貧養海域溶解態與膠體態砷物種研究

Abstract

砷為一類金屬元素，在環境中由於對生物體具有毒性而受到重視。砷在海洋中屬於類營養型的垂直分布，有光層裡受到生物吸收作用影響而濃度較低，但在深度大於有光層的海水中，砷的濃度穩定，約為20 nM。過去的研究已顯示，五價砷為海水中砷物種存在的主要型態；其餘砷物種如三價砷或甲基砷等，多出現於缺磷海域之有光層。然而過往的海洋砷物種研究，多是依據化學方式界定砷物種，並未從砷所存在分子量大小進行探討，即無從得知海水中是否存在膠體態（1 kDa–0.45 μm）砷。本研究即透過交流過濾法，搭配砷物種化學分析，探討西菲律賓海與南海的砷物種型態。 溶解態無機砷物種在西菲律賓海與南海，均呈現類似類營養鹽型的垂直分布。其中，10 – 20%的溶解態無機砷存於膠體態，而五價砷為膠體態無機砷的主要物種，三價砷則多存在於真溶解態（< 1 kDa）。南海的溶解態有機砷濃度在表水為3.2±1.1 nM，且濃度在上部有光層隨深度增加而降低；其中56%屬於膠體態有機砷。藉由分子量大小的物理分離，搭配化學物種分析，我們重新確立海水裡溶解態砷物種型態。 在西菲律賓海的時序觀測中，發現膠體態砷濃度在上部有光層有明顯的日夜變化現象，顯示膠體態砷在海水中的變化快速。此外，在西菲律賓海及南海的觀測中，都發現膠體態砷及膠體態有機磷於上部有光層具有正相關，表示磷對砷在海洋生地化循環相當重要，而膠體則在其中扮演著橋樑的角色。

Arsenic (As) is a metalloid element receiving major concern due to its toxicity. This element in the ocean shows nutrient-like vertical distributions, with a change of the concentrations mostly in the euphotic zone. Earlier studies had found that As(V) is the most dominant species, followed by As(III) and methylated As; the latter two are common in the surface of the P-stressed waters. However, there was no evidence of colloidal arsenic in the ocean, even though colloids are found labile to microbes and influence the partitioning of other bioactive elements in seawater. Here we used cross-flow ultrafiltration and chemical speciation methods to prove and provide the first evidence on the existence of colloidal arsenic and the molecular weight distributions of various dissolved arsenic species in the ocean. Dissolved inorganic arsenic (DIAs) showed typical nutrient-like depth profiles in the western Philippine Sea and the South China Sea. Moreover, 10–20% of the DIAs was composed of HMW-DIAs, which is discovered for the first time. Besides, nearly all the HMW-DIAs was As(V). Dissolved organic arsenic (DOAs) concentrations, in contrast, displayed decreasing trends with depth, with an average concentration of 3.2±1.1 nM in the surface. 56% of the DOAs was in colloidal phase. With molecular weight fractionation and chemical methods, dissolved arsenic species in seawater is redefined. In a short-term study in the western Philippine Sea, we found clear diel variations of the HMW-As in the upper euphotic zone, indicating a rapid turnover of arsenic in colloidal phase. Positive correlations between HMW-As and HMW-DOP in the upper euphotic zone were found in both the western Philippine Sea and the South China Sea, suggesting a close linkage with phosphorus. Overall, the results prove that colloids play a major role in biogeochemical cycle of arsenic in the ocean.