Lifeboat Foundation

In 2021, scientists at the Max Planck Institute for Marine Microbiology in Bremen, Germany, reported an astonishing new form of symbiosis: They found a unique bacterium that lives inside a ciliate—a unicellular eukaryote—and provides it with energy. The symbiont’s role is thus strongly reminiscent of mitochondria, with the key difference that the endosymbiont derives energy from the respiration of nitrate, not oxygen.

Now the researchers from Bremen set out to learn more about the environmental distribution and diversity of these peculiar symbionts. “After our initial discovery of this symbiont in a freshwater lake, we wondered how common these organisms are in nature,” says Jana Milucka from the Max Planck Institute for Marine Microbiology. “Are they extremely rare and therefore eluded detection so long? Or do they exist elsewhere and if so, what are their metabolic capacities?”