Every so often biology unveils an organism that demonstrates evolution in action. More rarely, it offers up a twofer. Deep in the world’s oceans, biologists have found a species of cyanobacterium that may be midway the process of becoming an organelle within a species of algae.
Candidatus Atelocyanobacterium thalassa is a cyanobacterium like no other. Its streamlined genome shows that it excels at grabbing nitrogen from the water and turning it into a form (ammonium) that organisms can handle. But it lacks the genes for photosynthesis that other marine cyanobacteria use to generate their food. Unusually for a cyanobacterium, it must rely on another organism for its food.
“It was hard to see how it could ever be a free-living organism,” says Jonathan Zehr at the University of California, Santa Cruz. With his colleagues, Zehr has now identified that mystery second organism. It’s a tiny type of algae called a prymnesiophyte.
What makes the marriage exciting from an evolutionary viewpoint is that it could turn out to be an example of a halfway stage towards the cyanobacterium eventually becoming an organelle within the alga – a huge evolutionary step for both organisms. Zehr’s team demonstrated through delicate experiments that the cyanobacterium is closely enough associated with the alga that the two remain together during cell sorting – but filtering can separate the pair.
That suggests the cyanobacterium lives on the surface of the alga, perhaps in little depressions. This distinguishes it from other cyanobacteria that also have formed symbiotic relationships with algae. Most famously, one photosynthetic cyanobacterium found its way inside an alga billions of years ago, ultimately becoming the first chloroplast – the organelles where all plants fix carbon through photosynthesis.
Zehr speculates that some undiscovered algae may already have fully assimilated nitrogen-fixing bacteria like Candidatus Atelocyanobacterium thalassa. If such an alga exists, it would be the first member of the plant kingdom discovered that could fix its own nitrogen without relying on external organisms. “What we’ve found is a model for the beginning stages of how organelles may have evolved on Earth,” he says.
And that could have implications for agriculture. It would make sense to genetically engineer staple crops to fix their own nitrogen and reduce the need for expensive and environmentally damaging fertilisers. Yet while some plants, including legumes, can rely on terrestrial root bacteria to provide them with ammonium, researchers have struggled to generate crops that can do the same. The signals that pass between the root bacteria and legumes are just too complex to interpret and copy.