Imachi, H., Nobu, M. K., Nakahara, N., Morono, Y., Ogawara, M., Takaki, Y., et al. & Takai, K. (2020). Isolation of an archaeon at the prokaryote–eukaryote interface. Nature 577(7791): 519–525. doi:10.1038/s41586-019-1916-6.
Key takeaways
From the reading, NOTE-1238.
- Dependent by way of life. MK-D1 grows only while a partner removes its hydrogen and formate. Inhibiting the partners impairs it, adding hydrogen or formate stops it, and it grows with either of two methanogens (THEORY-1108). Its dependence is on a service, not on a particular partner.
- Simple inside, elaborate outside. Cells about 550 nm across, no visible internal compartments, and long, often branching membrane protrusions.
- Closest cultured relative. Ribosomal-protein trees of cultured organisms place it sister to eukaryotes. Metagenome-derived genomes were left out of the trees.
- An inferred ancestor. Gene content across Asgard genomes suggests the Asgard ancestor was an amino-acid degrader that depended on metabolic partners. The authors keep other lifestyles open.
- A hypothesis, by its authors’ account. The entangle–engulf–endogenize model has the host wrap an oxygen-scavenging partner with its protrusions, then hand over its own energy metabolism to it. The authors call it “not definitive”, and favour an interaction other than hydrogen transfer as the driver of endosymbiosis.
Why this is here
The empirical case for the syntrophy explanation A1 named and did not cite. It is the first cultured Asgard archaeon, and it lives by syntrophy. Its model is not the hydrogen hypothesis (LIT-1597), with which the record groups it: here the host produces the hydrogen, and the authors favour another interaction as the driver of endosymbiosis. It bears only weakly on A2’s question of whether host complexity preceded the mitochondrion. The isolate shows no internal complexity, but the authors set about two billion years between it and the host lineage, and LIT-1566 says the same of cultivated Asgard archaea.