The consumption of rice has become a global food safety issue because rice paddies support the production of high levels of the potent neurotoxin, methylmercury. The production of methylmercury is carried out by chemotrophic anaerobes that rely on a diversity of terminal electron acceptors, namely sulphate. Sulphur can be a limiting nutrient in rice paddies, and sulphate amendments are often used to stimulate crop production, which can increase methylmercury production. Mercury (Hg) redox cycling can affect Hg methylation by controlling the delivery of inorganic Hg substrates to methylators in anoxic habitats. Whereas sulphur is recognized as a key substrate controlling methylmercury production, the controls sulphur exerts on other microbe‐mediated Hg transformations remain poorly understood. To explore the potential coupling between sulphur assimilation and anaerobic Hg^II reduction to Hg⁰, we studied Heliobacillus mobilis, a mesophilic anoxygenic phototroph representative from the Heliobacteriacea family originally isolated from a rice paddy. Here, we tested whether the redox state of the sulphur sources available to H. mobilis would affect its ability to reduce Hg^II. By comparing Hg⁰ production over a redox gradient of sulphur sources, we demonstrate that phototrophic Hg^II reduction is favoured in the presence of reduced sulphur sources such as thiosulphate and cysteine. We also show that cysteine exerts dynamic control on Hg cycling by affecting not only Hg's bioavailability but also its abiotic photoreduction under low light conditions. Specifically, in the absence of cells we show that organic matter (as yeast extract) and cysteine are both required for photoreduction to occur. This study offers insights into how one of the most primitive forms of photosynthesis affects Hg redox transformations and frames Heliobacteria as key players in Hg cycling within paddy soils, forming a basis for management strategies to mitigate Hg accumulation in rice.

中文翻译：

---

减少的硫源有利于Heliobacteria进行无氧光合作用过程中的HgII还原。

稻米的消费已成为全球食品安全问题，因为稻田支持高水平的有效神经毒素甲基汞的生产。甲基汞的生产是通过依赖于多种末端电子受体（即硫酸盐）的化学营养厌氧菌进行的。硫可能是稻田中的一种限制性营养素，硫酸盐改良剂通常用于刺激作物生产，从而可以增加甲基汞的产量。汞（Hg）氧化还原循环可通过控制无机Hg基质向缺氧生境中的甲基化剂的传递来影响Hg甲基化。尽管硫被认为是控制甲基汞生产的关键底物，但对硫施加于其他微生物介导的汞转化的控制方法仍知之甚少。^II还原为Hg ⁰，我们研究了运动发酵菌（Heliobacillus mobilis），这是一种嗜热的产氧营养养分，代表了最初从水稻中分离的Heliobacteriacea家族。在这里，我们测试了运动发酵单胞菌可用的硫源的氧化还原状态是​​否会影响其减少Hg ^II的能力。通过比较在硫源的氧化还原梯度上Hg ^0的产生，我们证明了光养性Hg ^II在还原的硫源如硫代硫酸盐和半胱氨酸的存在下，减少还原是有利的。我们还表明，半胱氨酸不仅会影响汞的生物利用度，而且还会影响其在弱光条件下的非生物光还原作用，从而对汞循环施加动态控制。具体而言，在没有细胞的情况下，我们表明有机物质（作为酵母提取物）和半胱氨酸都是光还原发生所必需的。这项研究提供了有关光合作用的最原始形式之一如何影响汞氧化还原转化以及将Heliobacteria构筑为稻田土壤中Hg循环的关键因素的见解，从而为减轻水稻中Hg积累的管理策略奠定了基础。