Nitric
oxide (NO) is a central molecule of the global nitrogen cycle. A study reveals
that microorganisms can grow on NO. Their results change our view of the
earth's nitrogen cycle and how microorganisms regulate the release of
greenhouse gases from natural and human-made environments.
Intriguingly,
long before there was oxygen on Earth, nitric oxide was available as a
high-energy oxidant, and might have played a fundamental role in the emergence
and evolution of life on Earth. A study by Max-Planck-scientist Boran Kartal
and colleagues now published in Nature Communications sheds a new light on
microbial transformations of this molecule. Yes they can -- with implications
for our climate.
One major question
about nitric oxide
remained unanswered up to now: Can organisms use it to grow? "One would
think so," Kartal explains, "as nitric oxide has been around since
the emergence of life on earth." However, no microbe growing on NO has
been found -- until now. Kartal and his colleagues from Radboud University in
the Netherlands have now discovered that the anaerobic ammonium-oxidizing
(anammox*) bacteria directly use NO to grow. In detail, these microorganisms
couple ammonium oxidation to NO reduction, producing nothing but dinitrogen gas
(N2) in the process.
Nitric
oxide is a central molecule in the global cycling of nitrogen. "These
findings change our understanding of the earth's nitrogen cycle. Nitric oxide
has been primarily thought of as a toxin, but now we show that anammox bacteria
can make a living from converting NO to N2," says Kartal. The present study
raises new questions. "Anammox, a globally important microbial process of
the nitrogen cycle relevant for the earth's climate, does not work the way we
assumed it did." Moreover, other microbes than the ones investigated here
could be using NO directly as well. Anammox bacteria are found all over the
planet. "In this sense, the anammox microbes growing on nitric oxide could
also be basically everywhere," Kartal continues.
Now,
Kartal and his group at Max Planck Institute in Bremen are exploring different
ecosystems from all around the world, hunting for specialized nitric oxide
converting microorganisms. They want to understand better how microbes use NO
in environments both with and without oxygen. This will probably pave the way
to the discovery of new enzymes involved in nitric oxide transformation.
"Basically, we want to understand how organisms can make a living on
NO."
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