Unusual symbiosis discovered among marine microorganisms

Tiny single-celled algae and nitrogen-fixing bacteria exchange carbon and nitrogen in a mutually beneficial relationship that helps fertilise the oceans

September 20, 2012

An international team of scientists from France, Germany and the USA have discovered an unusual symbiosis between tiny single-celled algae and highly specialised bacteria, the first symbiotic relationship known between these types of organisms. Their partnership plays an important role in marine ecosystems, fertilizing the oceans by taking nitrogen from the atmosphere and "fixing" it into a form that other organisms can use.

Photograph of seawater sampling during the BIOSOPE (Biogeochemistry and Optics South Pacific Experiment) cruise to the Southeast Pacific.

Details of the symbiosis emerged from the 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 the University of California, 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," says Anne Thompson, co-first author of the paper and a postdoctoral researcher in Zehr's lab at UC Santa Cruz. Rachel Foster of the Max Planck Institute for Marine Microbiology is the other lead author and contributed equally to this work.

In order to unravel the mystery, the researchers used techniques such as cell sorting and gene sequencing. 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 are sorted into the "photosynthetic picoeukaryote" population, meaning tiny single-celled algae in the 1 to 3 micron size range. The cyanobacteria are mostly seen in an indentation at one end of the host cell. 

"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," explains Zehr.

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