Half of world´s population is nourished by rice crops, but rice cultivation is harsh to the climate. The rice fields account for five per cent of global emissions of the greenhouse gas 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.
Now researchers from Aarhus University in Denmark and the University of Duisburg-Essen, Germany, have found that cable bacteria could be an important part of the solution to the methane problem. In the laboratory, they have grown rice in soil with and without cable bacteria and measured what happened.
“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 was published in the scientific journal Nature Communications on the 20th April 2020.
Cable bacteria used to be an unknown form of life until first identified in the Bay of Aarhus in 2012. They thrive on the bottom of the sea, lakes, groundwater and streams, often in large quantities. Each individual consists of thousands of cells in a centimetre-long chain surrounded by a common outer sheath with electric wires. One end is buried deep in the oxygen-poor sediment, while the other is so close to the water that it has contact with oxygen. This allows the bacteria to use electric power to burn the food in an oxygen-free environment. The process also changes the chemical composition in the field.
“Cable bacteria transport electrons over centimetre distances along their filaments, changing the geochemical conditions of the water-saturated soil. The cable bacteria recycle the soil's sulphur compounds, thus maintaining a large amount of sulphate in the soil. This has the consequence that the methane-producing microbes cannot maintain their activity,” explains Scholz.
It is already known that the rice growers can temporarily slow down the emission of methane by spreading sulphate 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. While the authors emphasise that they have made only the very first laboratory observation, it is tempting to speculate that enrichment of cable bacteria by sensible management of water and soil regime could become a sustainable and convenient solution for reducing methane emissions from rice fields. But now, seeing how cable bacteria can thrive in rice fields requires field studies.
(Universities of Aarhus and Duisburg-Essen/wi)
Scholz, V., Meckenstock, R.U., Nielsen, L.P. et al.: Cable bacteria reduce methane emissions from rice-vegetated soils. Nat Commun 11, 1878 (2020).