Some like it hotter

microbe of the month; Many bacteria thrive in extreme conditions, writes Bernard Dixon

Like some humans, some microbes function best in a middling environment and do not react well when confronted by change. Others, however, are highly adaptable. Indeed, certain microbes not only tolerate conditions that inhibit others but also positively thrive in extreme circumstances.

These exotic organisms are said to be much rarer than those that prefer moderation. Textbooks state, for example, that very few species of bacteria grow in boiling springs or grossly polluted rivers, as compared with the rich diversity of microscopic life in places such as garden soil or the human mouth. But the textbooks are wrong. The microbial populations of the world's most extreme environments are proving to be far more extensive than was ever imagined. That is clear from papers presented at a recent meeting held in The Hague to mark the work of the pioneer Dutch microbiologist, Martinus Beijerinck.

Beijerinck was the first to demonstrate our dependence on the beneficial activities of micro-organisms. It is apposite, then, that a conference 100 years after he began his work in Delft should mark a big advance in understanding the extent of microbial activity in the biosphere.

But why have decades of researchers drawn the wrong conclusions about the apparent paucity of microbes in extreme environments? The principal reason is simply that they have failed to grow many organisms in samples of water taken back to the laboratory and tested with techniques similar to those used to cultivate microbes from more moderate locations. It was faulty logic to assume that the non-appearance of living cells on nutrient media incubated in laboratory glassware demonstrated the lack of living cells in the original environment.

What has opened the floodgates to the discovery of hitherto unknown "extremophiles" is the advent of techniques by which nucleic acids (DNA and RNA) in living cells can be amplified and the constituent genes studied. One of the speakers at the Hague meeting, Norman Pace of Indiana University in the US, is using these methods to reveal the existence not only of particular types of microbe, but also entire families of related organisms, in hot springs in Yellowstone National Park. Although these cannot (yet) be grown in the laboratory, nucleic acid analysis demonstrates their presence and their relationship to relatives that are already cultivable in nutrient media.

Pace's finds include four different, hitherto unknown, bacteria in the outflow (at 84C) from the Octopus Spring at Yellowstone. One is related to Aquifex pyrophilus, previously described in a marine hot spring off Iceland. In sediment from Yellowstone's Obsidian Pool (at 74C), Pace made an even more amazing discovery - that of many new members of the Archaea. This is one of the three major groups, along with bacteria and Eucarya, into which all living things are divided. Pace's Archaea discovery includes an entire kingdom of previously undetected organisms.

Aside from the scientific interest in these finds, one motive for scouring extreme environments for micro-organisms is the possibility of harnessing them for industrial purposes. Bacteria that proliferate in near-boiling conditions may be exploited to effect chemical changes far more rapidly than those living at much lower temperatures. A similar argument applies to microbes that thrive in the presence of noxious chemicals, and which might therefore be used to break down those substances in a controlled way. Again, the conventional wisdom is that few microbes occur in environments that are, for example, very acidic or contain high levels of heavy metals. But again, findings presented at the Hague meeting show this is untrue. The new evidence comes from researchers at the Autonomous University of Madrid, who have been examining the Tinto River, one of the most polluted watercourses in the world. It arises in the midst of the Rio Tinto mining region of south-west Spain, and reaches the Atlantic Ocean 80km away at Huelva. The river is extremely acidic, contains high levels of various metals, and has other attributes that should make it inimical to life.

Wrong again. The Madrid team has found that the Tinto River contains some 349 different strains of filamentous fungi and at least 90 different yeasts, as well as various bacteria and other types of microbe.

Clearly, the world contains many more microbes than would have made sense to Beijerinck, or even to professional microbiologists as little as 10 years ago. One estimate given in The Hague was that only 0.1 per cent of micro-organisms in nature can be cultivated by conventional techniques. We are only just learning about the silent majority.