Nitrogen was plentiful for early life on Earth

Nitrogen was important in the evolution of life on early Earth, but its availability has been heavily debated. Although earlier hypotheses proposed that biologically available nitrogen was limited, recent studies contradict this notion.

Researchers, including scientists at RPTU University Kaiserslautern-Landau in southwestern Germany, have been studying how nitrogen affected the development and dissemination of life billions of years ago.

Nitrogen and the origins of life

Nitrogen is an essential element in all life. It is required for the formation of amino acids, the building blocks of proteins, and for other biological compounds.

Although the Earth’s atmosphere is full of nitrogen gas, it is not readily available to most living organisms.

Some types of microbes can convert atmospheric nitrogen into forms that are usable by other organisms – a process referred to as nitrogen fixation. But how did ancient microbes obtain nitrogen, and was it present in adequate amounts to facilitate the development of life?

This is where the research, led by RPTU researcher Michelle Gehringer, gives us some clues.

Nitrogen fixation remained stable over time

The scientists tracked nitrogen fixation stability under different environmental conditions. They used a method that detects nitrogen isotopes in two stable forms (¹⁵N and ¹⁴N) to track the process of nitrogen fixation by bacteria.

“Nitrogen gas is a mixture of the light atom ¹⁴N and the heavier atom, ¹⁵N. When modern microbes use nitrogen in their metabolism, they use these two isotopes in a certain ratio to each other. We measure this by burning nitrogen-containing biomass and collecting the nitrogen gas produced during combustion,” explained Gehringer.

So far, everyone assumed that microbes maintained the same ¹⁵N:¹⁴N ratio in any environmental conditions, even in a carbon dioxide-dense, oxygen-free ancient atmosphere. However, no one had ever tested this assumption.

Because metabolic rates depend on environmental conditions, the researchers theorized that nitrogen isotope ratios might be variable as well.

To test this, Gehringer and coworkers cultured cyanobacteria under conditions closely approximating those of the early Earth, with no oxygen present and lots of carbon dioxide.

“We found that the ¹⁵N:¹⁴N ratios of the cyanobacteria remain stable. Our results therefore support the assumption that this ratio was the same throughout the Earth’s history,” Gehringer explained.

Ancient rocks hold more clues

Following this research, Gehringer worked with Dr. Ashley Martin from Northumbria University and Dr. Eva Stüeken from the University of St Andrews. The experts set out to study nitrogen cycles in ancient stromatolites – microbial-generated sedimentary rock formations.

These rocks, dated to be approximately 2.7 billion years old, contain records of early life and offer a glimpse into Earth’s distant past.

“We gained access to pristine, unweathered rock, which we ground into a fine powder and analyzed for nitrogen isotopes,” said Gehringer.

The results were surprising. Unlike living stromatolites, which depend completely on cyanobacterial nitrogen fixation, the ancient samples indicated a secondary source of nitrogen – dissolved ammonium.

“And the most plausible source for this is hydrothermal activity on the sea floor,” stated Gehringer.

To test this hypothesis further, the researchers also analyzed sedimentary rocks in a volcanic basin that is also around 2.7 billion years old.

These findings supported the fact that hydrothermal vent-borne ammonium was a large contributor to the nitrogen cycle at the time.

What this means for life beyond Earth

Scientists have long explored the factors that shaped the spread of early life on Earth. However, this research challenges previous assumptions.

“Until now, it was assumed that life on the early Earth, before the atmosphere was enriched with oxygen, was limited by a lack of biologically available nitrogen,” said Gehringer.

“With the help of hydrothermal vents, nitrogen did not limit the spread of life on early Earth. Rather, life was able to flourish in both deep and shallow-water marine environments.”

This finding has broad implications – both for what it reveals about the history of our own planet and for whether life can or might exist elsewhere.

“Hydrothermal activity has been documented on Mars and probably also takes place on the icy moons in the outer solar system.”

If the same sources of nitrogen are present outside Earth, the conditions that nourished microbial life here may also exist elsewhere in the universe.

Ongoing research not only fills gaps in Earth’s history but also expands our understanding of where life might exist beyond our planet.

The full study was published in the journal Nature Communications.

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