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Last Updated: Tuesday, 25 July 2006, 12:35 GMT 13:35 UK
Methane makers yield to science
By Louisa Cheung

Rice paddy (AFP)
Flooded rice cultivation is a major source of global methane emissions
The genetic code of an important group of methane-producing microbes has been sequenced by German scientists.

The archaea are probably the major source of methane emanating from rice fields, contributing up to a quarter of global emissions of the gas.

The new genomic information reveals how the single-celled organisms have adapted to thrive in paddy soil.

The researchers believe the study, published by Science magazine, could lead to ways to control the microbes.

"But whether this really leads to a total reduction of methane emission in the rice paddy soil, it is very difficult to predict," says Dr Werner Liesack, from the Max-Planck Institute for Terrestrial Microbiology.

'Major producer'

Rice is grown on all continents except Antarctica. At least 114 countries are producers, with the greatest tonnages cultivated in China and India.

According to the International Rice Research Institute, the world's rice fields in 2004 covered some 1,532,570 sq km (600,000 square miles) - an area equivalent to more than six United Kingdoms.

Rice paddies give off substantial quantities of methane, a potent greenhouse gas; and eight years ago, microbiologists identified what they believed to be the key culprit: a organism group they called Rice Cluster I (RC-I).

The archaea live in the soil, in amongst the root system of the rice plant.

"This novel methanogen group has been detected repeatedly in a geographically diverse region: South East Asia, Japan, Philippines, Europe and the southern part of the US," Dr Liesack told the BBC News website.

Scientists have repeatedly struggled to isolate RC-I from the mass of other organisms living in the soil - and hence to get sufficient numbers of the microbes to work on cracking their genetic code.

The success of the German researchers has been to develop a technique to enrich the quantities of the methanogens available for study; and then to use special bioinformatics tools to identify only the target group's genetic code.

The team found the microbes' genome to comprise about three million "letters" of DNA wound onto one circular chromosome. The methanogens likely have a little over 3,000 genes controlling their biological functions.

'Ancient' life

The scientists say the genomic information illustrates how the RC-I group out-competes other methanogens in the tough living conditions of a rice paddy.

Methanogens are strictly anaerobic organisms - they live in an oxygen-free environment. But in rice paddies, minuscule amounts of oxygen (O2) will dissolve in the water and get into the soil. When this happens, it can be lethal to many methanogens.

But the RC-I microbes have evolved a set of antioxidant enzymes that protect them from the toxic effects of O2, and this allows them to thrive in the plant root system where they are exposed to even higher levels of oxygen. The RC-I group can also switch their metabolism in a neat trick that not only rids them of oxygen but gains them energy in the process.

"RC-I methanogens seem to be adapted to this situation, which provides them with a selective advantage over other methanogens," explained Dr Liesack, who added that the findings raised the possibility of developing agro-chemicals that could limit methane emissions.

"It might be possible in the future to develop bioactive compounds to specifically suppress the [methane producing] activity of RC-I. Then it is a question of whether other methanogens might occupy this niche and produce methane," he said.

The archaea are an ancient branch of microbial life on Earth first identified by scientists in 1977.

Many of their species live in extreme environments. Some scientists have suggested that as such, archaea may represent the earliest form of life and thus may be the most likely form of life existing on other planets.

Some researchers hold out hope that some of the methane traces observed on Mars, for example, may be coming from organisms like RC-I.

Plants revealed as methane source
11 Jan 06 |  Science/Nature
The end of India's green revolution?
29 May 06 |  South Asia
Scientists complete rice genome
10 Aug 05 |  Science/Nature
Rice yields dip as planet warms
29 Jun 04 |  Science/Nature
Methane theory gets frosty response
11 Apr 03 |  Science/Nature


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