Abstract:

The heavy metal mercury (Hg), especially methylmercury (MeHg), is highly prone to transfer from soil to rice plants, posing a significant threat to human health through the food chain. Element sulfur (S(0)) is widely used to reduce the accumulation of MeHg in rice grains. However, the effects of S(0) on rice growth traits and the regulatory mechanism of MeHg uptake and translocation remain unclear. Therefore, this study conducted a rice pot experiment, including the control treatment (Hg-contaminated soil from the Wanshan Hg mine in Tongren City, Guizhou Province, with a total mercury (THg) content of 40.3 mg·kg^-1) and the S(0) treatment (added at a content of 100 mg·kg^-1). A comparison was made between the two treatments in terms of soil properties (including redox potential, pH, dissolved organic carbon, and sulfate), MeHg levels in the soil and (NH₄)₂S₂O₃ extract, abundance of soil Hg transformation genes (dsrA and dsrB), rice growth conditions, leaf chlorophyll level, MeHg content and distribution in different tissues of rice plants. The results showed that S(0) treatment significantly promoted rice growth and increased leaf chlorophyll synthesis (an increase of 27.4%). Furthermore, S(0) treatment increased the accumulation of MeHg in rice roots and stems, reduced its transportation from roots, stems, and leaves to grains, and ultimately decreased the accumulation and content of MeHg in rice grains (a decrease of 14.7%). On the other hand, S(0) significantly increased the level of sulfate in the soil (an increase of 124%), stimulated the activity of sulfate-reducing bacteria, which may participate in both the Hg methylation process and the degradation of MeHg in anaerobic environment, thereby significantly reducing the MeHg level in soils (a decrease of 11.6%). The MeHg contents in rice grains and whole plants showed a significant positive correlation with the MeHg content in (NH₄)₂S₂O₃ extract (P<0.01), while the content of dissolved organic carbon and sulfate in the soil showed a significant negative correlation with the MeHg content in (NH₄)₂S₂O₃ extract (P<0.01). Therefore, this study demonstrated the important role of S(0) in the soil-rice system in mitigating heavy metal stress and provided a theoretical basis and practical evidence for the development of measures to reduce Hg pollution and achieve rice attenuation and detoxification.

Key words: sulfur, methylmercury, sulfate-reducing bacteria, Hg bioaccumulation, rice

摘要：

重金属元素汞，尤其甲基汞，极易从土壤中转运到水稻植株中，并通过食物链对人类健康构成严重威胁。单质硫被广泛用于降低水稻籽粒中甲基汞的累积，然而其对水稻生长性状的影响、对甲基汞的吸收和转运的调控机制尚不明确。采用水稻盆栽试验，设置了对照组（汞污染土采自贵州省铜仁市万山汞矿，总汞质量分数为40.3 mg·kg^-1）和单质硫修复组（添加量为100 mg·kg^-1），比较了两组土壤和硫代硫酸铵提取态甲基汞水平、土壤理化性质（包括氧化还原电位、酸碱度、溶解性有机碳、硫酸根）、土壤汞转化基因（dsrA和dsrB）丰度，以及水稻的生长状况、叶片叶绿素水平、不同组织甲基汞的质量分数和分配。结果表明，单质硫处理显著促进水稻的生长，提高叶片叶绿素的合成（增加了27.4%）；增加了水稻根和茎中甲基汞的累积，降低了根、茎和叶到籽粒的转运，最终降低了水稻籽粒中甲基汞的累积和质量分数（减少了14.7%）。同时，单质硫显著提高了土壤中硫酸根的水平（增加了124%），刺激了硫酸盐还原菌活性，其可能在参与汞甲基化过程的同时参与了厌氧环境下甲基汞的降解，从而显著降低了土壤甲基汞的水平（减少了11.6%）。水稻籽粒和整株水稻中的甲基汞质量分数与土壤硫代硫酸铵提取态甲基汞质量分数呈显著正相关（P<0.01），而土壤溶解性有机碳和硫酸根的质量分数与土壤硫代硫酸铵提取态甲基汞质量分数呈显著负相关（P<0.01）。该研究证明了单质硫在土壤-水稻体系中抵御重金属胁迫的重要作用，为制定降低汞污染以及实现稻米减毒脱毒的措施提供了理论基础和现实依据。

关键词: 硫, 甲基汞, 硫酸盐还原菌, 汞积累, 水稻