Ortiz Veronica L, Mason Robert P, Ward J Evan
University of Connecticut Marine Sciences Department 1080 Shennecossett Rd Groton, CT 06340.
Mar Chem. 2015 Dec 20;177(Pt 5):753-762. doi: 10.1016/j.marchem.2015.07.006.
In the marine environment, settling particulates have been widely studied for their role as effective vertical transporters of nutrients and metals scavenged from the euphotic zone to the benthos. These particulates are composed of transparent exopolymers, plankton and bacterial cells, detritus and organic matter, and form various size fractions from colloids (<0.2μm) to aggregates, and finally marine snow (>300 μm). As marine snow forms in the water column, anoxic layers form around and within the aggregation potentially creating a prime environment for the methylation of mercury (Hg), which occurs primarily in low oxygen environments. To examine this process, marine aggregates were produced from sieved estuarine seawater (100 μm) in 1-L glass bottles spiked with stable isotope enriched methylmercury (CHHg) and inorganic mercury (Hg(II)) at 18° C using a roller-table. After the rolling period, different particle-size fractions were collected and analyzed, including: visible marine snow (>300μm), particulates 8 to 300 μm, and particulates 0.2 to 8μm. Particulate analysis indicated higher incorporation of both forms of Hg into marine snow compared to unrolled treatments, with greater incorporation of Hg(II) than CHHg. In addition, inorganic Hg was methylated and CHHg was demethylated in the larger particulate fractions (>8μm). Methylation and demethylation rates were assessed based on changes in isotopic composition of Hg(II) and CHHg, and found to be comparable to methylation rates found in sediments. These results indicate that net Hg methylation can occur in marine snow and smaller aggregates in oxic coastal waters, and that this net formation of CHHg may be an important source of CHHg in both coastal and open ocean surface environments.
在海洋环境中，沉降颗粒物作为从真光层清除营养物质和金属并将其输送至底栖生物的有效垂直载体，已得到广泛研究。这些颗粒物由透明胞外聚合物、浮游生物和细菌细胞、碎屑及有机物组成，形成从胶体（<0.2μm）到聚集体，最终到海雪（>300μm）的各种粒径级分。当海雪在水柱中形成时，聚集体周围和内部会形成缺氧层，这可能为汞（Hg）的甲基化创造理想环境，汞的甲基化主要发生在低氧环境中。为研究此过程，在1升玻璃瓶中，使用转台在18°C条件下，用添加了稳定同位素富集甲基汞（CHHg）和无机汞（Hg(II)）的经筛滤的河口海水（100μm）制备海洋聚集体。滚动期结束后，收集并分析不同粒径级分，包括：可见海雪（>300μm）、8至300μm的颗粒物以及0.2至8μm的颗粒物。颗粒物分析表明，与未滚动处理相比，两种形式的汞在海雪中的掺入量更高，且Hg(II)的掺入量高于CHHg。此外，在较大粒径级分（>8μm）中，无机汞发生甲基化，CHHg发生去甲基化。基于Hg(II)和CHHg同位素组成的变化评估甲基化和去甲基化速率，发现其与沉积物中的甲基化速率相当。这些结果表明，在有氧的沿海水域，海雪和较小聚集体中可发生汞的净甲基化，且CHHg的这种净生成可能是沿海和开阔海洋表层环境中CHHg的重要来源。
