Rare Evolutionary Event: Two Lifeforms Merge

Scientists have witnessed a once-in-a-billion-year evolutionary event where two lifeforms merge, creating an organism with enhanced abilities. The last occurrence led to the emergence of plants on Earth.

Known as primary endosymbiosis, this phenomenon occurs when one microbial organism engulfs another, utilizing it as an internal organ. In return, the host cell provides nutrients, energy, protection, and other benefits to the symbiotic organism. Over time, the symbiote becomes indispensable to the host, essentially transforming into an organelle within microbial cells.

From Kidney Creatures to Self-Filtering

Imagine kidneys as tiny creatures scurrying about, and humans manually filtering their blood through dialysis machines. Then, one day, someone somehow traps one of these kidney critters internally (let’s not dwell on the how) and discovers they no longer require dialysis.

This change passes on to subsequent generations, and eventually, we’re all born with these beneficial little beings inside us. That’s somewhat akin to what’s happening in this scenario.

In Earth’s 4-billion-year history, primary endosymbiosis is thought to have happened only twice, marking major evolutionary milestones. The first, around 2.2 billion years ago, saw an archaea engulf a bacterium, resulting in the formation of mitochondria.

Evolutionary Impact of Specialized Organelles

Paving the way for the evolution of complex life forms, this specialized organelle actively produces energy and remains revered as the “powerhouse of the cell.”

The second occurrence occurred approximately 1.6 billion years ago when advanced cells incorporated cyanobacteria capable of harnessing solar energy. These transformed into chloroplasts, providing the ability to capture sunlight and imparting a vibrant green hue to a group of organisms we know as plants.

Scientists identified primary endosymbiosis in the algae Braarudosphaera bigelowii, which incorporated a cyanobacterium, enabling air-based nitrogen fixation, a rare function in algae and plants.

Initially believed to have a symbiotic relationship with UCYN-A, B. bigelowii’s association with the bacterium was further confirmed upon closer examination.

Size Ratios and Nutrient Exchange in Algal Symbiosis

A recent study revealed consistent size ratios between the algae and UCYN-A across related species, indicating growth regulation via nutrient exchange and interconnected metabolisms.

Jonathan Zehr noted that this relationship parallels that of organelles like mitochondria and chloroplasts, which scale with the cell size.

Using advanced X-ray imaging, researchers observed synchronized replication and cell division between the host and symbiotic organisms, confirming primary endosymbiosis.

UCYN-A’s Reliance on Algal Host for Essential Proteins

The team conducted a comparison of proteins from isolated UCYN-A with those found inside the algal cells. They discovered that the isolated bacterium can only produce approximately half of the required proteins, relying on the algal host for the remainder.

In fact, Jonathan Zehr noted, “This is characteristic of a transition from an endosymbiont to an organelle. As they reduce their DNA content and genome size, they increasingly depend on the host cell to transport gene products or proteins into the cell.”

The findings suggest that UCYN-A functions as a complete organelle, termed nitroplast. This evolution likely began around 100 million years ago, relatively recent compared to mitochondria and chloroplasts.

The team plans to further investigate nitroplasts to determine their presence in other cells and potential impacts. However, one potential benefit could be leveraging nitroplasts to introduce nitrogen-fixing capabilities into plants, enhancing crop growth.

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