Complex, eukaryotic cells that contain membrane-bound organelles evolved after simpler, prokaryotic cells like bacteria and archaea. It's thought that eukaryotes probably evolved after one microbe infected another, and took up residence there in an endosymbiotic relationship, behaving primarily as something like an energy-generating mitochondrion. The infected cell would have then had enough energy to acquire or construct other, new organelles to build a complex system.
While scientists cannot go back in time to verify this theory, they have discovered symbionts that infect cells, and provide evidence that endosymbiotic relationships can happen among microbes. In 2021, researchers reported the discovery of a bacterium that had taken up residence inside of a unicellular eukaryotic organism known as a ciliate. The symbiont also functions in a way that is similar to mitochondria. However, in this case, the energy comes from respiring nitrogen instead of oxygen.
The investigators that found this organism wanted to know more about these symbionts, how common they are, and what kinds of abilities they may have, noted Jana Milucka of the Max Planck Institute for Marine Microbiology.
After searching through genetic sequences in public databases that hold information about many different types of environmental samples and the organisms they contain, the researchers identified symbionts in around 1,000 different datasets. The found their original symbiont as well as two new species that are part of a novel genus.
"We were surprised how ubiquitous they are. We could find them on every inhabited continent," said Milucka. "Moreover, we learned that they can live not only in lakes and other freshwater habitats but also in groundwater and even wastewater."
After studying the new symbionts, they discovered that along with anaerobic respiration (also known as denitrification), they can also generate an enzyme called a terminal oxidase. This enzyme enables these organisms to respire oxygen as well as nitrogen, and allows the symbionts to live in a huge range of environments, including some that could be toxic. The findings have been reported in Nature Communications.
"By performing denitrification, this symbiosis impacts the nitrogen cycle of their respective habitat and has the potential to remove nutrients, such as nitrogen oxides, as well as produce greenhouse gases, such as nitrous oxide," noted first study author Daan Speth.
Sources: Max Planck Society, Nature Communications