一氧化氮的海洋生物地球化學(xué)研究進展?

劉春穎, 田 野, 姜源慶，李培峰, 簡慧敏, 張升輝

(1.中國海洋大學(xué)海洋化學(xué)理論與工程技術(shù)教育部重點實驗室, 化學(xué)化工學(xué)院, 山東 青島 266100；2.國家海洋局 煙臺海洋環(huán)境監(jiān)測中心站，山東 煙臺 264006)

1 海洋中NO的測定方法研究

由于NO的不穩(wěn)定性、易逸性、低含量，目前關(guān)于海水中NO測定方法的報道不多(見表1)，主要有電化學(xué)方法[11-13]，熒光法[14-16]和化學(xué)發(fā)光法[17-19]，這些方法可用于NO的光解產(chǎn)生和生長調(diào)控作用機制的研究。而能用于現(xiàn)場NO觀測的只有吹掃-捕集熒光法[19]和吹掃-捕集化學(xué)發(fā)光法[17-18]，可分別用于近海和大洋的NO分布和產(chǎn)生速率的觀測。吹掃-捕集化學(xué)發(fā)光法也逐漸從大體積開放式變成小體積密閉式。除了測定方法，NO的采樣過程對測定結(jié)果有一定的影響，用傳統(tǒng)的Niskin采水器采樣可能有一定的誤差，用水泵直接采水測定更為準(zhǔn)確[18]。

表1 海水中NO的測定方法

2 海洋中NO的來源與分布

海水中NO的產(chǎn)生途徑有光化學(xué)過程[22]、大氣交換[23]、熱過程[24]、生物學(xué)過程[25]含硝化與反硝化過程[2]、化學(xué)過程等。目前關(guān)于海洋NO的確切來源和轉(zhuǎn)化還不清楚[26]，研究較多的是光降解過程，微生物硝化與反硝化過程和浮游植物生長過程(見圖1)。

圖1 NO的海洋生物地球化學(xué)過程[1-3,11-14]Fig.1 Marine biogeochemistry process of nitric oxide

海洋中生物體釋放NO的研究主要集中在植物上。Morrall等[32-33]研究表明，熱帶海洋中銀蓮花的一氧化氮合成酶(NOS)活性明顯受到環(huán)境溫度、銅離子濃度的影響。在深?；虮砻嫠拇罅扛∮沃参?藻類)生長處，因缺氧條件而產(chǎn)生NO[17]。Kim等[34-35]觀測到Chattonellamarina,C.ovata和Heterosigmaakashiwo三種海洋赤潮藻釋放NO的現(xiàn)象。劉春穎等[1]對海洋微藻Platymonassubcordiformis、Skeletonemacostatum、Gymnodiniumsp.和Chaetoceroscurvisetu藻液中NO的濃度和來源進行了初步研究，結(jié)果表明，不管是赤潮藻還是非赤潮藻體系都發(fā)現(xiàn)NO的客觀存在，濃度大約在10-8～10-9mol/L；NO是微藻生長狀況的信息因子，也是微藻應(yīng)激反應(yīng)的信號分子；藻密度達最大值之前，伴隨著NO濃度曲線也出現(xiàn)一個峰值，此現(xiàn)象為赤潮化學(xué)預(yù)報研究提供了實驗基礎(chǔ)。除了實驗室的觀測，在野外藻類培養(yǎng)池中也檢測到NO的存在[20,34-35]。此外，海洋動物產(chǎn)生NO的研究也開始有報道。Jeong等[37]發(fā)現(xiàn)潮間帶橈足類虎斑猛水蚤受到免疫挑戰(zhàn)時可誘發(fā)體內(nèi)的NOS；Xian等[38]檢測到對蝦不同部位血細胞中NO濃度有所不同。

3 海洋中NO的遷移與轉(zhuǎn)化

4 NO在海洋生態(tài)系中的作用

研究表明，生物體不僅自身會產(chǎn)生NO，同時會對外界的NO做出不同的響應(yīng)，當(dāng)然這依賴于濃度的大小[20, 43-45]，并有種間差異。Yoshihara等[46]和Nagase等[11]報道綠藻在吸收利用爐氣時，能吸收其中的NO，在NO與爐氣的體積比為(100～300)×10-6的氣體中，綠藻能正常生長。從20世紀(jì)80年代后期開始，由于生物化學(xué)和分子生物學(xué)的研究發(fā)展，逐漸發(fā)現(xiàn)和證實NO是生物體內(nèi)一種重要的信使分子和效應(yīng)分子，在生命過程中起著舉足輕重的作用。外源NO對海洋浮游植物的生長有顯著的影響，它可能是浮游植物(藻類)生長的一種信息因子[8, 11, 46-47]；NO能參與調(diào)節(jié)微藻的其他生理活動[48-50]；并能在一定程度上抵御非生物的生長脅迫[51]，包括金屬，非金屬，紫外照射，農(nóng)藥等的脅迫。NO在水產(chǎn)養(yǎng)殖生物病害方面具有抵抗病原體的作用[7]；Chen等[52]報道了熱帶海參體內(nèi)NO參與調(diào)節(jié)鈣調(diào)素的證據(jù)；越來越多的證據(jù)表明，NO在海洋生態(tài)系扮演著重要的角色。

5 NO在大氣中轉(zhuǎn)化與作用

在平流層臭氧光化學(xué)過程中，NO通過反應(yīng)NO+O3→NO2+O2直接破壞臭氧；另一個反應(yīng)NO2+O→NO+O2不僅使大氣中氧原子減少，不利于O3形成，同時還產(chǎn)生NO，進一步破壞O3。另一方面，平流層NO2還可以光化分解為NO和氧原子。NOx在對流層中的化學(xué)行為較為復(fù)雜，NO和NO2可轉(zhuǎn)化為HNO2、HNO3、HO2NO2、NO3、N2O5和有機硝酸鹽，它們當(dāng)中大部分分子又被光解和熱分解為NOx。觀測表明，NO和NO2主要轉(zhuǎn)化為HNO3、過氧乙酰硝酸酯(Peroxyacetyl Nitrate，PAN)和顆粒物硝酸鹽。HNO3在白天很快被光解,在晚上能和NO2生成N2O5，而N2O5的濕清除可能是NOy(NOy=NOx+HNO2+HNO3+HO2NO2+NO3+2N2O5+PAN+顆粒物硝酸鹽)的一個重要的匯。HNO3再以其高度水溶性溶入雨滴降落地面，也可能粘結(jié)在氣溶膠或懸浮顆粒上，最后干降于地表，所以NOx最終的匯是地面或地面水系。NOx既能生成二次無機氣溶膠，也能生成二次有機氣溶膠(SOA)[53]。人類活動排放的CH4、CO、NMHC等氣體和大氣中的OH和O2反應(yīng)生成過氧基，這些過氧基使大氣中的NO轉(zhuǎn)化為NO2。NO2光解生成O原子，基態(tài)氧原子和O2反應(yīng)生成O3，誘發(fā)城市光化學(xué)煙霧。工業(yè)革命后對流層中O3濃度每年以2%～4%的速度增長，研究發(fā)現(xiàn)，這主要是由于人類活動排放的NOx所致的。由于NOx在對流層大氣化學(xué)過程中的重要地位,所以近些年來對NOx在對流層中的分布及變化規(guī)律進行了許多測量和研究。觀測表明，白天NOy的15%～30%是以(HNO3+PAN+NO3(p))形態(tài)存在的,夜晚90%以上的NOy是以(NO+NO2)的形態(tài)存在的。白天硝酸占有較高的比例，是因為NOx的光化學(xué)反應(yīng)所致;在夜間有霧的情況下，40%的NOy是以硝酸鹽的形態(tài)存在的。大氣中NOx的濃度取決于NOx的排放與輸送，在城市和工業(yè)區(qū)主要是人為的排放，濃度范圍為1×10-9～10×10-9(v/v)；沿海地區(qū)為0.1×10-9～1.0×10-9(v/v)；在沒有受到人為影響的遠海和極地地區(qū)NOx的濃度為0.001×10-9～0.01×10-9(v/v)。洋面上NOx的濃度從海平面到平流層頂隨著高度的增加而增大，這與陸地上NOx垂直梯度的變化正相反。夏季NOx的壽命主要由NO2和OH的反應(yīng)所決定，且隨著NOx濃度的變化而變化，同時也取決于NOx和NMHC的比值。當(dāng)NOx的濃度在1×10-9(v/v)時，NOx的壽命約為0.5天，冬季NOx的壽命很不確定，約為1～2天[54]。衛(wèi)星返回的數(shù)據(jù)可以檢測到大氣中NO2濃度變化，目前陸地上空的觀測較多，而海洋上空的觀測還未深入開展。

6 結(jié)論

綜上所述，NO對全球氣候與環(huán)境有著重要影響，海洋生態(tài)系釋放NO越來越受到人們的重視；此外，NO在海洋生態(tài)系中扮演著重要的角色，對海洋生物生長具有重要的調(diào)控作用。然而NO的海洋生物地球化學(xué)研究剛剛起步，許多重要的科學(xué)問題和研究手段尚待解決。

(1)建立起適合于現(xiàn)場觀測的易操作、高靈敏、低檢出限的NO分析方法，才能廣泛開展NO的海洋生物地球化學(xué)研究。目前還沒有一套大家認可的NO分析方法。

(2)隨著人們對NOx危害的重視以及化石燃料的減排，生態(tài)系統(tǒng)釋放NO的問題越來越受到人們的關(guān)注。開展海洋尤其是近岸海域釋放NO的過程及機制研究，才能完善海洋氮循環(huán)的研究，進一步對全球氮的收支平衡做出定量評估，為應(yīng)對全球氣候和環(huán)境變化提供參考數(shù)據(jù)。

(3)開展海洋生態(tài)系中NO的產(chǎn)生途徑、釋放速率及濃度范圍，才能進一步認識NO的生態(tài)效應(yīng)。

(4)深入研究NO對海洋生物生長，抗逆抗病等作用規(guī)律和機制，才能認識NO在海洋生態(tài)系中的地位與作用。

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