"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.
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. In the newly discovered nitrogen-fixing partnership, the cyanobacteria are mostly seen in an indentation at one end of the host cell.
"At this point, it's unclear exactly how the cyanobacteria are associated with the host cells. It looks like there may be a little groove in the host cell where the cyanobacteria fits," said Thompson. "The association is robust enough to go through the cell sorter and other preparations, but delicate enough that they separate if they're filtered or frozen and thawed."
In previous work, Zehr's team had studied the cyanobacteria, which they called UCYN-A, in samples processed at sea and brought back to the lab for cell sorting and genetic analysis. Despite being unable to grow it in the lab, they were able to sequence the microbe's complete genome and discover that it was missing the genes for several key metabolic pathways, suggesting that it might live in association with another organism. Thompson said researchers were only able to see the symbiotic partners together when they sorted freshly collected seawater samples on board the ship.
"Our collaborators at the University of Hawaii, Dave Karl and Ken Doggett, put a cell sorter into a portable laboratory, like a lab in a box, so now we can take the machine to sea and sort cells that minutes before were in their natural environment. That's how we found the association," Thompson said.
The exchange of carbon and nitrogen between the two partners was demonst
|Contact: Tim Stephens|
University of California - Santa Cruz