ZJU scholars discover a novel methane-driven pathway to arsenic transfer

Methane is the second most abundant greenhouse gas which can be found in natural habitats such as wetlands and oceans as well as in artificial systems such as rice paddies and anaerobic digesters. Methane is also an effective biological carbon source and energy which can sustain microbial growth and metabolism, drive biogeochemical cycles of such elements as carbon, nitrogen, and sulfur, and promote the evolution and development of the biosphere.

Recently, scholars from Zhejiang University, the CAS Institute of Urban Environment and the Center for Applied Geosciences at the University of Tübingen in Germany, pioneered in the discovery of coupled anaerobic methane oxidation and reductive arsenic mobilization in wetland soils. Their findings are published in the journal of Nature Geoscience.

The research team led by Prof. ZHAO Heping from the Zhejiang University College of Environmental and Resource Sciences has long engaged in relevant research into water pollution control. Nitrate nitrogen is a major contaminant in urban wastewater and groundwater, and microbes can use COD as a carbon source to reduce it to nitrogen via denitrification, thus achieving removal of pollutants. However, when there is no sufficient carbon source, it is essential to add an external carbon source for the microorganisms to achieve denitrification, which not only increases the cost but also contributes to the hazard of secondary pollution.

In 2013, scientists discovered by happenchance that some microbes can use methane as their sole carbon source and electron donor for nitrate reduction. “This discovery brought us a flash of inspiration. As an intermediate product of wastewater treatment, methane is also sufficiently present in the natural environment,” ZHAO Heping said, “It will serve dual purposes if more of these microorganisms can be found to utilize greenhouse methane and remove a range of oxidizing contaminants such as nitrate nitrogen.”

On this basis, the research team extracted samples from methane-rich wetlands and used the isotopic tracer method to culture more similar microbes. In experiments, they discovered coupled anaerobic methane oxidation and reductive arsenic mobilization during which methane was oxidized to carbon dioxide and arsenate was converted into arsenite in a more soluble and toxic form.

“Unlike the reduction of other oxidizing pollutants, this valence shift is far from a good thing,” ZHAO Heping said, “Methane may give rise to arsenic reduction, a phenomenon that has gone unnoticed. The generation of large amounts of water-soluble arsenic in the environment may incur a higher risk of ecological toxicity due to increased mobility and transfer. For example, the presence of water-soluble arsenic in large quantities will lead to grains with too much arsenic, thereby causing food safety problems. This phenomenon is worth noting.”

Researchers also found that a particular type of microbes can metabolize methane and provide electrons for symbiotic arsenic-reductive bacteria, but little is known about the interspecies electron propagation of different microbes. Using comparative gene quantification and metagenomic sequencing, researchers from Zhejiang University found that the coupled pathway is facilitated by anaerobic methanotrophs, either independently or synergistically with arsenate-reducing bacteria through reverse methanogenesis and respiratory arsenate reduction.

In other words, electrons can run from cell to another, and the polyhemoglobin pigment protein plays the role as a “ferry” to transport electrons. “This protein is a prerequisite for interspecies collaboration among microorganisms. In the future, microorganisms with this protein can be utilized to remedy environmental pollution in an efficient manner,” ZHAO Heping commented.

In microcosm incubations with natural wetland soils, researchers found that the coupled pathway of anaerobic methane oxidation and arsenate reduction contributed 26.4 to 49.2% of total arsenic release from soils, exerting a deleterious impact on ecological health and crop safety. Further bioinformatic analyses showed that genes coding for reverse methanogenesis and respiratory arsenate reduction are universally co-distributed in nature. This suggested that coupling of anaerobic methane oxidation and arsenate reduction is a potentially global but previously overlooked process, with implications for arsenic mobilization and environmental contamination.

“Our team will carry out follow-up research into the biological mechanism from the perspective of genes and enzymology, and conduct a more systematic and comprehensive assessment of the impact of the biological metabolic pathway on human production and life. Hopefully we will provide some theoretical and experimental support for the formulation of prevention and control policies and the development of environmental pollution control technologies,” said ZHAO Heping.

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