Nitrogen Fixation Via Primary Endosymbiosis Observed in Braarudosphaera bigelowii
JoeMerchant writes:
Braarudosphaera bigelowii is a species of algae, a coastal coccolithophore in the fossil record going back 100 million years. It has recently been found to have engulfed a cyanobacterium that lets them do something that algae, and plants in general, can't normally do: "fixing" nitrogen straight from the air, and combining it with other elements to create more useful compounds.
Nitrogen is a key nutrient, and normally plants and algae get theirs through symbiotic relationships with bacteria that remain separate. At first it was thought that B. bigelowii had hooked up this kind of situation with a bacterium called UCYN-A, but on closer inspection, scientists discovered that the two have gotten far more intimate.
In one recent study, a team found that the size ratio between the algae and UCYN-A stays similar across different related species of the algae. Their growth appears to be controlled by the exchange of nutrients, leading to linked metabolisms.
"That's exactly what happens with organelles," said Jonathan Zehr, an author of the studies. "If you look at the mitochondria and the chloroplast, it's the same thing: they scale with the cell."
In a follow-up study, the team and other collaborators used a powerful X-ray imaging technique to view the interior of the living algae cells. This revealed that the replication and cell division was synchronized between the host and symbiote - more evidence of primary endosymbiosis at work.
And finally, the team compared the proteins of isolated UCYN-A to those inside the algal cells. They found that the isolated bacterium can only produce about half of the proteins it needs, relying on the algal host to provide the rest.
"That's one of the hallmarks of something moving from an endosymbiont to an organelle," said Zehr. "They start throwing away pieces of DNA, and their genomes get smaller and smaller, and they start depending on the mother cell for those gene products - or the protein itself - to be transported into the cell."
Altogether, the team says this indicates UCYN-A is a full organelle, which is given the name of nitroplast. It appears that this began to evolve around 100 million years ago, which sounds like an incredibly long time but is a blink of an eye compared to mitochondria and chloroplasts.
The researchers plan to continue studying nitroplasts, to find out if they're present in other cells and what effects they may have. One possible benefit is that it could give scientists a new avenue to incorporate nitrogen-fixing into plants to grow better crops.
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Hopefully this behavior has been going on for tens of millions of years, if this is a newer development it could become a significant evolutionary advantage - radically changing the biome in a short time.
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