Cable Bacteria Offer a Greener Way To Grow Rice

Half of the world's population gets their nutrition from rice, but rice cultivation is harsh by the climate. The rice fields account for five percent of global greenhouse gas emissions of methane, which is 25 times stronger than CO2 .

This is because the rice plants grow in water. When the fields are flooded, the soil becomes poor in oxygen, creating the right conditions for microorganisms to produce methane.

Researchers from Aarhus University and the University of Duisburg-Essen have now found that cable bacteria can be an important part of the solution. In the lab, they have grown rice in soil with and without cable bacteria and measured what came out.

Cable bacteria were an unknown way of life until they were first identified in the Gulf of Aarhus in 2012. They thrive on the bottom of the sea, lakes, groundwater and streams and often in large quantities. Each individual consists of thousands of cells in a centimeter-long chain surrounded by a common outer sheath with electrical wires. One end is buried in the oxygen-poor sediment, the other is so close to the water that it has contact with oxygen. This allows the bacteria to use electrical power to burn the food in an oxygen-free environment. The process also changes the chemical composition of the soil.

“And the difference was far beyond my expectations. The pots with cable bacteria emitted 93 per cent. less methane than the pots without cable bacteria, ”says Vincent Valentin Scholz, who conducted the experiments as a PhD student at the Center for Electromicrobiology (CEM) at Aarhus University.

The result is published today in the scientific journal Nature Communications.

Increases sulfate and attenuates microbes

“Cable bacteria carry electrons over centimeter distances along their filaments, which changes the geochemical conditions of the water-saturated soil. The cable bacteria recycle the soil's sulfur compounds, thus maintaining a large amount of sulfate in the soil. This has the consequence that the methane-producing microbes reduce their activity, ”explains Vincent Valentin Scholz.

It is already known that the rice growers can temporarily slow down the emission of methane by spreading sulfate on the rice fields. Apparently, the cable bacteria can do this for them - and not just temporarily.

This finding adds a new angle to the role of cable bacteria as ecosystem engineers. Vincent Valentin Stolz and the other authors emphasize that they still have only the very first laboratory observation. But they find it tempting to speculate that enrichment with cable bacteria through sensible management of water and soil conditions can become a sustainable and convenient solution to reduce methane emissions from rice fields - although of course it requires field studies to see how cable bacteria thrive in rice fields. 

Reference
Scholz, V., Meckenstock, R.U., Nielsen, L.P. et al. Cable bacteria reduce methane emissions from rice-vegetated soils. Nat Commun 11, 1878 (2020). https://doi.org/10.1038/s41467-020-15812-w.

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