They are also called power plants of the cells: the mitochondria. They are present in almost all eukaryotic cells and they supply the cells with energy. Scientists at the Max Planck Institute for Marine Microbiology have now discovered that symbiotic bacteria can fulfil this function too. Their findings shed a completely new light on the survival of simple eukaryotes in oxygen-free environments. These results have just been published in the renowned scientific journal Nature.
Researchers from Bremen, together with their colleagues from the Max Planck Genome Center in Cologne and the aquatic research institute Eawag from Switzerland, have discovered a unique bacterium that lives inside a unicellular eukaryote and provides it with energy. Unlike mitochondria, this so-called endosymbiont derives energy from the respiration of nitrate, not oxygen. In general, among eukaryotes, symbioses are rather common. Eukaryotic hosts often co-exist with other organisms, such as bacteria.
Some of the bacteria live inside the host cells or tissue, and perform certain services, such defense or nutrition. In return, the host provides shelter and suitable living conditions for the symbiont. An endosymbiosis can even go that far that the bacterium loses its ability to survive on its own outside its host. This was also the case with the symbiosis discovered by the Bremen scientists in Lake Zug in Switzerland.
So far, it has been assumed that eukaryotes in oxygen-free environments survive through fermentation, since mitochondria require oxygen in order to generate energy. The fermentation process is well documented and has been observed in many anaerobic ciliates. However, microorganisms cannot draw as much energy from fermentation, and they typically do not grow and divide as quickly as their aerobic counterparts.
Since then, evolution has further deepened this intimate partnership. The evolution of mitochondria has proceeded in a similar way. It is believed that more than a billion years ago when an ancestral archaeon engulfed a bacterium, these two started a very important symbiosis: this event marked the origin of the eukaryotic cell.
