Daily news

25 August 2005

Millions of bacterial species revealed underfoot

The soil beneath our feet may be teeming with a hundred times more species of bacteria than previously thought, according to biologists in New Mexico, US. Their calculations reveal that one gram of dirt can harbour a million microbial species – and that metal pollution kills 99% of these as-yet unknown germs.

Measuring the bacterial biodiversity of soil is difficult because only a few species can be cultured in the lab, according to Jason Gans of Los Alamos National Laboratory, New Mexico, US. Fortunately, biologists can also estimate biodiversity using a technique called DNA reassociation. This involves chemically unzipping the two strands of all the bacterial DNA in a sample, mixing them up and seeing how long they take to join up again with matching partners.

If all the DNA strands were the same, they would find matching partners very quickly. But the more diverse the DNA strands, the longer this match-making takes, allowing researchers to estimate how many different species there are in the sample.

When this technique was applied to soils in the late 1990s, it suggested that a gram of dirt contained about 16,000 species. But this estimate contained a simplification that the populations of all the different species in the soil were roughly equal in size. So Gans and his colleagues have developed new equations to reanalyse the same DNA reassociation data but without this size assumption.


Their results reveal that there are a few very common species in soil but lots of rare species. “There is a very large number of low abundance species,” says Gans. So many rare species, in fact, that the estimate of bacterial biodiversity rises to one million species per gram of soil.

Sewage sludge

These rare species appear to be absent in soil contaminated with heavy metals, however. The team also reanalysed the DNA reassociation pattern of soil experimentally polluted with metal-rich sewage sludge. Gans suggests that the contamination may have killed 99% of the bacterial species.

But the consequences of losing so much bacterial biodiversity in polluted plots of land are unknown. “Now that we have a way to measure it, the next thing is to correlate species diversity with how well plants grow,” he says.

As the new calculations reveal far more bacterial species in soil than anyone realised, the next challenge is to identify those species and the roles that they play in ecosystems.

“They might have some key functions that are known, or even unknown,” says Ruth-Anne Sandaa of the University of Bergen in Norway, who measured the original DNA reassociation patterns used in Gans’ analysis.

Journal reference: Science (vol 309 p 1387)