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, the soil, and your gut, they could fight and eat each other, exchange dnacompete for nutrients, or eat each other’s by-products. Sometimes they become even more intimate: One a cell could slip into another and make himself comfortable. If the conditions are just right, it might stay and be welcomed, triggering a relationship that could last for generations—or billions of years. This phenomenon of one cell living inside another, called endosymbiosis, fueled the development of complex life.
Examples of endosymbiosis are everywhere. Mitochondria, the energy factories in your cells, there were once free-living bacteria. Photosynthetic plants owe their sun-spun sugars to the chloroplast, which was also originally an independent organism. Many insects receive essential nutrients of bacteria that live in them. And last year researchers discovered the “nitroplast”, an endosymbiont that helps some algae process nitrogen.
So much of life depends on endosymbiotic relationships, but scientists have struggled to understand how they happen. How does an internalized cell avoid digestion? How does it learn to reproduce within its host? What makes a chance fusion of two independent organisms into a stable, lasting partnership?
Now, for the first time, researchers have watched the initial choreography of this microscopic dance of inducing 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 get cooperation going without killing the bacteria or the host. Their observations offer a glimpse into the conditions that allow the same thing to happen in the microbial wilderness.
The cells even adapted to each other faster than expected. “To me, this means that organisms really 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 that’s big, big news for me and for this world.”
Early attempts that fell short reveal that most cellular love affairs are unsuccessful. But by understanding how, why and when organisms accept endosymbionts, researchers can better understand key moments in evolution, and also potentially develop synthetic cells engineered with superpowered endosymbionts.
The Cell Wall Breakthrough
Julia Vorholtmicrobiologist at the Swiss Federal Institute of Technology Zurich in Switzerland, has long puzzled over the circumstances of endosymbiosis. Researchers in the field theorized that once a bacterium sneaks into a host cell, the relationship vacillates between infection and harmony. If the bacterium 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 replicates too slowly, it won’t get established in the cell. Only in rare cases, they thought, does the bacterium reach a Goldilocks reproductive rate. Then, to become a true endosymbiont, it must infiltrate the reproductive cycle of its host to drive the next generation. Finally, the host genome must later mutate to accommodate the bacterium – allowing the two to evolve as a unit.
“They become addicted to each other,” Vorholt said.