海洋生态与环境科学重点实验室知识库

溶解有机质在痕量金属的海洋生物地球化学循环过程中扮演着重要角色，痕量金属的迁移转化过程及生态学功能等都强烈依赖于其在不同分子量溶解有机质中的配分及相互作用。本论文聚焦海水中的不同分子量有机质结合总溶解态(<0.45 μm)的痕量金属，通过测定不同分子量溶解有机碳的浓度、分析其荧光光谱和紫外-可见吸收光谱特征以及比较不同痕量金属在不同盐度下的配分特征，解析了溶解有机质分子量特征、溶解有机质组成和结构特征、痕量金属特性等内部因素以及盐度等外部因素对痕量金属-溶解有机质配分特征的影响研究，揭示了胶州湾海水中溶解态Cu，Cr，Cd，Pb，Ni和Co等在不同分子量溶解有机质中的配分特征，并初步探讨了其配分机理。本研究为进一步了解胶州湾海水痕量金属的生物地球化学行为，评估其生物可利用性和毒性效应等提供了一定的理论支持，获得的主要研究结论和认识如下：

1. 胶州湾海水中的痕量金属多与<1 kDa的低分子量溶解有机质相配合，平均占总溶解态痕量金属的70.1%，配分比例按Cu，Cd，Cr，Ni，Co，Pb的顺序逐渐降低。胶州湾海水中的溶解有机质也以<1 kDa的低分子量组分为主，源于浮游植物释放及微生物降解等过程原位产生的海源有机质，主要为腐殖化和芳构化程度较低的类富里酸和一些小分子的游离氨基酸。

胶州湾海水中的痕量金属主要配分于低分子量溶解有机质（<1 kDa）中，平均有70.1%（47.5-86.6%）的痕量金属与1 kDa以下的溶解有机质相配合，10 kDa-0.45 μm溶解有机质中的配分比例平均仅为5.4%。与低分子量溶解有机质相配合的痕量金属占总溶解态痕量金属的比例按Cu，Cd，Cr，Ni，Co，Pb的顺序逐渐降低，Cu（79.0%）和Cd（77.6%）在低分子量溶解有机质中的配分比例明显高于其余痕量金属，Cr（71.5%），Ni（67.3%）和Co（66.9%）配分比例稍低，Pb（58.2%）则最低。在水平方向上，胶州湾北部水体中痕量金属在低分子量溶解有机质中的配分比例（64.2%）普遍低于湾口及湾外水体（74.0%）。在垂直方向上，胶州湾底层海水中多数痕量金属在低分子量溶解有机质中的配分比例（73.1%）高于表层海水（68.1%）。

胶州湾海水中溶解有机碳的浓度范围为0.58-1.36 mg/L，其中有62.1-86.0%分布于1 kDa 以下，10 kDa以上的组分所占比例仅为2.36-10.37%，低分子量组分在胶州湾海水溶解有机碳中占主要优势。在空间分布上，胶州湾湾口及湾外水体中分布于1 kDa以下的溶解有机碳所占平均比例略高于湾内北部水体，底层海水与表层海水相比1 kDa以下的溶解有机碳所占平均比例略高，而1-3 kDa，3-10 kDa和10 kDa -0.7 μm分子量范围内溶解有机碳的空间分布规律较弱。

胶州湾溶解有机质中共检测出3种类蛋白质（低激发波长处的类酪氨酸D，低激发波长处的类色氨酸S，高激发波长处的类酪氨酸B）与3种类腐殖质（类腐殖酸A，海源类腐殖质M，类富里酸C）荧光组分，且随滤膜孔径的减小，类蛋白质荧光强度明显下降，而类腐殖质荧光强度下降趋势十分平缓，其中类富里酸随逐级过滤变化极不明显，表明胶州湾海水中高分子量溶解有机质主要由类蛋白质以及部分类腐殖酸和海源类腐殖质组成，低分子量溶解有机质则主要为类富里酸，也含有一些小分子的游离氨基酸。此外，胶州湾海水中溶解有机质的紫外-可见吸收光谱斜率S_275-295随分子量减小而逐渐升高，这表明低分子量溶解有机质中含有更多由浮游植物释放及微生物分解等过程原位产生的海源有机质，还代表着溶解有机质中富里酸与腐殖酸含量的比值逐渐升高，富里酸的调控作用逐渐凸显。特定波长下吸光度的比值E₂:E₃随分子量降低也表现出升高趋势，表明低分子量溶解有机质的芳构化程度比较低。

2. 溶解有机质分子量分布、化学结构及其与痕量元素的配合能力是控制胶州湾海水痕量金属-溶解有机质配分特征的最主要因素，同时痕量金属自身特性和盐度也具有一定影响。低分子量溶解有机质的优势比例，富含富里酸的化学结构和更高的痕量元素配合能力是胶州湾海水中总溶解态痕量金属主要配分于1 kDa以下低分子量溶解有机质中的主要原因。

胶州湾海水中痕量金属在不同分子量溶解有机质中的配分比例与该分子量有机质占总溶解态有机质的比例相对应，这表明溶解有机质的性质是控制痕量金属在不同分子量溶解有机质中配分的最关键因素。胶州湾海水中的溶解有机质以<1 kDa的低分子量组分为主，其配合容量远远高于其余分子量范围内的有机质。此外，<1 kDa的溶解有机质主要组分为类富里酸，富含羧基、酚羟基、醇羟基及酮羟基等官能团，氧化程度高，缩合程度低，芳构化程度低，可为痕量金属提供丰富的配合位点，痕量金属配合能力较高。1 kDa以上的溶解有机质中类蛋白质金属离子配合能力极低，类腐殖酸也因腐殖化程度较高而不利于痕量金属的配合和吸附。低分子量溶解有机质（<1 kDa）的高丰度与高配合能力共同决定了胶州湾海水中的痕量金属主要配分于低分子量溶解有机质中。

与低分子量溶解有机质相配合的痕量金属占总溶解态痕量金属的比例按Cu，Cd，Cr，Ni，Co，Pb的顺序逐渐降低，不同痕量金属与溶解有机质之间的配合方式和配合强度存在差异，进而影响着痕量金属-溶解有机质的配分特征。在配合方式上，Cu的配合为多分子层作用，可生成稳定的内部配合物，Cd的配合为单分子层作用，Ni和Co也更易与溶解有机质表面的配合位点作用生成外界配合物。在配合强度上，Cu与浮游植物及细菌释放的低分子量溶解有机质的配合位点之间亲和力极强，且依赖微弱的疏水作用便可以维持有机配合物的稳定，不会进一步连接聚集生成高分子量的聚集体。Pb对溶解有机质中弱配合位点的竞争力较高，与强配合位点间的配合强度则较弱，易受其他金属干扰破坏。

娄山河口高盐度水样（S=27.5）中的痕量金属在>1 kDa溶解有机质中的配分比例（30.0-42.4%）明显低于上游低盐度水样（S=1.1，37.7-47.9%），表明盐度升高不利于水体中痕量金属与高分子量溶解有机质的配合。盐度升高会导致水体中的高分子量溶解有机质发生絮凝沉淀，从水体中去除，低分子量组分则受影响较小。胶州湾海水盐度较高且差别较小，水体中的絮凝作用变得不明显，盐度对痕量金属-溶解有机质的主要作用方式发生改变，可能通过影响痕量金属的吸附上限pH值、干扰海洋生物活动等途径间接对痕量金属的配分过程产生影响，但作用结果与絮凝作用仍是一致的。此外，随着盐度升高，离岸距离增加，水体中由陆生或高等水生植物所产生的溶解有机质逐渐减少，来源于海洋浮游植物和微生物活动的溶解有机质逐渐增多，而前者的分子量通常高于后者。

Dissolved organic matters (DOM) are of great significance in the biogeochemical cycles of trace metals in aquatic systems. The speciation, transformation and ecological functions of trace metals are closely related to their distribution and mutual transformation among DOM with different molecular size. It is likely that different sized components may have various sources, characteristics and behaviors. In this thesis, water samples from Jiaozhou Bay were collected to analyze the size partitioning characteristics and controlling mechanisms of dissolved (<0.45 μm) Cu，Cr，Cd，Pb，Ni and Co. Besides, molecular size spectra, fluorescence spectra and ultraviolet-visible absorption spectra of DOM were measured to study the influence of compositions and structures of DOM to trace metal distribution characteristics. Influence of trace metal speciation and salinity were also investigated by comparing the size distribution characteristics of different metals at different salinities. The main results and conclusions obtained are as follows:

1. Dissolved race metals in Jiaozhou Bay seawater were mostly partitioned in the <1 kDa fraction, with an average percentage of 70.1% and a decreased order of Cu，Cd，Cr，Ni，Co and Pb. DOM in Jiaozhou Bay seawater was also mainly distributed in the <1 kDa size range, mostly including free amino acids and fluvic acids with lower degree of humification and aromatization.

About 47.5-86.6% (70.1% on average) of the trace metals in Jiaozhou Bay seawater were measured in the <1 kDa fraction, while the proportion of the 10 kDa-0.45 μm fraction was only 5.4% on average. The proportion of low molecular weight fractions (<1 kDa) in the total dissolved trace metals gradually decreased according to the order of Cu, Cd, Cr, Ni, Co and Pb. The proportions of Cu (79.0%) and Cd (77.6%) distributed in small molecular size DOM were significantly higher than that of other trace metals, and then Cr (71.5%)，Ni (67.3%) and Co (66.9%), and lastly Pb (58.2%). In the horizontal direction, seawater samples from northern Jiaozhou Bay possessed lower proportions of <1 kDa fraction (64.2%) than that from the mouth and outside of the bay (74.0%). In the vertical direction, surface seawater samples seemed to have lower proportions (68.1%) of <1 kDa fraction than bottom seawater samples (73.1%).

The concentrations of dissolved organic carbon in Jiaozhou Bay seawater ranged from 0.58 mg/L to 1.36 mg/L, of which 62.1-86.0% were distributed below 1kDa and 2.36-10.37% were distributed above 10 kDa, showing a considerable abundance of the smallest size fraction among the total size range. Seawater from the mouth and outside of the bay were characterized with higher proportions of smaller size components than that from northern area, while bottom seawater had higher proportions than surface seawater. The spatial distribution pattern of trace metals among 1-3 kDa，3-10 kDa and 10 kDa - 0.45 μm was not clear.

Fluorescent assay revealed that six types of fluorescence were detected according to the position of fluorescence peaks based on fluorescence of model compounds, involving three types of protein-like components (D, S, B) and three types of humic-like components (A, M, C). A pronounced decrease for fluorescent intensities of protein-like fractions appeared along with the ultrafiltration, while the intensities of humic-like fluorescence varied little with the decreased of membrane cut-offs, among which C peak possessed the gentlest trend. It was suggested that the high molecular size DOM in Jiaozhou Bay seawater was mainly composed of proteins, humic acids and marine humic substances, while the small molecular size DOM was comprised of fulvic acids and amino acids. The absorption spectral slope S_275-295 gradually increased with the decrease of the molecular size of DOM, suggesting that there were more in-suit generated organics from activities of phytoplankton and microorganisms in small molecular size fractions. It also indicated that the ratio of fulvic acids to humic acids was higher for small molecular size DOM. The effect of fulvic acids became more prominent along with the decrease of molecular size of DOM. What’s more, the E₂:E₃ ratio showed a rising trend as molecular size of DOM decreased, demonstrating a lower aromatization degree for small molecular DOM.

2. The size distribution, chemical structure and complexing ability of DOM were the major factors controlling the size spectra of trace metals in Jiaozhou Bay seawater, while the trace metal speciation and salinity also made some difference. The low molecular size dominated distribution of trace metals was a result of the higher abundance and stronger metal binding capacity of low molecular size DOM.

The size spectra of trace metals and dissolved organic carbon were basically the same, indicating that the molecular size distribution of DOM played a dominant role in shaping the size spectra of trace metals in the ocean. Besides, DOM in the <1 kDa size range was mainly composed of fulvic acids, which were abundant in carboxyl, phenolic hydroxyl, alcohol hydroxyl, ketone hydroxyl groups and had higher oxidation degrees and lower condensation degrees, thus had higher complexing abilities to trace metals. Proteins and highly humified humic acids in the >1 kDa fraction only played limited roles in the complexation of trace metals. The higher abundance and stronger metal binding capacity of low molecular size dissolved organic matters determined that the trace metals in Jiaozhou Bay seawater were mainly associated with the lower size range.

The proportion of low molecular weight fractions (<1 kDa) in the total dissolved trace metals gradually decreased according to the order of Cu, Cd, Cr, Ni, Co and Pb, indicating the influence of metal species to trace metal distributions. The complexation forms and degrees were different for metals with different properties, thus resulted in various size distribution characteristics. In terms of complex mechanism, the complexation of Cu was multilayer interaction and could form stable internal complexes, while Cd, Ni and Co were more likely to react with the complexing sites on the surface of DOM and formed external complexes. As for complex strength, Cu had a higher affinity with the coordination sites of low molecular size DOM and the stability of complexes could be maintained without further connection and aggregation, while Pb possessed a higher competitiveness to the coordination sites of high molecular size DOM.

Proportions of high molecular size components were 37.7-47.9% and 30.0-42.4% at the salinity of 1.1 and 27.5 respectively, indicating that increasing salinity was detrimental to the complexation between high molecular size DOM and trace metals. DOM with high molecular size were flocculated and precipitated due to the increasing ion strength and removed from the water in the end, while small molecular size fractions were less affected by the increasing salinity. The flocculation of DOM in seawater became insignificant when the salinity rose to a certain degree, and then salinity might indirectly affect the distribution processes of trace metals by influencing the upper adsorption pH value of them and interfering with biological activities. Furthermore, DOM derived from phytoplankton and microbial activities increased as salinity increased, which had a smaller size than DOM originating from terrestrial higher plants.