Reduced sulphur sources favour Hg^II reduction during anoxygenic photosynthesis by Heliobacteria

Abstract

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 HgII 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 HgII. By comparing Hg0 production over a redox gradient of sulphur sources, we demonstrate that phototrophic HgII 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.

Common physiological processes control mercury reduction during photosynthesis and fermentation

“…Although this lag did not affect final cell density, the slow-growing culture from replicate #1 initiated Hg 0 production earlier than the culture in replicate #2 (Fig 1E, F). These results may be attributable to cells in replicate #1 maintaining a higher Hg to cell ratio in the first 12 hours of the experiment, which suggests that Hg is being reduced intracellularly in line with our previous work on Heliobacteria (23,54).…”

The biogeochemistry of ferruginous lakes and past ferruginous oceans

“…However, recent work has documented that Fe 3+ reducing bacteria such as Geobacteraceae can also methylate mercury (Bravo et al, 2018;Kerin et al, 2006;Si et al, 2015). Furthermore, a variety of anoxygenic photosynthetic bacteria have recently been shown to mediate Hg 2+ reduction under anoxic conditions (Grégoire et al, 2018;Grégoire and Poulain, 2016;Lavoie et al, 2020).…”

Bacterial Diversity and CAZyme Potential Revealed in Pandanus Rich Thermal Spring Cluster of India: A Non-cultivable 16S rRNA Sequencing Approach

“…The differential location of phylotypes in different sectors in the CCA plot and Pearson heatmap supports the idea that environmentally relevant factors other than temperature also contribute to bacterial divergence in these hot springs ( Klatt et al, 2013 ; Badhai et al, 2015 ; Sahoo et al, 2017 ; Kumar and Sharma, 2019 ). A significant positive correlation was depicted in Pearson heatmap and CCA analysis between sulfur content and sulfur reducing bacteria such as Heliobacterium , Thermodesulfovibrio , Thermodesulfobacterium , and Herpetosiphon ( Lobo et al, 2012 ; Lavoie et al, 2020 ) by using them as electron acceptors to sustain several activities such as respiration, conserving energy and growth, in the absence of oxygen ( Florentino et al, 2016 ).…”