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Tuesday, 2 July, 2002, 09:27 GMT 10:27 UK
Measuring the unmeasurable
Soil, Eyewire
Thousands of species in a gram of soil
Scientists think they may have hit upon a good method for estimating the number of species of bacteria that live around us.

Finding out the number of different types of microbe, let alone the number of actual cells, on Earth is impossible - there are just too many to count. Science has only managed to study and classify a tiny, tiny fraction.

But researchers think the latest statistical techniques can at least give them a handle on the diversity that exists in, for example, a tonne of soil and make sensible guesses on even larger scales.

Such information could prove invaluable when scientists try to understand the role bacteria play in different environments, such as in helping to break down nutrients to make some areas of agricultural land more productive than others.

Water treatment

Dr Tom Curtis, from the University of Newcastle upon Tyne, UK, and colleagues tackled the problem of estimating bacterial populations because, as environmental engineers, they want to design better sewage treatment works.

"These systems need to hold a certain amount of diversity in order to break down the amount of pollutants that come in," he told BBC News Online.

"So, knowing the number of bacterial species has, on that level, profound practical importance. If we can make these systems more diverse, they may be able to clean up more pollutants."

But how can you have any idea of the number of different species of bacteria when that figure is, essentially, incalculable?

The Newcastle team approached the problem by counting individual cells in small samples and then measuring the abundance of the predominant species in those samples to estimate diversity on the larger scale.

"The smaller the proportion of the individuals taken up by the most abundant thing, the more uncounted things there are," Dr Curtis said. "It's quite straightforward really."

'Make a start'

Using this method, the Newcastle team estimates that typical samples of ocean contain 160 species per millilitre, soil contains 6,400-38,000 per gram, and sewage systems contain surprisingly fewer, only 70 species per millilitre.

According to the team's calculations, the entire ocean may contain about two million different bacterial species, while the average backyard (just one tonne of soil) could contain double that number.

By comparison, insects, the most diverse group of animals on the planet, may number no more than four to six million species in total.

The Newcastle team hopes the new estimation technique will prove useful in studying the effect of bacterial diversity on all sorts of environments.

"At the moment, for example, we don't know the relationship between the productivity of agricultural soils and the diversity in those soils - because we haven't been able to measure the diversity. It's been regarded as so difficult that no-one has really bothered. Now, we can at least make a start," Dr Curtis said.

"To use an analogy, you can't measure the distance to the Sun with a very long piece of string, but you can work it out by knowing how the Solar System works. And that's essentially what we're doing."

See also:

05 Jun 02 | Science/Nature
28 Sep 01 | Science/Nature
25 Apr 02 | Science/Nature
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