Scientists have discovered a remarkable new form of symbiosis - a bacterium that lives inside and provides energy to a single-celled organism called a ciliate. Unlike mitochondria, which use oxygen, the microbe provides energy to its host by breathing nitrate.

Illustration of the originally discovered endosymbiotic bacterium Candidatus Azoamicusciliaticola and its ciliate host. The image is a composite of a scanning electron microscope image (SEM, gray) and a fluorescence image. Endosymbionts (yellow) and bacterial prey can be seen in the food vacuole and large nuclei (blue). The external structure of the weakly fluorescent ciliates as well as the cilia are also clearly visible. Image credit: S.Ahmerkamp/Max Planck Institute for Marine Microbiology

The researchers first discovered the microbe in a freshwater lake and then set out to determine its distribution range. To their surprise, they found these microbes in a variety of environments around the world, from lakes and groundwater to even wastewater. The discovery challenges our understanding of microbial partnerships and reveals how these tiny organisms play a hidden but important role in ecosystems around the world.

In 2021, scientists at the Max Planck Institute for Marine Microbiology in Bremen, Germany, made a major discovery: a unique bacterium lives in and provides energy to ciliates, a single-cell eukaryotic organism. This symbiotic relationship is similar to the role that mitochondria play in cells, but with one major difference: Instead of using oxygen, this endosymbiotic bacterium generates energy through nitrate respiration.

To better understand the distribution and diversity of these unusual microorganisms, researchers in Bremen expanded the scope of their study. Now, researchers in Bremen are beginning to learn more about the environmental distribution and diversity of these strange symbionts. Yana Miluca from the Max Planck Institute for Marine Microbiology explains: "After discovering this symbiont for the first time in a freshwater lake, we wanted to know how common these organisms are in nature. Are they extremely rare and therefore have remained undiscovered for a long time? Or do they exist elsewhere, and if so, what are their metabolic capabilities?"

To find out, the scientists searched large public sequencing databases containing genetic data from numerous environmental samples. What they found was surprising: These symbionts appeared in approximately 1,000 different data sets. "We were surprised by how ubiquitous they are," Miluca said. "We can find them on every inhabited continent, and furthermore, we learned that they can live not only in lakes and other freshwater habitats, but also in groundwater and even wastewater."

Scientists discovered not only the original symbionts in these data sets, but also some new relatives. "We ultimately identified four new species, two of which actually form a new genus," explains first author Daan Speth, "because the symbionts in this new genus may be related to the originally discovered Azoamicus (the name of which Meaning "Nitrogen Friend") has a similar role and we named the new genus Azosocius ("Nitrogen Friend"). We were lucky enough to find a new Azosocius species in groundwater samples from Heinich, Germany, not far from Bremen."

Now, scientists want to delve deeper into the lives of these new species. Working with Kirsten Küsel and Will Overholt of Friedrich-Schiller University Jena, Germany, who originally collected Heinich's samples, the scientists were able to access the sampling sites and study metatranscriptomic data—data that describes gene expression in a sample and shows microbial activity.

Speth continued: "Here we get another surprise - these respiratory symbionts can play new tricks. Unlike the original symbiont species, which can only perform anaerobic respiration (i.e. denitrification), all the new symbiont species actually encode a terminal oxidase enzyme that allows them to perform oxygen respiration in addition to nitrogen. This may explain why we also find these symbionts in environments that are completely or partially anoxic."

The results, now published in the journal Nature Communications, answer scientists' open questions about the biogeography of symbionts. "Thanks to the discovery of these new species, we can now start to think more about their evolution as well," Miluca says. "We hope to better understand how these beneficial symbionts began and how they evolved over time." "

There are also ecological aspects to the study, Speth added: "By carrying out denitrification, this symbiotic relationship has an impact on the nitrogen cycle in their respective habitats and has the potential to remove nutrients such as nitrogen oxides while producing greenhouse gases such as nitrous oxide."

Last but not least is a simple appreciation of the microbial world. "This creature is a miracle of nature," Miluca said excitedly. "Protists are often able to carry out such amazing metabolic innovations because they are so easily related to prokaryotes. To me, that's really fascinating. These organisms are an important piece of the puzzle in understanding eukaryotic evolution."

Compiled from /ScitechDaily