Thousands of samples of algae that could hold the key to cleaning up sewage pollution will have to be re-examined because they have been wrongly classified by scientists.

A so-called 'algae bank' at the Scottish Association for Marine Science in Oban is embarking on the mammoth task of checking all 3,000 of its colourful, slimy samples to make sure that they have been correctly identified.

This is crucial, researchers say, so that the "amazing capacity" of algae to combat water contaminated by human or animal wastes can be realised. Algae also has "huge potential" to produce energy, food and medicines, they claim.

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A new study in the science journal, Nature, has revealed that 32 strains of algae that were catalogued under the same heading were actually four distinct sub-groups. They also included a completely new species.

Now scientists at the Culture Collection of Algae and Protozoa at Oban are preparing to reanalyse and re-label all their samples. "We're only just beginning to understand the vast unlocked potential of algae as an energy source, a clean-up tool and as a food," said Dr Gary Caldwell, an expert from Newcastle University involved in the project.

"And part of the reason for this is that we still know relatively little about them. Properly identifying and categorising the different strains is a key step towards unlocking that potential and that is why research like this is so important."

According to Caldwell, algae act as a "biological filter" to help clean up contamination. "A lot of the pollutants in sewage discharge are nutrients such as nitrates and phosphates," he explained.

"Algae are hungry for nutrients and they will try to take as many from their environment as they can," he continued. "They are also extremely effective at removing metals from water and could therefore be used to strip out metal contaminants from industrial discharges."

He cautioned, though, that care was needed on what to do with algae that have absorbed pollutants. "Entry into the human food chain is problematic but the algae could, for instance, be turned into a fuel or a source of chemicals for manufacturing," he said.

Algae can grow very fast, look unsightly and produce lethal toxins, he pointed out. "So there is a delicate road to be travelled between having algae as a valued pollution clean-up technology and forcing algae to become a problem in their own right."

As well as metabolising domestic and industrial waste, algae can convert sunlight into biomass. According to Professor John Day from the Scottish Association of Marine Science, they could also provide new drugs to overcome the growing problem of superbug resistance to antibiotics.

There was a greater diversity of life in the oceans than on land, he pointed out, and scientists were now starting to look for pharmaceuticals in algae. "It's early days but the indications are positive," he told the Sunday Herald.

"Algae have real potential to do the final stage of cleaning waste water before it is discharged back into the environment. Depending on the composition of the waste water, the algae then have the potential to be harvested and re-used, for example as a component of animal feed."

Understanding of the diversity and relationships of algae was "very fluid", Day said. Properly identifying the 3,000 samples at Oban was vital to working out how they could be useful.

He added: "This fingerprinting will give us much more precise identification and even tell us where these strains have come from, in terms of family links. It can tell us what a cell is doing and what it's made of."