Bacteria show new route to making oxygen

Discovery adds to the few known biological pathways

Microbiologists have discovered bacteria that can produce oxygen by breaking down nitrite compounds, a novel metabolic trick that allows the bacteria to consume methane found in oxygen-poor sediments.

Previously, researchers knew of three other biological pathways that could produce oxygen. In photosynthesis, microbes or plants containing chlorophyll grow by gleaning energy from the sun, releasing oxygen as a waste product. In the two other schemes, cells generate oxygen — typically for their own internal use — by using enzymes to break down oxygen-containing substances such as chlorates, says Katharina Ettwig, a microbiologist at Radboud University Nijmegen in the Netherlands.

The newly discovered pathway  opens up new possibilities for understanding how and where oxygen can be created, Ettwig and her colleagues report in the March 25 Nature.

“This is a seminal discovery,” says Ronald Oremland, a geomicrobiologist with the U.S. Geological Survey in Menlo Park, Calif., who was not involved with the work. The findings, he says, could even have implications for oxygen creation elsewhere in the solar system.

Ettwig’s team studied bacteria cultured from oxygen-poor sediment taken from canals and drainage ditches near agricultural areas in the Netherlands. The scientists found that in some cases the lab-grown organisms could consume methane — a process that requires oxygen or some other substance that can chemically accept electrons — despite the dearth of free oxygen in their environment. The team has dubbed the bacteria species Methylomirabilis oxyfera, which translates as “strange oxygen-producing methane consumer.”

When the team gave the bacteria nitrates, a common oxygen-bearing component of fertilizers, no methane was consumed. But when nitrites, close chemical relatives of nitrates, were added to the mix, the bacteria fed on the methane and released nitrogen gas. That combination suggests that the microbes were breaking down nitrites, using the oxygen to consume the methane and releasing nitrogen as waste.

Although the team has sequenced the full genome of the microbe, the researchers don’t know which of the enzymes it produces actually drives the oxygen-producing reaction. “It’s like looking for a needle in a haystack,” says Ettwig. “These cells make hundreds of unknown proteins, and all of them are candidates.”

Certain sequences of genetic material suggest that the bacteria share metabolic pathways seen in many other microbes, including those used for denitrification and for the consumption of methane in an oxygen-rich environment. The M. oxyfera bacteria, however, “are combining those pathways in a previously unexpected way,” says Ettwig.

The team’s findings “are exciting and new,” says Julia Vorholt, a microbiologist at the Swiss Federal Institute of Technology Zurich. The next big step, she notes, is to isolate the enzyme or enzymes that enable oxygen production and show how they work.

By producing their own oxygen, the bacteria can take advantage of a much more energy-efficient method of consuming methane and can therefore grow and proliferate more quickly, says Martin Klotz, an evolutionary microbiologist at the University of Louisville in Kentucky.

It isn’t clear, Oremland says, whether the newly identified oxygen-making technique is a recent biological adaptation that allows proliferation in fertilizer-polluted sediments or an ancient pathway that evolved when Earth was young and its atmosphere was methane-rich but oxygen-poor.

Many of the planets and moons in the outer solar system are lousy with methane, so the new oxygen-producing pathway would allow life there a way to feed in an environment that lacks free oxygen. When looking for life beyond Earth, he notes, “NASA’s always had this mantra: ‘Follow the water.’ Maybe we should think about following the methane.”

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