Archaea are among the most commonly found single-celled organisms in the ocean, and they play an important role in the global nitrogen cycle. They generate energy by oxidizing ammonia, which is produced when dead algae decompose. Up to now, it was a big question where the carbon came from that they use to build their cells; is it by fixation of CO2 (carbon dioxide) that is dissolved in the water, just as with algae that use sunlight as an energy source, or by incorporating organic compounds? The idea that ammonia-oxidizing archaea (AOA) use organic compounds came up after cultivation experiments showed that adding specific organic acids, such as tartaric acid or oxaloacetic acid, stimulated AOA growth. Another possibility was that they can choose between two metabolic routes, which is common among microorganisms.

Archaea ‘feeds on’ CO2, not acid ​
A team of Korean scientists from Chungbuk National University and scientists from the NIOZ Royal Netherlands Institute for Sea Research studied the role played by these organic acids by adding specific acid molecules labelled with the 13C isotope to a new AOA strain that had been isolated from seawater. The research team showed that this 13C isotope was not incorporated into the cell material of their experimental strain, but that all of its carbon came from (unlabelled) dissolved CO2. This proves unambiguously what the NIOZ scientists had been suspecting for a while: these AOA use CO2 as a source of carbon for building their cells in the deeper waters of the oceans.

Role Played by Organic Acids
But what makes these organic acids so effective in the laboratory? The oxidation process of ammonia also produces hydrogen peroxide. This unstable compound – mostly known as a bleaching agent and disinfectant – toxifies the cell if the concentration becomes too high. The organic acids, however, neutralize the hydrogen peroxide by turning it into water. Many bacteria have a built-in enzyme mechanism that neutralizes hydrogen peroxide, but genetic research into AOA shows that they cannot produce this enzyme (catalase) themselves. This means that in real life and in the absence of these acids, they depend on other bacteria that are in fact capable of neutralizing hydrogen peroxide.

Bibliography and Link to the Article
 Kim J.G, Park S.J., Sinninghe Damsté J.S., Schouten S., Rijpstra W.I.C., Jung M.Y., Kim S.J., Gwak J.H., Si O.J., Lee S.H., Madsen E.L., and Rhee S.R. (2016) ‘Hydrogen Peroxide Detoxification is a Key Mechanism for Growth of Marine Ammonia-Oxidizing Archaea’. Proc. Nat. Acad. Sci. USA, in press. http://www.pnas.org/content/early/2016/06/22/1605501113.full.pdf 

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