The original version of this story appeared in Quanta Magazine.
Far from being solo operators, most single-cell microbes are in complex relationships. In the ocean, soil, and your gut, they might fight and eat each other, exchange DNA, compete for nutrients, or feed on each other. Sometimes they become even more intimate: one cell can slip inside another and make itself comfortable. If the conditions are right, she could stay and be welcomed, sparking a relationship that could last generations – or 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-living bacteria. Photosynthetic plants owe their sugars in the sun to the chloroplast, which was also originally an independent organism. Many insects obtain 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 a random merger of two independent organizations into a stable and lasting partnership?
Now, for the first time, researchers have looked at the opening choreography of this microscopic dance by induce endosymbiosis in the laboratory. After injecting bacteria into a fungus—a process that required creative problem solving (and a bicycle pump)—the researchers managed to trigger cooperation without killing the bacteria or the host. Their observations offer insight into the conditions that allow the same thing to happen in the microbial wild.
The cells even adjusted to each other faster than expected. “To me, this means that organisms actually want to live together, and symbiosis is the norm,” said Vasilis Kokkorisa mycologist who studies the cell biology of symbiosis at VU University in Amsterdam and was not involved in the new study. “So this is big, big news for me and for this world.”
Early failed attempts reveal that most cellular romances 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 superpowerful endosymbionts.
Breakthrough of the cell wall
Julia Vorholta microbiologist at the Swiss Federal Institute of Technology Zurich in Switzerland, has long puzzled over the circumstances of endosymbiosis. Researchers in the field have theorized that once a bacteria sneaks into a host cell, the relationship wavers 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 establish itself in the cell. Only in rare cases, they thought, does the bacteria achieve Goldilocks-like reproduction rates. Then, to become a true endosymbiont, it must infiltrate its host’s reproductive cycle to make a turn for the next generation. Finally, the host genome must eventually mutate to adapt to the bacteria, causing the two to evolve as a unit.
“They become dependent on each other,” Vorholt said.
