A new powerhouse.


Breaking down the microbiology world one bite at a time

A new powerhouse.

The oxygenation of Earth started 2.4 billion years ago, which enabled eukaryotic life to evolve 1 – 1.9 billion years ago. The diversification of eukaryotes is due to their ability to harvest a lot of energy from oxygen respiration in their mitochondria: the powerhouse of the cell. 

According to the endosymbiotic theory, mitochondria come from the assimilation of bacteria as symbionts, two organisms living closely together. These mitochondria, like other membrane-bound compartments in the cell, are called organelles.

Endosymbiotic theory: Modified from Wikicommons

However, not all eukaryotes have mitochondria: some anaerobic eukaryotes, organisms that live without oxygen, generate energy from fermentation in hydrogenosomes (little hydrogen factories to make energy). Some facultative anaerobes can use anaerobic respiration by denitrification, which means they make energy out of hydrogen or nitrates instead of oxygen.

Oxygen preference of aerobes / facultative aerobes and anaerobes

In their study, Graf et al. investigated Lake Zug (in Switzerland) at 200m depth where there is no oxygen. They found a high abundance of motile unicellular eukaryotes that were obligate anaerobes, as there is no oxygen in their environment. Some of these organisms seemed to have endosymbionts in them but not the expected usual hydrogenosomes.

The authors sequenced the genome and found a new type of organism: Candidatus Azoamicus ciliaticola. This organism had a very small genome but was able to self-replicate. Most of its genome was dedicated to production and conversion of energy (as other mitochondria-type organelles), but no genes for aerobic respiration were found in the bacteria or the host. 

Instead, the organism had the complete set of genes for respiratory denitrification. Therefore, nitrates or nitrites are being used through the respiratory denitrification to power the cell, just like oxygen fuels our cells in our mitochondria. To date, it is the only known endosymbiont to only possess the denitrification process without also having the possibility to use oxygen respiration like found in facultative aerobes.

This new partnership enables us to better understand the original symbiotic relationship that allowed eukaryotes to evolve, as well as uncovering different energy systems. Who knows maybe other powerhouses exist and we still have to discover them.


Link to the original post: Graf, J.S., Schorn, S., Kitzinger, K. et al. Anaerobic endosymbiont generates energy for ciliate host by denitrification. Nature 591, 445–450 (2021).

Featured image: https://www.flickr.com/photos/stiefkind/4280496233