An international team of scientists says it has found a marine alga with an "early evolutionary stage" cell part that can harvest nitrogen from the atmosphere.
The researchers reported the findings to the US magazine Science. They are from institutions including Japan's Kochi University and the University of California.
Nitrogen makes up roughly 80 percent of Earth's air. But only some bacteria and other microorganisms are known to have the ability to pull the gas from the atmosphere and convert it into a biologically useful form.
The ability, called nitrogen fixation, has so far been discovered in none of the eukaryotes -- any cell or organism that has a clearly defined nucleus. Eukaryotes include animals and plants.
The research team developed the first-ever method to cultivate a species of ocean alga, which is a eukaryote measuring about 20 micrometers long.
The team says a nitrogen-fixing bacterium was thought to live inside the alga in a relationship termed as endosymbiosis. The phenomenon involves two dissimilar organisms providing each other with necessary conditions for its continued existence.
But the team says the bacterium has evolved beyond endosymbiosis and now functions as an "early evolutionary stage" nitrogen-fixing cell organ.
Kyoto University Professor Yoshida Takashi, a marine microbiologist, says the findings are important in studying how life forms have evolved.
He adds that the nitrogen-fixing ability could be used to develop ways to grow agricultural crops without the application of fertilizers.