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Scientists recently identified a rare fourth instance of endosymbiosis, a phenomenon where one species lives inside another for mutual benefit, with profound implications for understanding evolution, New Scientist reported.

Endosymbiosis is not uncommon in nature. Notable examples include nitrogen-fixing bacteria found within the roots of legumes, which facilitate their growth, and specialized bacteria that produce essential nutrients residing within cockroach cells.

The organisms remain distinct in the majority cases, but previous studies have documented only three instances where endosymbionts merged with their hosts to become integral components of their cellular machinery.

These instances include the formation of mitochondria, responsible for energy production, and chloroplasts, pivotal in photosynthesis within plant cells.

For decades, researchers suspected that that the cyanobacterium UCYN-A living within the alga single-celled alga Braarudosphaera bigelowi had become the latter’s organelle.

In their new study, a research team confirmed those suspicions using soft X-ray tomography to watch how the algal cells divide.

They noticed that the bacterium divides in concert with its host’s cells, with each algal cell inheriting one UCYN-A. Moreover, about 50 percent of the 2,000 proteins within UCYN-A originated from the algal host, instead of the bacterium.

The team explained that the bacterium had become a nitroplast, an organelle that helps the B. bigelowi fix its nitrogen levels.

While the findings hint that organisms become organelles are not so uncommon, they also extend beyond our understanding of cellular evolution

Other researchers suggested that the study could transform agriculture by potentially enabling crop plants to fix their own nitrogen, reducing reliance on nitrogen fertilizers.

However, the dependency of UCYN-A on B. bigelowii complicates direct application in crop modification. Instead, researchers propose starting with cyanobacteria to develop nitroplasts that could be integrated into various crop plants more easily.

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