Early Signs of Nitrogen Fixation

Nitrogen is an essential element for cells

Life on Earth is described as being carbon based. Carbon's chemical versatility allows it to form up to four bonds with other atoms and so act as the skeleton for complex organic molecules. Carbon often goes hand in hand with hydrogen and oxygen in organic molecules. Sulphur and phosphorus make smaller appearances here and there, but one element which is as essential is nitrogen.

It is more accurate to say that life is protein- based, relying on bonds between carbon and nitrogen atoms. In ecosystems there exists what is termed the nitrogen cycle. This process governs how different organisms obtain their nitrogen.

Animals eat plants and animals eat other animals. This is how they obtain their nitrogen. How nitrogen enters an ecosystem in the first place is another matter. Nitrogen molecules contain a strong triple bond which is very difficult to break. However, some bacteria, nitrogen fixers, contain enzymes which allow them to strip nitrogen directly from the atmosphere. All ecosystems are ultimately reliant on nitrogen-fixing bacteria and now geological evidence has shown that nitrogen fixation is an ancient process, dating back at least 3.2 billion years.

'People always had the idea that the really ancient biosphere was just tenuously clinging on to this inhospitable planet, and it wasn't until the emergence of nitrogen fixation that suddenly the biosphere become large and robust and diverse,' said Roger Buick, professor of Earth and Space Sciences at the University of Washington. Our work shows that there was no nitrogen crisis on the early Earth, and therefore it could have supported a fairly large and diverse biosphere.' Genetic analysis has shown that nitrogen fixing enzymes existed at least 2.2 billion years ago, but now geochemical evidence suggests that it may be a billion years older still.

3.2 billion year old sedimentary rocks in Australia

Nitrogen exists in two forms known as isotopes. N-14 is the more common and the lighter variety. N-15 is heavier and less common. Nitrogen-fixing bacteria will favour N-14 because it is lighter and so thermodynamically requires less energy to capture. This leads to a fractionation, a process where there is a higher concentration of N-15 relative to N-14 in soils and sediments.

By analysing 52 samples of sedimentary rocks dating from 2.75 to 3.2 billion years old, collected in South Africa and Australia, a team of researchers led by Buick found that the same ratio of nitrogen isotopes observed today existed 3.2 billion years ago - a ratio expected if nitrogen fixation were taking place.

Evidence of nitrogen fixation so far back in evolutionary history gives us a snapshot of life on the early Earth. Nitrogen is an essential component of proteins and life has evolved a myriad of ways to extract it from various sources, but nitrogen fixation is the first step in the process. The enzymes involved use molybdenum. What is interesting is that a source of molybdenum on the early Earth has yet to be identified.

Today it reacts with oxygen to form insoluble oxides which are washed into the ocean by rivers, but 3.2 billion years ago the atmosphere contained no oxygen. Instead cyanobacteria on the land may have produced enough oxygen to generate small quantities of molybdenum oxides. 'We'll never find any direct evidence of land scum one cell thick, but this might be giving us indirect evidence that the land was inhabited,' said Buick. 'Microbes could have crawled out of the ocean and lived in a slime layer on the rocks on land, even before 3.2 billion years ago.'