U. WASHINGTON (US)—It’s not every day you find clues to the planet’s inner workings in aquarium scum. But that’s what happened when researchers cultured a tiny organism from the bottom of a Seattle Aquarium tank and found it can digest ammonia.
Results, published in the journal Nature, show this minute organism and its relatives play a more central role in the planet’s ecology than previously suspected. These microorganisms, members of the ancient lineage called archaea, beat out all other marine life in the race for ammonia.
Ecologists have assumed that ammonia in the upper ocean is first gobbled up by phytoplankton to make new cells, leaving very little ammonia for microbes to turn into nitrate.
“Our data suggests that it’s the other way around,” says coauthor Willm Martens-Habbena, a University of Washington postdoctoral researcher. “Archaea are capable of stealing the ammonia from other organisms and turning it into nitrate. Then it’s the phytoplankton that take up that nitrate once again.”
Ammonia is a waste product that can be toxic to animals. But plants, including phytoplankton, prize ammonia as the most energy-efficient way to build new cells. The new paper also shows that archaea can scavenge nitrogen-containing ammonia in the most barren environments of the deep sea, solving a long-running mystery of how the microorganisms can survive in that environment.
Archaea therefore not only play a role, but are central to the planetary nitrogen cycles on which all life depends.
Archaea were discovered only about 30 years ago and were first thought to exist only in extreme environments, such as hot springs or hydrothermal vents. They are now known to be more widespread.
The new experiments show that the organism can survive on a mere whiff of ammonia—10 nanomolar concentration, equivalent to a teaspoon of ammonia salt in 10 million gallons of water. In the deep ocean there is no light and little carbon, so this trace amount of ammonia is the organism’s only source of energy.
That finding has two important implications for ocean ecosystems. Scientists knew that something was turning ammonia into nitrate in the deep ocean, but could not fathom what organism might be responsible. Now it appears archaea are those mysterious organisms.
And in the sun-dappled upper ocean waters, it appears that archaea can out-compete phytoplankton for ammonia. The same may be true in soil environments, the researchers say.
The archaea in question are small even by the standards of single-celled organisms. At 0.2 micrometers across, about 8 millionths of an inch, the only life forms smaller are viruses. Martens-Habbena speculates that archaea’s size could explain how they are able to survive on such a scant energy supply. The strain used in these experiments is named Nitrosopumilus maritimus, which means “tiny ammonia-oxidizer of the sea.”
A better understanding of archaea’s lifestyle and role in nitrogen cycles not only would rewrite ecology textbooks. It could also have practical applications, such as devising natural ways to boost a soil’s nitrogen content without needing to use chemical fertilizers, or designing sewage treatment plants that employ microbes to remove nitrogenous waste more efficiently, or understanding which microbes produce global-warming gases such as nitrous oxide.
The new findings will also affect the equations used in global climate models, researchers say. Computer models use global cycles of nitrogen and other chemicals to estimate how much carbon dioxide the oceans will absorb and ultimately sink to the bottom of the sea. The new findings suggest that most of the nitrate in the surface water comes from recycling of biomass, and not from the deep water as currently assumed.
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