A Rare Evolutionary Event That Enabled Plant Life on Earth Has Occurred Again

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An extraordinary event, only known to have happened three times in Earth’s history, has been documented once again.

A marine bacterium has integrated into its algal host, evolving over time into an organelle within the algal cell.

This new discovery marks the first time a eukaryote (an organism with DNA in a membrane-bound nucleus) contains an organelle capable of fixing nitrogen.

A Rare Evolutionary Milestone

“This is a very rare occurrence,” says Tyler Coale, the lead author of the study. Historically, similar events have been pivotal in the evolution of life.

The first such event led to the creation of mitochondria, the powerhouse of the cell.

Another, over a billion years ago, gave rise to chloroplasts, enabling the dawn of plant life on Earth.

The Discovery Journey

The foundation for this discovery was laid nearly 30 years ago.

UC Santa Cruz Professor Jonathan Zehr and his team found a new cyanobacterium in the Pacific Ocean, capable of nitrogen fixation.

This bacterium, named UCYN-A, pulls nitrogen from the environment to create essential compounds for life.

Meanwhile, in Japan, paleontologist Kyoko Hagino was cultivating a marine alga, which turned out to be the host for UCYN-A.

Over the years, scientists discovered that UCYN-A and the alga, Braarudosphaera bigelowii, were not just coexisting but had co-evolved.

UCYN-A had become an integral part of the algal cell, now considered an organelle.

New Evidence and Findings

In March 2024, researchers published a paper showing that UCYN-A and its algal host have intertwined metabolisms.

The size ratios of the two organisms indicate their close metabolic relationship, similar to that of other organelles like mitochondria and chloroplasts.

A second paper provided more conclusive evidence.

It showed that UCYN-A imports proteins from its host cell, a key sign of organelle development.

Through proteomics analysis, Coale confirmed that many proteins essential for UCYN-A’s function are produced within the algal host.

Introducing the Nitroplast

This newly discovered organelle, named the “nitroplast,” evolved around 100 million years ago.

Its discovery offers new insights into how nitrogen fixation affects ocean ecosystems and could have significant implications for agriculture.

“This new system provides a fresh perspective on nitrogen fixation,” Coale explained.

It may also offer clues on engineering similar organelles into crop plants, potentially revolutionizing farming.

Zehr believes that UCYN-A is not unique and that other similar organisms are yet to be discovered. This finding will likely be a focal point of research for decades to come.


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