Researchers led by University of California, Santa Cruz found that tiny single-celled algae and highly specialised bacteria exchange carbon and nitrogen in a mutually beneficial relationship.
This partnership plays an important role in marine ecosystems, fertilising the oceans by taking nitrogen from the atmosphere and "fixing" it into a form that other organisms can use.
The discovery emerged after investigation of a mysterious nitrogen-fixing microbe with a drastically reduced genome.
First detected in 1998 by Jonathan Zehr, a professor of ocean sciences at UC, Santa Cruz, it now appears to be the most widespread nitrogen-fixing organism in the oceans.
The microbe belongs to a group of photosynthetic bacteria known as cyanobacteria, but it lacks the genes needed to carry out photosynthesis and other essential metabolic pathways.
Apparently, its association with a photosynthetic host cell makes those genes unnecessary.
"The cyanobacterium is a nitrogen-fixer, so it provides nitrogen to the host cell, and the host cell provides carbon to the cyanobacterium, which is lacking the metabolic machinery to get its own carbon," said lead author, Anne Thompson.
Although the partners in the symbiosis have not been grown in the laboratory, the researchers were able to characterise both partners using cell sorting, gene sequencing, and other techniques.
The host cell is a type of single-celled algae in a class known as "prymnesiophytes," which are found throughout the world's oceans.
In seawater samples sorted by flow cytometry, which separates cells by size and colour, the host cells were discovered among the "photosynthetic picoeukaryotes," a mixed population of tiny single-celled algae in the 1- to 3-micron size range.
"Aside from the importance of nitrogen fixation in marine ecosystems, this is such an interesting symbiosis from an evolutionary perspective, because it can be seen as analogous to an early stage in the endosymbiosis that led to chloroplasts," Zehr said in a statement.
Chloroplasts, which carry out photosynthesis in all plants, evolved from symbiotic cyanobacteria that eventually became incorporated into their host cells in a process known as endosymbiosis.
The study was published in the journal Science.
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