的動(dòng)態(tài)有機(jī)碳儲(chǔ)庫(kù)(約662 Gt碳), 海洋中DOM的生物地球化學(xué)行為對(duì)碳循環(huán)以及全球氣候變化有著重要的作用[2]。DOM根據(jù)其生物可利用性可分為: 活性DOM(Liable DOM, LDOM)、半活性DOM(Semi-Liable DOM, SLDOM)和惰性DOM(Recalcitrant DOM, RDOM)。其中RDOM的含量最高, 約占海洋DOM的95%(約624Gt碳), 與大氣中的CO2的碳量(約750 Gt碳)相當(dāng), 是一個(gè)巨大的碳匯。由于其難降解, 在海洋中有極長(zhǎng)的停留時(shí)間, 焦念志等[3]認(rèn)為RDOM是海洋重要的儲(chǔ)碳物質(zhì), 并提出“微型生物碳泵(Microbial carbon pump, MCP)”的概念, 即有機(jī)質(zhì)在微型生物的作用下形成RDOM的過(guò)程。經(jīng)過(guò)MCP過(guò)程而形成的RDOM有較高的碳: 氮: 磷(3511: 202: 1)[4], 從而使碳以有機(jī)物的形態(tài)長(zhǎng)期保存在海洋中, 而氮, 磷則以無(wú)機(jī)的形態(tài)被生產(chǎn)者重新吸收利用。因而, MCP不僅有儲(chǔ)碳的作用, 且能促進(jìn)營(yíng)養(yǎng)鹽的循環(huán)和生產(chǎn)力。

工業(yè)革命以來(lái), 受人類(lèi)活動(dòng)的影響, 臭氧層被削薄使得更多的紫外線(xiàn)(Ultraviolet radiation, UV)能達(dá)到地球的表面。近年來(lái), 甚至在一些熱帶地區(qū)也出現(xiàn)了臭氧洞, UV對(duì)水生生態(tài)系統(tǒng)的影響越來(lái)越突出, 特別是紫外線(xiàn)B波段(UV-B)[5-7]。Kieber等[8]在《Nature》上報(bào)道了海洋中高分子量的RDOM在紫外線(xiàn)作用下能發(fā)生光化學(xué)反應(yīng), 生成分子質(zhì)量更小且具有生物活性的光降解產(chǎn)物, 可被浮游生物吸收利用, 從而影響海洋中碳的轉(zhuǎn)移以及浮游生物食物鏈傳遞動(dòng)力學(xué)。因此, 紫外線(xiàn)對(duì)DOM的光降解作用可對(duì)MCP的慢速循環(huán)過(guò)程進(jìn)行內(nèi)容補(bǔ)充[3]。國(guó)內(nèi)也有研究指出, DOM的光降解可以延長(zhǎng)藍(lán)藻水華的持續(xù)時(shí)間[9]。可見(jiàn)DOM的光降解在海洋中元素的循環(huán)和海洋生態(tài)過(guò)程中均起著重要的作用。

在全球氣候變化的大環(huán)境下, 因紫外輻射的增強(qiáng)而導(dǎo)致的生態(tài)效應(yīng)無(wú)疑是一個(gè)亟需研究的科學(xué)問(wèn)題。生態(tài)系統(tǒng)中作為基礎(chǔ)生物的浮游細(xì)菌和浮游植物無(wú)疑對(duì)紫外線(xiàn)輻射增強(qiáng)引起的反應(yīng)最為敏感, 為了加深了解DOM的光降解對(duì)浮游生物生態(tài)系統(tǒng)的影響, 很有必要對(duì)DOM光降解的基本過(guò)程、產(chǎn)物以及對(duì)浮游生物的影響進(jìn)行詳細(xì)闡述。

1 影響DOM光降解的環(huán)境因素

由于人類(lèi)活動(dòng)引起的全球氣候變化(如: 紫外線(xiàn)增強(qiáng)、海水溫度升高、海洋酸化等), 使海洋生態(tài)環(huán)境遭受?chē)?yán)重影響。海洋中DOM的光降解勢(shì)必也會(huì)受到這些環(huán)境因素變化的影響。1)近年來(lái), 河口區(qū)的鐵含量呈升高的趨勢(shì)[10], 鐵含量的增加有助于DOM吸收UV, 海水pH的降低和鐵濃度增加均能提高DOM的光降解速率, 而pH的降低對(duì)DOM光降解的影響更為顯著。Molot等[11]指出: 在pH低于7時(shí), 光降解過(guò)程主要是由羥基自由基所激發(fā)。2)在鹽度高的水體中, 陸源DOM光降解產(chǎn)生溶解無(wú)機(jī)碳(Dissolvedinorganic carbon, DIC)的速率減慢, 光漂白作用減弱[12],但光銨化以及DOM中UV254-發(fā)色團(tuán)的光降解速率提升, 從而改變了DOM在光降解過(guò)程中的光吸收特性[13-14]。3)室內(nèi)受控培養(yǎng)實(shí)驗(yàn)表明: 在氧飽和濃度下, DOM光降解產(chǎn)生DIC的速率加快, 光漂白作用增強(qiáng)[15-16], 這與直接光降解過(guò)程需要氧有關(guān)[17]。4)不同波長(zhǎng)的紫外線(xiàn), 其所含的能量不同, 對(duì)DOM光降解的影響也不同。Wang等[16]報(bào)道了UV-B、UV-A和可見(jiàn)光三個(gè)波段對(duì)DOM光降解產(chǎn)生溶解無(wú)機(jī)物(Dissolved inorganic matter, DIM)的貢獻(xiàn)分別為31.8%、32.6%、25.6%。此外, 在不同季節(jié)和不同海區(qū), UV輻射強(qiáng)度存在很大差異, 這無(wú)疑對(duì)DOM的光降解產(chǎn)生很大的影響。5)Shirokova等[18]認(rèn)為在異常高溫的水域中溶解有機(jī)碳(Dissolved organic carbon, DOC)的濃度降低了30%, 很可能是DOM光降解速率加快的緣故。Porcal等[19]探討了DOM光降解產(chǎn)生DIC的兩種可能途徑, 包括DOM直接光降解產(chǎn)生DIC以及DOM先降解產(chǎn)生顆粒有機(jī)碳(Particulate organic carbon, POC)等中間產(chǎn)物, 再降解成DIC, 前者受低溫控制, 而后者受高溫控制并起到主要作用。Ren等[20]也指出了水溫升高在很大程度上影響著DOM光降解產(chǎn)生CO的速率。此外, 海水的溫度上升還會(huì)使混合層變淺, 溫躍層更加明顯, 上下層海水垂直混合更加困難, 導(dǎo)致上層海水將接收更多的太陽(yáng)輻射, 使得DOM光降解和光漂白將更加劇烈[21-22]。

2 DOM的主要光降解過(guò)程和產(chǎn)物

2.1 DOM的主要光降解過(guò)程

DOM的主要光降解過(guò)程可分為: 直接光降解、間接光降解和Fe3+-DOM復(fù)合物的光降解[23]。

2.1.1 直接光降解

直接光降解是一種較為簡(jiǎn)單的光化學(xué)反應(yīng), 指的是DOM自身作為主要的發(fā)色團(tuán), 其直接吸收光而進(jìn)行的化學(xué)反應(yīng), 其初級(jí)產(chǎn)物芳香性降低[24], 并能進(jìn)行二次分解反應(yīng)生成分子質(zhì)量更小的物質(zhì)[25-26]。DOM是否通過(guò)直接光降解途徑取決于DOM的化學(xué)組成及其來(lái)源。有研究指出: 陸源DOM比藻源DOM展示出更強(qiáng)的光反應(yīng)活性, 而且陸源DOM在光降解過(guò)程中產(chǎn)生更多具有生物活性的DOC[27-28]。

2.1.2 間接光降解

間接光降解比直接光降解常見(jiàn), 指的是DOM自身不能作為發(fā)色團(tuán)而直接吸收光, 需要水體中存在的天然物質(zhì)(如: 腐殖質(zhì)或微生物等)被光激發(fā)后, 將激發(fā)態(tài)的能量轉(zhuǎn)移給化合物而導(dǎo)致的分解反應(yīng)。其中一個(gè)重要的途徑是通過(guò)羥基自由基激發(fā)[29]。由于間接光降解能改變自然水體中阻礙光降解的化學(xué)物質(zhì)的分子結(jié)構(gòu), 因此它在水體中有著特別重要的作用和意義。

2.1.3 Fe3+-DOM復(fù)合物的光降解

指的是在富含鐵元素的表層水中, Fe3+-DOM的羧酸鹽復(fù)合體通過(guò)配體到金屬的電荷轉(zhuǎn)移而形成的分解反應(yīng)[27, 30]。反應(yīng)包括了Fe3+到Fe2+的轉(zhuǎn)化和脫羧過(guò)程, 可見(jiàn)DOM光降解過(guò)程對(duì)鐵離子的氧化還原反應(yīng)有重要影響[15]。

2.2 主要光解產(chǎn)物

水體中的DOM光降解改變了其原有的物理性質(zhì)和化學(xué)組成, 如: 芳香性降低[31]、pH值下降[32]、疏水性[33]、吸收光譜性質(zhì)[34]以及分子質(zhì)量大小的改變[32]。其主要的產(chǎn)物可以分成以下3類(lèi)[35]: 1)最為常見(jiàn)的一類(lèi)產(chǎn)物就是含碳?xì)怏w, 包括: CO2、CO、CH4、DIC等[36-37]。2)低分子質(zhì)量的有機(jī)化合物, 包括: 氨基酸、尿素、甲醛、乙醛、丙酮酸等[38-39]。3)氮和磷等無(wú)機(jī)鹽, 包括: NH4+、NO2–、PO43–等[40-41]。

3 DOM光降解對(duì)浮游細(xì)菌的影響

3.1 DOM光降解對(duì)浮游細(xì)菌生長(zhǎng)的促進(jìn)與抑制作用

DOM具有吸收UV的特性, 因而富含DOM的水域無(wú)疑更能阻礙UV在水層中的穿透[42]。一方面, 高濃度的DOM可以對(duì)UV敏感的浮游細(xì)菌起到保護(hù)作用; 另一方面, UV輔射引起的DOM光化學(xué)反應(yīng)可產(chǎn)生活性物質(zhì), 同時(shí)光漂白作用又改變加劇了UV在水域中的穿透深度?？梢?jiàn), DOM的光降解從多方面影響浮游細(xì)菌的群落結(jié)構(gòu)和功能。UV促進(jìn)了細(xì)菌生長(zhǎng)的可能機(jī)制是在一定程度上, DOM光降解產(chǎn)生的活性物質(zhì)在促進(jìn)生長(zhǎng)方面抵消了UV對(duì)細(xì)菌的損傷作用[8, 43]。接種在事先用UV處理過(guò)的海水中的浮游細(xì)菌可以達(dá)到更高的豐度, 表明細(xì)菌吸收利用了DOM的光化學(xué)分解產(chǎn)物[44]。在表層海水, 由于較高強(qiáng)度的UV輻射使得細(xì)菌活動(dòng)受到嚴(yán)重的抑制, 在深5 m的表層沿岸海水, 細(xì)菌活動(dòng)抑制率達(dá)到40%左右; 在貧營(yíng)養(yǎng)鹽的大洋水域, 抑制作用延伸到10 m以上, 因而UV輻射使表層海水富含活性有機(jī)物質(zhì)[45]。當(dāng)隨著深度的增加或者通過(guò)垂直(或湍流)混合等使水層UV強(qiáng)度減弱, 且UV對(duì)浮游細(xì)菌的損傷由UV-A誘導(dǎo)的酶促反應(yīng)得以修復(fù)時(shí), 浮游細(xì)菌便能更有效地吸收光降解產(chǎn)物而得到更好的生長(zhǎng)[44, 46]。

光化學(xué)轉(zhuǎn)化能使DOM轉(zhuǎn)化為更具活性的物質(zhì), 但也會(huì)產(chǎn)生相反的效應(yīng)。研究表明, DOM光化學(xué)轉(zhuǎn)化產(chǎn)生的羰基化合物, 如: 羰酸, 成為細(xì)菌分解代謝的底物[47]。此外, DOM光降解還能產(chǎn)生具有生物活性的NH4+-N[48]。然而, Keil和Kirchman[49]發(fā)現(xiàn)DOM在太陽(yáng)光的照射下, 加速了活性蛋白的“老化”, 即轉(zhuǎn)化為難降解的狀態(tài)。UV加速“老化”的報(bào)道在藻源DOM上出現(xiàn)較多, 這無(wú)疑對(duì)浮游細(xì)菌的生長(zhǎng)起到抑制作用[50]。此外, Kramer和Herndl[51]指出浮游細(xì)菌在培養(yǎng)過(guò)程中產(chǎn)生RDOM, 這種RDOM的光降解產(chǎn)物仍然不具生物活性, 二次培養(yǎng)不會(huì)促進(jìn)浮游細(xì)菌的生長(zhǎng)。一般來(lái)說(shuō), 在呈弱酸性、離子強(qiáng)度和葉綠素含量低、腐殖質(zhì)含量高的水域, 由UV誘導(dǎo)的DOM光降解對(duì)細(xì)菌的生長(zhǎng)起促進(jìn)作用[52]。

由DOM的光化學(xué)轉(zhuǎn)化造成營(yíng)養(yǎng)物質(zhì)結(jié)構(gòu)的改變不僅影響細(xì)菌的生理功能, 還會(huì)改變細(xì)菌的群落結(jié)構(gòu)[53-54]。總的來(lái)說(shuō), DOM的光降解可以改變微食物環(huán)的物質(zhì)循環(huán)和能量流動(dòng), 進(jìn)而影響食物鏈的結(jié)構(gòu)與功能。

Chrost和Faust[55]指出在伯利茲珊瑚礁保護(hù)區(qū)中, 由于DOM光降解, 浮游細(xì)菌的生長(zhǎng)率和二次生產(chǎn)得到提高。在北冰洋的邊緣海—波弗特海, DOM光降解產(chǎn)生的DIM可達(dá)細(xì)菌呼吸消耗量的10%, 由于冰川的融化, DOM的光降解作用將更加顯著[56]。在波羅的海, 平均每年DOC的光降解量超過(guò)了河流輸入的具有光活性的DOC量, 其中用于支持浮游細(xì)菌生物量的活性光反應(yīng)產(chǎn)物占DOC光降解產(chǎn)物的20%, 表明波羅的海光降解作用是陸源DOC的匯[14]。然而, 國(guó)內(nèi)雖有見(jiàn)對(duì)DOM與污染物、抗生素和重金屬化合物毒性的報(bào)道[57-58], 卻鮮有DOM光降解與浮游細(xì)菌耦合的報(bào)道。

3.2 DOM光降解影響浮游細(xì)菌生長(zhǎng)的機(jī)理

由于DOM中含有光化學(xué)和生物活性成分, 光化學(xué)和生物過(guò)程對(duì)DOM的降解起到了競(jìng)爭(zhēng)作用。Obernosterer等[28]指出了由于富含糖類(lèi)物質(zhì), 藻源DOM(以培養(yǎng)過(guò)程中的產(chǎn)物為主)比陸源DOM(以腐殖質(zhì)為主)更具生物活性, 而陸源DOM則含有豐富的芳香性碳, 更具光化學(xué)活性。在陸源DOM的光轉(zhuǎn)化反應(yīng)過(guò)程中, 生物活性DOC含量提高了7%, 而藻源DOM沒(méi)有產(chǎn)生生物活性DOC。此外, 生物和光對(duì)DOM的降解也有互利的作用, Amado等[59]報(bào)道了在富含腐殖酸的瀉湖中, 細(xì)菌礦化作用使DOM光降解效率提高了13%, 而光降解可使細(xì)菌礦化效率提高300%。他認(rèn)為在這個(gè)過(guò)程中起關(guān)鍵功能的物質(zhì)為富含電子的氨基酸(如: 組氨酸、蛋氨酸、酪氨酸、色氨酸和半胱氨酸等)[60]。Amado等[60]還提出了新的模型: DOM光降解對(duì)細(xì)菌生長(zhǎng)的影響除了與DOM的來(lái)源有關(guān)之外, 還與DOM的濃度有關(guān)。DOM在光降解過(guò)程中會(huì)產(chǎn)生一些強(qiáng)氧化性物質(zhì), 如: 單線(xiàn)態(tài)氧(Singlet oxygen), 其生成量與DOM的濃度成正相關(guān)[61-62], 而且有研究指出水域中單線(xiàn)態(tài)氧的含量處于被低估的狀態(tài)[63]。單線(xiàn)態(tài)氧可以反過(guò)來(lái)降解氨基酸和其他DOM分子, 影響DOM的組成結(jié)構(gòu)并抑制細(xì)菌的生長(zhǎng)[60-61, 64]。單線(xiàn)態(tài)氧對(duì)浮游細(xì)菌還有毒性作用并影響細(xì)菌代謝及其種群動(dòng)力學(xué)[65]。

4 DOM光降解對(duì)浮游植物的影響

普遍認(rèn)為大洋區(qū)的浮游植物初級(jí)生產(chǎn)力表現(xiàn)為氮限制, 在貧營(yíng)養(yǎng)鹽的東地中海, 光銨化速率約為40 mmol/(m2·a), 與該海區(qū)大氣氮沉降量相近, 可提供新生產(chǎn)力氮需求的12%[66]。Morell和Corredor[67]發(fā)現(xiàn)近海葉綠素濃度的增加與DOM的光降解有著密切的聯(lián)系, 在富含DOM的河口區(qū), 光降解過(guò)程釋放大量的銨鹽, 為浮游植物的生長(zhǎng)提供了豐富的無(wú)機(jī)氮, 他們估計(jì)由光銨化作用產(chǎn)生的氮鹽可達(dá)到浮游植物需氮量的50%。在波羅的海, 由RDOM光降解產(chǎn)生的生物活性氮可支持浮游植物1.2%的新生產(chǎn)力和3.6%需氮量[68]。而在智利中部上升流區(qū)的研究表明: 在春、夏季, 光銨化產(chǎn)物能支持50%~178%的浮游植物NH4+需求量[69]。可見(jiàn), DOM光降解是對(duì)海洋營(yíng)養(yǎng)鹽動(dòng)力學(xué)起著極其重要的作用, 特別是在貧營(yíng)養(yǎng)鹽的大洋區(qū), 是海洋中的無(wú)機(jī)氮重要的來(lái)源之一。在光銨化對(duì)浮游植物生長(zhǎng)的影響方面, 有學(xué)者[68]把波羅的海原有的浮游植物接種到DOM完全光降解的海水中培養(yǎng)發(fā)現(xiàn), 受氮限制影響的浮游植物生物量得到提高。通過(guò)模型計(jì)算得出, 夏季DOM光降解產(chǎn)生活性氮的速率為22~26 μmol/(m2·d), 使海區(qū)葉綠素含量提高12~14 μg/(m2·d)。同時(shí), DOM的光降解產(chǎn)物DIC(CO2、HCO3–、CO32–)也可以促進(jìn)浮游植物的生產(chǎn)力。當(dāng)形成藻類(lèi)水華時(shí), DOM的光降解生成DIC的速率降低, 其產(chǎn)量?jī)H能支持小于3%的生產(chǎn)力。但研究人員認(rèn)為在藻華過(guò)程中產(chǎn)生的DOM(即藻源DOM)更具有光化學(xué)活性, 由于海水平流交換使外源DOM成為主要成分才導(dǎo)致了降解速率的降低[70]。很明顯, 這與目前的主流相悖, 因而在DOM光降解的耦合機(jī)制上還亟待更系統(tǒng)和深入的研究。此外, DOM光降解產(chǎn)生的活性氧產(chǎn)物也會(huì)對(duì)浮游植物產(chǎn)生損傷作用[71]。

近年來(lái), 有害藻華、底層缺氧等水域生態(tài)災(zāi)害頻繁發(fā)生, 嚴(yán)重地影響水域生態(tài)系統(tǒng)結(jié)構(gòu)的穩(wěn)定性, 污染水域環(huán)境, 最終危害人類(lèi)健康。長(zhǎng)期以來(lái), 人類(lèi)活動(dòng)所造成的水體富營(yíng)養(yǎng)化被認(rèn)為是引起藍(lán)藻水華的最主要因素。而且, 由于水溫上升導(dǎo)致的躍層的擴(kuò)大、風(fēng)速的減弱、光照強(qiáng)度和時(shí)間的增加均有助于藍(lán)藻水華的爆發(fā)[72]。有研究指出, DOM的光降解也會(huì)延長(zhǎng)藍(lán)藻水華的持續(xù)時(shí)間[9]。一方面, 相對(duì)于其他藻類(lèi), 藍(lán)藻對(duì)于太陽(yáng)光輻射具有更強(qiáng)的耐受性[73], 另一方面, 在適應(yīng)不斷惡化的生態(tài)環(huán)境過(guò)程中, 藍(lán)藻已形成一定的自我保護(hù)機(jī)制, 如: 遷移到更深的水層避開(kāi)高強(qiáng)度的輻射[74]; 超氧化物歧化酶(Superoxide Dismutase, SOD)、過(guò)氧化氫酶(Catalase, CAT)等抗氧化機(jī)制清除細(xì)胞內(nèi)的過(guò)氧化合物[75-76]; 合成細(xì)胞外多糖[77]; 分泌具有吸收UV作用的化學(xué)物質(zhì), 如: 類(lèi)菌胞素氨基酸(Mycosporine-like amino acids, MAAs)[78-79]以及偽枝藻素(Scytonemin, SCY)[80]; 通過(guò)修復(fù)和更新?lián)p傷的DNA和蛋白質(zhì), 如: 切除修復(fù)[81-82]、SOS反應(yīng)[83]、光合系統(tǒng)II(PSII)蛋白的重新合成[84]; 通過(guò)細(xì)胞凋亡清除損傷嚴(yán)重的細(xì)胞[85]等。

5 DOM光降解對(duì)食物網(wǎng)的影響

浮游植物和浮游細(xì)菌作為海洋生態(tài)系統(tǒng)中的主要生產(chǎn)者和分解者, DOM光降解對(duì)其生物量和群落產(chǎn)生的變化必然會(huì)通過(guò)食物網(wǎng)傳遞影響上級(jí)營(yíng)養(yǎng)級(jí)。這方面的研究報(bào)道先見(jiàn)于湖泊, De Lange等[86]通過(guò)培養(yǎng)實(shí)驗(yàn)表明: 由紫外線(xiàn)造成的DOC光降解提高了微食物環(huán)中浮游細(xì)菌、低級(jí)的異養(yǎng)以及兼養(yǎng)生物的生物量。但其用于培養(yǎng)實(shí)驗(yàn)的浮游生物來(lái)源于實(shí)驗(yàn)室, 因此不能很好地指示自然環(huán)境狀態(tài)。Daniel等[87]研究了湖泊DOM光降解產(chǎn)物對(duì)異養(yǎng)微食物環(huán)的影響, 其研究發(fā)現(xiàn)DOM光降解提高了湖泊中總的浮游細(xì)菌、原生以及后生浮游動(dòng)物的生物量。對(duì)于富營(yíng)養(yǎng)鹽的水體, DOM光降解沒(méi)有引起明顯的群落變化, 但對(duì)于腐殖質(zhì)水體, 鞭毛蟲(chóng)、輪蟲(chóng)、無(wú)節(jié)幼體以及枝角類(lèi)的生物量有顯著提高。而在海洋方面的報(bào)道, V?h?talo等[88]研究了波羅的海近岸海域DOM光降解對(duì)浮游細(xì)菌為起點(diǎn)的三個(gè)營(yíng)養(yǎng)級(jí)的影響, 其研究也表明了DOM光降解的促進(jìn)作用。Stepanauskas等[89]在圣華金河口三角洲的研究指出, 由于具有生物活性DOC的含量夠低, 以DOC為基礎(chǔ)的微食物網(wǎng)每年僅能支持小于0.6×109g C的原生動(dòng)物生產(chǎn)力, 相當(dāng)于17×109g C的初級(jí)生產(chǎn)力, 即使DOC的光降解使其生物活性降低40%也不會(huì)對(duì)浮游動(dòng)物以及魚(yú)類(lèi)的營(yíng)養(yǎng)需求產(chǎn)生重要的影響。目前, 人類(lèi)活動(dòng)所導(dǎo)致的污染物質(zhì)過(guò)多排放、冰川和凍土的融化、極端氣候現(xiàn)象發(fā)生頻率的升高等提高了河流和海洋中的DOM含量[90-92]。DOM光降解對(duì)水域生態(tài)系統(tǒng)功能和結(jié)構(gòu)的影響將更加顯著, 特別是沿岸海域。然而, DOM光降解對(duì)浮游植物食物網(wǎng)以及更高營(yíng)養(yǎng)級(jí)的影響的研究仍然很缺乏。

6 研究展望

一方面, 由于全球氣候變化和人類(lèi)活動(dòng)的影響, 使河流以及海洋中DOM的濃度不斷升高; 另一方面, 由于臭氧層的削薄, 使更多的紫外線(xiàn)能達(dá)到地球表面。紫外線(xiàn)使DOM發(fā)生的光降解反應(yīng)在碳、磷等生源要素的生物地球化學(xué)循環(huán)以及海洋生態(tài)過(guò)程有著越來(lái)越重要的影響。國(guó)外學(xué)者在這方面開(kāi)展的研究相對(duì)較多, 主要包括: DOM光降解與浮游細(xì)菌和浮游植物的耦合及其對(duì)海洋生產(chǎn)力以及微食物網(wǎng)結(jié)構(gòu)的影響、DOM光降解機(jī)理和產(chǎn)物、還有在污水處理方面的應(yīng)用等[93]。然而國(guó)內(nèi)相關(guān)的工作較少, 多見(jiàn)于DOM與污染物和重金屬化合物的毒性以及陸地土壤DOM遷移、轉(zhuǎn)化方面的報(bào)道。近年來(lái), 國(guó)內(nèi)雖有對(duì)水域生態(tài)系統(tǒng)中, DOM光降解產(chǎn)物、降解速率、對(duì)藻華的影響以及利用三維熒光光譜分析手段對(duì)DOM光降解特征和動(dòng)力學(xué)方面的研究[94-95], 但仍有不少問(wèn)題亟待進(jìn)一步研討:

(1) 由于DOM的成分復(fù)雜, 對(duì)于來(lái)源與化學(xué)組成不同的DOM, 其光降解過(guò)程不同, 產(chǎn)物也不一致。此外, DOM的光降解過(guò)程與環(huán)境因素直接相關(guān), 而且環(huán)境因子對(duì)于DOM的光降解是聯(lián)合起作用的。因此, 需要對(duì)DOM的來(lái)源和組成成分進(jìn)行分類(lèi)、對(duì)影響DOM的光降解的環(huán)境因子進(jìn)行整合, 篩選主要環(huán)境因子, 構(gòu)建其反應(yīng)過(guò)程模型。

(2) 需要針對(duì)典型海區(qū)開(kāi)展DOM光降解及其生態(tài)效應(yīng)研究(如: 熱帶珊瑚礁海區(qū))。DOM具有吸收UV的作用可以減輕珊瑚礁生態(tài)系統(tǒng)的環(huán)境壓力, 同時(shí)活性光降解產(chǎn)物為其帶來(lái)營(yíng)養(yǎng)物質(zhì), 通過(guò)食物網(wǎng)的傳遞促進(jìn)其生產(chǎn)力。DOM的光降解作用可為珊瑚礁生態(tài)系統(tǒng)“低營(yíng)養(yǎng)鹽, 高生產(chǎn)力和生物多樣性”的特征研究提供新思路。

(3) 以往的研究著重于對(duì)不同來(lái)源DOM的光化學(xué)反應(yīng), 很少有考慮到水體中DOM濃度的變化。然而, 由于人類(lèi)活動(dòng)和全球氣候變化使淡水流域中DOM濃度升高, 進(jìn)而隨著江河流入海洋, 對(duì)海洋DOM的含量和組成成分有重要影響。因此, 有必要對(duì)陸源的DOM進(jìn)行精確的成分分析, 并對(duì)DOM濃度升高和光降解對(duì)水域生態(tài)系統(tǒng)的影響進(jìn)行研究。

(4) 對(duì)DOM的光降解進(jìn)行長(zhǎng)期觀察和大尺度的研究, 從而更準(zhǔn)確地把握其生態(tài)效應(yīng)和對(duì)水域生態(tài)系統(tǒng)響應(yīng)的預(yù)測(cè)。

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Photodegradation of dissolved organic matter and its effect on bacterioplankton and phytoplankton

ZHOU Wei-hua1, 2, LIAO Jian-zu1, 2, 3, GUO Ya-juan1, 2, 3, YUAN Xiang-cheng1, 2, HUANG Hui1, 2, LIU Sheng1, 2, LI Tao1, 2

(1. Key Laboratory of Tropical Marine Bio-resources and Ecology, South China Sea Institute of Oceanology, Chinese Academy of Sciences, Guangzhou 510301, China; 2. Tropical Marine Biological Research Station in Hainan, Chinese Academy of Sciences, Sanya 572000, China; 3. University of Chinese Academy of Sciences, Beijing 100049, China)

As the largest dynamic reservoir of organic carbon in the ocean, photodegradation of dissolved organic matter (DOM) under ultraviolet radiation (UV) has important effects on the biogeochemical cycles of biogenic elements, as well as on the structure and function of the marine ecosystem. This article summarizes the environmental factors that affect the photodegradation processes and products of DOM. In addition, the effects of photodegradation of DOM on bacterioplankton and phytoplankton are discussed. Owing to the different sources and complex compositions of DOM, the ecological effects of photodegradation are spatially different. Hence, further comprehensive studies are crucially needed to evaluate the ecological effects of photodegradation of DOM in different sea areas.

dissolved organic matter; photodegradation; ultraviolet radiation; bacterioplankton; phytoplankton

P76

A

1000-3096(2017)02-0136-09

10.11759/hykx20151214003

2015-12-14;

2016-04-17

國(guó)家自然科學(xué)基金(No.31370500, No.40806050, No.31370499)

周偉華(1976-), 男, 浙江東陽(yáng)人, 博士, 研究員, 主要從事海洋生態(tài)環(huán)境研究, 電話(huà), 020-89023225, Email: whzhou@scsio.ac.cn

Dec. 14, 2015

[National Natural Science Foundation of China, No. 31370500, No. 40806050, No. 31370499]

(本文編輯: 康亦兼)