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Urea keeps archaea alive in cold, dark polar seas

3 October 2012

The Canadian icebreaker CSGS Amundsen

The Canadian icebreaker CSGS Amundsen

Single-celled archaea, along with eukaryotes and bacteria, represent a third domain in the tree of life. In a new study directed by Uppsala University, researchers show that the growth and proliferation of archaea in polar seas are a result of their capacity to take up urea to satisfy their energy needs. This capacity enables them to survive and thrive in one of the most inhospitable natural environments on earth.

From having been regarded as odd and unusual organisms that exist primarily in extremely salty, warm or anaerobic environments, where they produce the greenhouse gas methane, among other things, they have more recently been mapped by researchers in previously unknown groups that exist in large numbers in most water and land environments. The metabolism and function of these archaea were unknown until eight years ago, when scientists managed to isolate and culture so-called Thaumarchaea in pure cultures. Researchers then found that the organisms could satisfy their energy needs by oxidizing ammonia to nitrite and create biomass from inorganic carbon: nitrification.

In a study being published today in the journal PNAS, the Uppsala scientists show that these Thamarchaea thrive in the northern and southern polar seas, above all during the coldest and darkest months of the year, and that their growth and success in this seemingly inhospitable environment can be explained by their uptake and exploitation of urea.

Urea, which is a central molecule in the nitrogen metabolism of various organisms (including the urine of mammals), normally occurs in higher concentrations than ammonia in these cold waters and is therefore a more reliable source of energy. By converting urea to carbon dioxide and ammonia, which are then used to extract energy, archaea manage to grow despite the lack of sunlight and organic compounds.

In this research study, directed by Dr Laura Alonso-Saez and Professor Stefan Bertilsson at the Department of Ecology and Genetics, scientists have managed to use new large-scale sequencing methods to reconstruct these cold-loving archaea’s genes and thereby described their function and metabolism. With various isotope-based methods, they have moreover demonstrated the Thaumarchaea’s uptake of urea and other compounds and charted the connection between their occurrence and the specific genes that enable the exploitation of urea and oxidation of ammonia in both the northern and southern polar seas.

‘This research into the genes of archaea helps us understand the biogeochemical metabolism of nitrogen, carbon, and other elements and is a striking example of how microorganisms use various adaptations to manage to grow successfully in the cold and dark polar seas, which represent some of the most inhospitable environments on earth’, says Stefan Bertilsson.