Archaea to re-write nitrogen cycle?
Researchers from Norway have made a startling discovery which could re-write one of the most fundamental biochemical cycles on Earth - the nitrogen cycle. Nitrogen makes up 78 per cent of the atmosphere, but it is far more useful when 'fixed' in the Earth. There it can nourish plants, which can in turn be eaten and ultimately end up inside us to make complex proteins and amino acids - including DNA and RNA - which are not so much essential for life, but are life itself. According to the received wisdom, nitrogen in the air is 'fixed' to the soil, making it available as a food for plants, thanks to various bacteria. This process is the most important step in the nitrogen cycle: ammonia is oxidised to nitrates and nitrites, which can be easily absorbed by plants. But it may in fact be of peripheral importance. Professor Christina Schleper from the University of Bergen in Norway believes that the key biological players are not bacteria at all but another group entirely, known as archaea. Her findings, co-authored with colleagues in the UK, US and Germany, are published in the journal Nature. Classified as distinct group of living creatures only in 1977 by researchers Carl Woese and George Fox, archaea have many similarities to bacteria, and 3.8 billion years ago, the Earth would have been populated exclusively by bacteria and archaea. Archaea differ from bacteria primarily in their molecular make-up and in their biochemistry. These differences give archaea the ability to live in extreme environments, which is where they were first discovered. Archaea can live in highly saline or high temperature environments, and can be found amongst geological fissures and in geysers. Although, archaea are being found in more and more environments, including the sea and soil, very little is known about them. Recent estimates calculate that archaea represent 40 per cent of the microbial biomass found in the sea. 'Many of them are difficult to cultivate in the laboratory, since the soil is a complicated system that is impossible to copy in the laboratory. Only archaea that live in hot springs or other extreme environments thrive under such artificial conditions,' explains Professor Schleper. 'In one gram of soil there are an average of at least 10,000 species of micro-organisms,' she says. Looking at the soil, Professor Schleper used a technique for extracting DNA from the organisms in soil, and then using the sequences to identify what is there. 'That is basically how scientists discovered that archaea are also present in soil samples and not just in hot springs or in deep oceans,' she says. Part of this process of DNA sequencing gave Professor Schleper clues as to the nature of the archaea - they found genes that in bacteria are responsible for 'fixing' nitrogen. The team found the archaea everywhere, and at surprising depths underground, long after the bacteria had petered out, making the archaea some 3,000 times more abundant. The research is not yet complete. Professor Schleper has found the gene which oxidises ammonia - the key process - in large quantities in the archaea samples. What she does not yet know, and therefore the next stage for research, is the extent to which archaea or bacteria contribute to this oxidisation process. 'It is still too early, however, to talk about rewriting the textbooks,' she says. Next she has to test her theory, and compare the bacteria and archaea. 'Either by trying to halt the process in the one group and seeing what effect that has on overall oxidisation, or trying to characterise the biochemistry that underlies each of them.' Until now, the nitrogen cycle has only been considered in terms of bacteria, and bacteria have been the focus for areas where nitrogen is of key importance, such as in sewage treatment or agriculture - particularly in areas where there has been over-fertilisation. If archaea are indeed responsible for oxidising nitrogen compounds, and can live in extreme environments, then these ancient, mysterious and largely forgotten organisms may yet prove vital in cleaning up and managing our fragile planet.
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