Having bacteria as endosymbionts is fairly common in life on Earth: almost all eukaryotes have them in the form of mitochondria and sometimes chloroplasts. These former bacteria somehow got inside the ancestral eukaryote, either as parasites or as prey, and ended up as integral parts of their host's metabolic functions. Some organisms, especially insects, obtained bacterial endosymbionts more recently, that help them balance their metabolic needs when living on limited diets.
Algae have been known to be endosymbionts also, performing photosynthesis for their host. But in this study, a ciliate with both algae and purple photosynthetic bacteria as endosymbionts was discovered. Purple bacteria as symbionts is rare, and this combination has not been observed before. Interestingly, though algae produce oxygen through their photosynthesis, the ciliate prefers living in low-oxygen sediment at the bottom of a pond. The symbionts and their host seem to adjust their metabolisms as needed depending on the needs at the time; they may each perform photosynthesis, fermentation, or respiration if light, organic carbon, or oxygen are available.
Muñoz-Gómez SA, Kreutz M, Hess S. 2021.
. Sci Adv 7:eabg4102.
