The original version of this story appeared in Quanta Magazine.
Far from being solo operators, most of them unicellular microbes are in complex relationships. In the ocean, the ground and your intestines, they could fight and eat each other, exchange DNAcompete for nutrients or feed on each other’s by-products. Sometimes they become even more intimate: a cell could slip into another and get comfortable. If conditions are right, he could stay and be welcomed, sparking a relationship that could last generations or even billions of years. This phenomenon of one cell living inside another, called endosymbiosis, has fueled the evolution of complex life.
Examples of endosymbiosis are everywhere. Mitochondria, the energy factories of your cells, were once free bacteria. Photosynthetic plants owe their sun-spun sugars to the chloroplast, which was also originally an independent organism. Many insects receive essential nutrients bacteria that live inside. And last year, researchers discovered the “nitroplast” an endosymbiont that helps certain algae process nitrogen.
Much of life relies on endosymbiotic relationships, but scientists have struggled to understand how they occur. How does an internalized cell escape digestion? How does it learn to reproduce inside its host? What makes the random merger of two independent organizations a stable and lasting partnership?
Now, for the first time, researchers have observed the opening choreography of this microscopic dance of induce endosymbiosis in the laboratory. After injecting bacteria into a fungus—a process that required creative problem solving (and a bicycle pump)—the researchers were able to elicit cooperation without killing either the bacteria or the host. Their observations offer insight into the conditions that allow the same thing to happen in microbial nature.
The cells even adapted to each other faster than expected. “To me, this means that organisms actually want to live together and that symbiosis is the norm,” said Vasilis Kokkorisa mycologist who studies the cell biology of symbiosis at VU University Amsterdam and was not involved in the new study. “So this is very big news for me and for this world.”
Early failed attempts reveal that most cellular romantic relationships fail. But by understanding how, why, and when organisms accept endosymbionts, researchers can better understand key moments in evolution and potentially develop synthetic cells engineered with super-powered endosymbionts.
Breakthrough of the cell wall
Julia Voerholtmicrobiologist at the Swiss Federal Institute of Technology in Zurich, Switzerland, has long wondered about the circumstances of endosymbiosis. Researchers in the field have hypothesized that once a bacteria infiltrates a host cell, the relationship oscillates between infection and harmony. If the bacteria reproduces too quickly, it risks depleting the host’s resources and triggering an immune response, resulting in the death of the guest, the host, or both. If it reproduces too slowly, it will not take hold in the cell. Only in rare cases, they believed, did the bacteria achieve a Goldilocks reproduction rate. Then, to become a true endosymbiont, it must infiltrate the reproductive cycle of its host to pass to the next generation. Finally, the host genome must eventually mutate to adapt to the bacteria, allowing the two to evolve as a unit.
“They become dependent on each other,” Vorholt said.