It’s time to welcome a new type of cell to the club of living things that can harvest nitrogen from the atmosphere.
Until now, the only lifeforms thought to pull nitrogen from the air and turn it into a biologically useful form were bacteria and archaea. But the discovery of a special ammonia factory inside a single-celled ocean alga adds eukaryotes — organisms with membrane-bound structures called organelles — to the list, researchers report in the April 12 Science.
That factory, the scientists say, was once a bacterium that started living inside the alga about 100 million years ago and has since evolved into a nitrogen-harvesting machine for its host. Once a symbiont, it’s now one of the cell’s many organelles.
Nitrogen fixation, where atmospheric nitrogen gas is converted into ammonia, is an important process for life (SN: 4/28/17). Organisms require access to nitrogen-containing compounds to synthesize essential biochemicals. The bacteria and archaea that have this ability often work their gas-refining in soil or in aquatic environments like the ocean.
One such bacteria, dubbed UCYN-A, is widely distributed in the world’s oceans and is important to oceanic nitrogen fixation, says marine ecologist Jonathan Zehr of the University of California, Santa Cruz. These bacteria are also known to be symbionts living within the unicellular algae Braarudosphaera bigelowii and its relatives.
However, the line between symbiont and organelle can be fuzzy. Zehr and colleagues set out to better understand where UCYN-A falls on that spectrum.
Using X-ray imaging, the team first learned that when the algal cells divide, all its organelles arrange themselves in a line and take turns dividing in a well-defined sequence. “This symbiont participates in that sequence,” says marine biologist Tyler Coale, also of UC Santa Cruz. “It’s somehow getting the cue to divide right on time with the other organelles.”
Next, the researchers analyzed the full sets of genetic instructions and proteins — the genomes and proteomes — made by the algae and the UCYN-A symbionts. “About half of the proteins that are physically present inside this symbiont are derived from the host genome,” Coale says. These supplemental proteins appear to fill gaps in the bacterium’s crucial metabolic pathways, suggesting it relies on the algal cell’s proteins to function.
In line with this, many of the bacterial proteins harbor special amino acid chains. Molecular biologist John Archibald, who did not participate in the work, describes them as “postage stamps” for trafficking proteins within the cell. A similar system exists for routing proteins that are encoded by the host cell’s genome into mitochondria and chloroplasts — organelles thought to have evolved from symbiotic microbes (SN: 11/5/18).
“The data clearly show that the two cells have been coevolving for some time,” says Archibald, of Dalhousie University in Halifax, Nova Scotia.
The researchers argue that all these features show UCYN-A isn’t merely a symbiont but has evolved into an organelle: the nitroplast.
“Protein import is really the smoking gun,” says Oliver Caspari, a molecular biologist at the University of Bonn in Germany who was not involved with the study. That import implies a degree of interdependence that marks the bacterium as an organelle, he says.
The nitroplast is one of just four known instances of symbiotic microbes evolving into cogs in a host’s cellular machinery. In particular, chloroplasts and mitochondria evolved from microbial symbionts as much as 2 billion years ago. Previous research on the evolutionary history of UCYN-A showed its relationship with algae is far more recent — around 100 million years old.
That means nitroplasts may provide a snapshot of how mitochondria and chloroplasts evolved into organelles (SN: 11/5/18). Researchers have long thought that this process involved symbiont genomes migrating into the nuclear genome of the host, but there doesn’t seem to be evidence of this in the nitroplast, Coale says. Instead, the host’s genome may support the symbiont to the point that the symbiont’s own genome withers away.
“If genes are targeted and their proteins are imported into these organelles, then their genomes are free to lose those genes,” Coale says. “Maybe this is the mechanism by which symbionts are domesticated.”
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