Scientists have discovered a groundbreaking new form of symbiosis—bacteria living inside a single-celled organism (a ciliate) and providing it with energy through nitrate respiration. This finding revolutionizes our understanding of microbial partnerships and underscores the crucial role these minuscule organisms play in global ecosystems.
Initially found in a freshwater lake, researchers sought to determine how widespread these unique microbes are. Their investigation revealed these symbionts in diverse environments worldwide, from lakes and groundwater to even wastewater. This discovery challenges our current knowledge of microbial interactions and highlights the hidden yet significant impact of these tiny life forms.
A New Symbiotic Partnership
In 2021, scientists from the Max Planck Institute for Marine Microbiology in Bremen, Germany, made a remarkable discovery: a unique bacterium that resides inside a ciliate and provides it with energy. This symbiotic relationship parallels the role of mitochondria in cells, but differs crucially in its energy source. Instead of using oxygen, this endosymbiont generates energy by respiring nitrate.
Curious about the prevalence and diversity of these microbes, the researchers expanded their study. As they explored massive public sequencing databases, they uncovered a surprising amount of data on these unusual symbionts. “We were stunned to find them on every inhabited continent,” notes Jana Milucka from the Max Planck Institute for Marine Microbiology. “Their ubiquity challenged our preconceived notions about their rarity.”
Global dwellers
The study revealed that these symbionts are not limited to freshwater environments. They can thrive in groundwater and even wastewater. Milucka adds, “This adaptability to various environments underscores the significant role these microorganisms play in nutrient cycling and ecosystem balance.”
A Family of Symbionts with Diverse Abilities
As the researchers delved deeper into the data, they not only identified the original symbiont but also discovered four new closely related species. Two of these new species formed a new genus, named Azosocius, meaning “nitrogen associate.” “This discovery provides insight into the genomic and metabolic diversity within this fascinating group of symbionts,” explains first-author Daan Speth.
The researchers analyzed a groundwater sample from Hainich, Germany, where one of the new Azosocius species was found. This sample offered a unique opportunity to study the symbionts’ metabolic processes in more detail.
Adaptive Metabolic Abilities
In collaboration with Kirsten Küsel and Will Overholt from Friedrich Schiller University in Jena, Germany, the researchers gathered meta-transcriptomic data from the Hainich samples. This data revealed a surprising capability of the new symbiont species. “Unlike the original species, which can only perform anaerobic respiration, these new species can also respire oxygen,” says Speth.
This versatility expands our understanding of the symbionts’ environmental niches. “The ability to respire both nitrate and oxygen broadens their potential habitats and ecological roles,” notes Speth.
Evolutionary and Ecological Implications
The findings, published in Nature Communications, offer valuable insights into the evolutionary and ecological significance of these symbionts. “By understanding how these beneficial symbioses evolve, we can better appreciate their historical and contemporary relevance,” Milucka remarks.
Ecologically, these symbionts play a critical role in the nitrogen cycle of their habitats. “Through denitrification, they influence nitrogen oxide levels and may produce greenhouse gases like nitrous oxide,” adds Speth. This underscores the symbionts’ importance in nutrient cycling and climate regulation.
Pioneers of Microbial Symbiosis
The discovery of these symbionts highlights the remarkable adaptability and metabolic innovation of microorganisms. “Protists like ciliates are capable of such fascinating metabolic innovations, often due to their partnerships with prokaryotes,” Milucka observes. “This symbiosis exemplifies the intricate relationship between eukaryotes and prokaryotes in shaping evolutionary processes.”
Understanding these relationships is crucial for comprehending the evolution of eukaryotes and the broader functioning of ecosystems. “These microorganisms are like biological marvels, providing us with new perspectives on life’s strategies and capabilities,” Milucka concludes.
Reference: “Genetic potential for aerobic respiration and denitrification in globally distributed respiratory endosymbionts” by Daan R. Speth, Linus M. Zeller, Jon S. Graf, Will A. Overholt, Kirsten Küsel, and Jana Milucka, Nature Communications, 8 November 2024, DOI: 10.1038/s41467-024-54047-x
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