Fourth time in nature organelle evolution has been identified

Scientists have identified the fourth time that a rare, but crucial, biological process has occurred in nature – the formation of a tiny organelle in a eukaryotic cell.

Known as ‘primary endosymbiosis,’ this process has been foundational for complex organisms (like humans). It describes how a single-celled organism is engulfed by a larger, more complex eukaryotic cell and evolves into a functional organelle.

It’s the same process by which mitochondria and chloroplasts – the organelles responsible for converting energy in the cells of plants, animals and fungi – and another chloroplast-like structure called a chromatophore found in the amoeba species Paulinella chromatophora evolved billions of years ago.

This new, fourth example of primary endosymbiosis is being called a ‘nitroplast’ and has been found within an algal species called Braarudosphaera bigelowii. It appears to have evolved just 100 million years ago. The findings lead the current issue of the journal, Science.

The unicellular but eukaryotic B. bigelowii has been known to effectively ‘fix’ or convert nitrogen into ammonia and similar compounds for some time, likely due to a symbiotic relationship with a bacterium. Prokaryotes like bacteria were previously thought of as the only organisms capable of performing such tasks.

And there are plenty of examples of this in nature. Plants, which require nitrogen to grow, borrow the fixing capability of soil bacteria that dwell inside their roots. As with other symbiotic relationships, plant and bacterium remain separate organisms.

But not in B. bigelowii, according to researchers from the University of California Santa Cruz, who working with colleagues across the Pacific have observed the role a tiny nitrogen-fixing cyanobacterium called Atelocyanobacterium Thalassa (UCYN-A) plays in its algal host.

Last month, Jonathan Zehr, a professor of marine sciences from UC Santa Cruz, who first identified genetic material from then unknown UCYN-A in Pacific seawater in 1998, identified that the organelle candidate and its algal host remained proportional across different examples of B. bigelowii. This was a strong sign that UCYN-A might act like organelle within these algal cells.

Organelle 2
The new organelle, indicated by an arrow. Pic: UCSC

Now using soft X-ray tomography to observe the morphology and process of cell division within the cell, Zehr and his collaborators have seen the UCYN-A evenly split during the division of these algal cells.
At the same time, UCYN-A appears to have discarded parts of its genome in order to take on proteins supplied by the algal host.

“That’s one of the hallmarks of something moving from an endosymbiont to an organelle,” says 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.”

Zehr’s colleague Tyler Coale, a postdoctoral scholar specialising in the physiology of marine eukaryotes, identified cellular processes that manufacture and ship specific proteins into the nitroplast.

While research is ongoing into the precise mechanics of UCYN-A within marine algae, Coale believes there may be an opportunity to consider new methods of modifying crops to harness the power of self-provided nitrogen fixation.

“This system is a new perspective on nitrogen fixation, and it might provide clues into how such an organelle could be engineered into crop plants,” Coale says.

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