A new study of methane-munching microbes adds weight to the idea that bacteria were producing oxygen on Earth before photosynthesis evolved.
Margaret Butler of the Australian Institute for Bioengineering and Nanotechnology at the University of Queensland, and colleagues, report their study of a new kind of bacteria this week in the journal Nature.
"We're proposing that it makes oxygen within itself," said Butler, who did the research while at the Radboud University Nijmegen in the Netherlands.
"It's quite unusual. This is only the fourth known mechanism of producing oxygen in the environment," she said.
Butler and colleagues studied samples of a type of anaerobic bacteria that was suspected of being able to convert methane to carbon dioxide, using nitrite.
The bacteria, collected from ditches in the Netherlands, belongs to a group called NC10 that was first discovered in the Nullarbor Caves, near the southern coast of Western Australia.
Butler and colleagues carried out experiments in which they were able to trace labelled oxygen and nitrogen entering and leaving a chamber containing the bacteria.
This told them that oxygen was being produced via a completely new biochemical process.
Butler and colleagues propose a previously unknown enzyme is able to produce oxygen and nitrogen gas from two molecules of nitric oxide, produced from nitrites in the environment.
Some of this oxygen is then used to burn methane for energy, said Butler.
Like many environmental microbes, scientists don't yet know how to make a pure culture of NC10 bacteria in the laboratory.
Instead, Butler and colleagues grew the bacteria with other organisms in a bioreactor and then reconstructed it from DNA extracted from the mix.
As well as sequencing a genome for the proposed bacteria, called Candidatus Methylomirabilis oxyfera, they also probed the link between this genome and proteins it was producing.
Based on the proteins that were being most highly expressed in the bacteria, Butler and colleagues then proposed there was an enzyme that was producing the oxygen.
Cycling of methane
"We don't have an enzyme but we have possible genes in the genome that might be that enzyme," she said.
Butler said the findings have implications for what happened on early Earth.
"The bacteria could have been producing little bits of oxygen before photosynthesis actually started," she said.
Butler said the findings also have relevance for scientists studying the cycling of methane on Earth.
"This pathway would have been overlooked in previous calculations," she said.
Apart from photosynthesis, bacteria are known to be able to produce oxygen via chlorate respiration and detoxification of reactive oxygen species by bacteria.
Microbial ecologist Tony O'Donnell of the University of Western Australia describes the research as a "nice piece of work."
Although he said the team doesn't have definitive evidence of the organism they've described, their evidence for a new pathway for producing oxygen is "pretty good."
"The experiments they did with the labelled nitrogen and labelled oxygen are very elegant," said O'Donnell.
O'Donnell said the idea that oxygen was available from microbes earlier than previously thought is not new.
But he said previous evidence for this was mainly geological, and the new paper provides a possible mechanism for production of oxygen at an earlier stage.
"I think the exciting thing about this is that it adds some biological evidence to the geological evidence," said O'Donnell.