一氧化氮的海洋生物地球化学研究进展❋

刘春颖, 田 野, 姜源庆，李培峰, 简慧敏, 张升辉

(1.中国海洋大学海洋化学理论与工程技术教育部重点实验室, 化学化工学院, 山东 青岛 266100；2.国家海洋局 烟台海洋环境监测中心站，山东 烟台 264006)

1 海洋中NO的测定方法研究

由于NO的不稳定性、易逸性、低含量，目前关于海水中NO测定方法的报道不多(见表1)，主要有电化学方法[11-13]，荧光法[14-16]和化学发光法[17-19]，这些方法可用于NO的光解产生和生长调控作用机制的研究。而能用于现场NO观测的只有吹扫-捕集荧光法[19]和吹扫-捕集化学发光法[17-18]，可分别用于近海和大洋的NO分布和产生速率的观测。吹扫-捕集化学发光法也逐渐从大体积开放式变成小体积密闭式。除了测定方法，NO的采样过程对测定结果有一定的影响，用传统的Niskin采水器采样可能有一定的误差，用水泵直接采水测定更为准确[18]。

表1 海水中NO的测定方法

2 海洋中NO的来源与分布

海水中NO的产生途径有光化学过程[22]、大气交换[23]、热过程[24]、生物学过程[25]含硝化与反硝化过程[2]、化学过程等。目前关于海洋NO的确切来源和转化还不清楚[26]，研究较多的是光降解过程，微生物硝化与反硝化过程和浮游植物生长过程(见图1)。

图1 NO的海洋生物地球化学过程[1-3,11-14]Fig.1 Marine biogeochemistry process of nitric oxide

海洋中生物体释放NO的研究主要集中在植物上。Morrall等[32-33]研究表明，热带海洋中银莲花的一氧化氮合成酶(NOS)活性明显受到环境温度、铜离子浓度的影响。在深海或表面水的大量浮游植物(藻类)生长处，因缺氧条件而产生NO[17]。Kim等[34-35]观测到Chattonellamarina,C.ovata和Heterosigmaakashiwo三种海洋赤潮藻释放NO的现象。刘春颖等[1]对海洋微藻Platymonassubcordiformis、Skeletonemacostatum、Gymnodiniumsp.和Chaetoceroscurvisetu藻液中NO的浓度和来源进行了初步研究，结果表明，不管是赤潮藻还是非赤潮藻体系都发现NO的客观存在，浓度大约在10-8～10-9mol/L；NO是微藻生长状况的信息因子，也是微藻应激反应的信号分子；藻密度达最大值之前，伴随着NO浓度曲线也出现一个峰值，此现象为赤潮化学预报研究提供了实验基础。除了实验室的观测，在野外藻类培养池中也检测到NO的存在[20,34-35]。此外，海洋动物产生NO的研究也开始有报道。Jeong等[37]发现潮间带桡足类虎斑猛水蚤受到免疫挑战时可诱发体内的NOS；Xian等[38]检测到对虾不同部位血细胞中NO浓度有所不同。

3 海洋中NO的迁移与转化

4 NO在海洋生态系中的作用

研究表明，生物体不仅自身会产生NO，同时会对外界的NO做出不同的响应，当然这依赖于浓度的大小[20, 43-45]，并有种间差异。Yoshihara等[46]和Nagase等[11]报道绿藻在吸收利用炉气时，能吸收其中的NO，在NO与炉气的体积比为(100～300)×10-6的气体中，绿藻能正常生长。从20世纪80年代后期开始，由于生物化学和分子生物学的研究发展，逐渐发现和证实NO是生物体内一种重要的信使分子和效应分子，在生命过程中起着举足轻重的作用。外源NO对海洋浮游植物的生长有显著的影响，它可能是浮游植物(藻类)生长的一种信息因子[8, 11, 46-47]；NO能参与调节微藻的其他生理活动[48-50]；并能在一定程度上抵御非生物的生长胁迫[51]，包括金属，非金属，紫外照射，农药等的胁迫。NO在水产养殖生物病害方面具有抵抗病原体的作用[7]；Chen等[52]报道了热带海参体内NO参与调节钙调素的证据；越来越多的证据表明，NO在海洋生态系扮演着重要的角色。

5 NO在大气中转化与作用

在平流层臭氧光化学过程中，NO通过反应NO+O3→NO2+O2直接破坏臭氧；另一个反应NO2+O→NO+O2不仅使大气中氧原子减少，不利于O3形成，同时还产生NO，进一步破坏O3。另一方面，平流层NO2还可以光化分解为NO和氧原子。NOx在对流层中的化学行为较为复杂，NO和NO2可转化为HNO2、HNO3、HO2NO2、NO3、N2O5和有机硝酸盐，它们当中大部分分子又被光解和热分解为NOx。观测表明，NO和NO2主要转化为HNO3、过氧乙酰硝酸酯(Peroxyacetyl Nitrate，PAN)和颗粒物硝酸盐。HNO3在白天很快被光解,在晚上能和NO2生成N2O5，而N2O5的湿清除可能是NOy(NOy=NOx+HNO2+HNO3+HO2NO2+NO3+2N2O5+PAN+颗粒物硝酸盐)的一个重要的汇。HNO3再以其高度水溶性溶入雨滴降落地面，也可能粘结在气溶胶或悬浮颗粒上，最后干降于地表，所以NOx最终的汇是地面或地面水系。NOx既能生成二次无机气溶胶，也能生成二次有机气溶胶(SOA)[53]。人类活动排放的CH4、CO、NMHC等气体和大气中的OH和O2反应生成过氧基，这些过氧基使大气中的NO转化为NO2。NO2光解生成O原子，基态氧原子和O2反应生成O3，诱发城市光化学烟雾。工业革命后对流层中O3浓度每年以2%～4%的速度增长，研究发现，这主要是由于人类活动排放的NOx所致的。由于NOx在对流层大气化学过程中的重要地位,所以近些年来对NOx在对流层中的分布及变化规律进行了许多测量和研究。观测表明，白天NOy的15%～30%是以(HNO3+PAN+NO3(p))形态存在的,夜晚90%以上的NOy是以(NO+NO2)的形态存在的。白天硝酸占有较高的比例，是因为NOx的光化学反应所致;在夜间有雾的情况下，40%的NOy是以硝酸盐的形态存在的。大气中NOx的浓度取决于NOx的排放与输送，在城市和工业区主要是人为的排放，浓度范围为1×10-9～10×10-9(v/v)；沿海地区为0.1×10-9～1.0×10-9(v/v)；在没有受到人为影响的远海和极地地区NOx的浓度为0.001×10-9～0.01×10-9(v/v)。洋面上NOx的浓度从海平面到平流层顶随着高度的增加而增大，这与陆地上NOx垂直梯度的变化正相反。夏季NOx的寿命主要由NO2和OH的反应所决定，且随着NOx浓度的变化而变化，同时也取决于NOx和NMHC的比值。当NOx的浓度在1×10-9(v/v)时，NOx的寿命约为0.5天，冬季NOx的寿命很不确定，约为1～2天[54]。卫星返回的数据可以检测到大气中NO2浓度变化，目前陆地上空的观测较多，而海洋上空的观测还未深入开展。

6 结论

综上所述，NO对全球气候与环境有着重要影响，海洋生态系释放NO越来越受到人们的重视；此外，NO在海洋生态系中扮演着重要的角色，对海洋生物生长具有重要的调控作用。然而NO的海洋生物地球化学研究刚刚起步，许多重要的科学问题和研究手段尚待解决。

(1)建立起适合于现场观测的易操作、高灵敏、低检出限的NO分析方法，才能广泛开展NO的海洋生物地球化学研究。目前还没有一套大家认可的NO分析方法。

(2)随着人们对NOx危害的重视以及化石燃料的减排，生态系统释放NO的问题越来越受到人们的关注。开展海洋尤其是近岸海域释放NO的过程及机制研究，才能完善海洋氮循环的研究，进一步对全球氮的收支平衡做出定量评估，为应对全球气候和环境变化提供参考数据。

(3)开展海洋生态系中NO的产生途径、释放速率及浓度范围，才能进一步认识NO的生态效应。

(4)深入研究NO对海洋生物生长，抗逆抗病等作用规律和机制，才能认识NO在海洋生态系中的地位与作用。

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