Armstrong Grace J, Janssen Sarah E, Poulin Brett A, Tate Michael T, Krabbenhoft David P, Hurley James P
U.S. Geological Survey Upper Midwest Water Science Center, Madison, Wisconsin 53726, United States.
Environmental Chemistry and Technology Program, University of Wisconsin-Madison, Madison, Wisconsin 53706, United States.
ACS Earth Space Chem. 2023 Dec 6;7(12):2382-2392. doi: 10.1021/acsearthspacechem.3c00154. eCollection 2023 Dec 21.
Isotope fractionation related to photochemical reactions and planktonic uptake at the base of the food web is a major uncertainty in the biological application of mercury (Hg) stable isotopes. In freshwater systems, it is unclear how competitive interactions among methylmercury (MeHg), dissolved organic matter (DOM), and phytoplankton govern the magnitude of mass-dependent and mass-independent fractionation. This study investigated how DOM alters rates of planktonic MeHg uptake and photodegradation and corresponding Hg isotope fractionation in the presence of freshwater phytoplankton species, . Outdoor sunlight exposure experiments utilizing were performed in the presence of different DOM samples using environmentally relevant ratios of MeHg-DOM thiol groups. The extent of ΔHg in phytoplankton incubations (2.99‰ St. Louis River HPOA, 1.88‰ Lake Erie HPOA) was lower compared to paired abiotic control experiments (4.29 and 2.86‰, respectively) after ∼30 h of irradiation, resulting from cell shading or other limiting factors reducing the extent of photodemethylation. Although the ΔHg/ΔHg ratio was uniform across experiments (∼1.4), ΔHg/δHg slopes varied dramatically (from -0.96 to 15.4) across incubations with and DOM. In addition, no evidence of Hg isotope fractionation was observed within cells. This study provides a refined examination of Hg isotope fractionation markers for key processes occurring in the lower food web prior to bioaccumulation, critical for accurately accounting for the photochemical processing of Hg isotopes across a wide spectrum of freshwater systems.
与光化学反应和食物网底部浮游生物吸收相关的同位素分馏是汞（Hg）稳定同位素生物应用中的一个主要不确定因素。在淡水系统中，甲基汞（MeHg）、溶解有机物（DOM）和浮游植物之间的竞争相互作用如何控制质量依赖和质量独立分馏的程度尚不清楚。本研究调查了在淡水浮游植物物种存在的情况下，DOM如何改变浮游生物对MeHg的吸收和光降解速率以及相应的Hg同位素分馏。利用不同DOM样品，以与环境相关的MeHg-DOM硫醇基团比例，进行了户外阳光照射实验。在照射约30小时后，浮游植物培养中的ΔHg程度（圣路易斯河HPOA为2.99‰，伊利湖HPOA为1.88‰）与配对的非生物对照实验（分别为4.29‰和2.86‰）相比更低，这是由于细胞遮光或其他限制因素降低了光脱甲基化的程度。尽管跨实验的ΔHg/ΔHg比率是一致的（约为1.4），但跨 与DOM的培养中，ΔHg/δHg斜率变化很大（从-0.96到15.4）。此外，在 细胞内未观察到Hg同位素分馏的证据。本研究对生物累积之前在较低食物网中发生的关键过程的Hg同位素分馏标记进行了精细研究，这对于准确核算广泛淡水系统中Hg同位素的光化学过程至关重要。
