Eukaryotes originated from the symbiosis of an Asgard archaeon, the alphaproteobacterial ancestor of mitochondria, and possibly additional bacterial contributions.
This transition occurred in redox-transition environments such as microbial mats or shallow sediments ∼2 billion years ago, when atmospheric oxygen was far lower than today. We investigated Asgard-enriched microbial mats from the low-oxygen, sulfidic Catherine volcano lake (Afar region, Ethiopia), mimicking early Proterozoic conditions.
16S rRNA gene metabarcoding, metagenomics, and metagenome-assembled genome analyses across redox-stratified layers of in situ and mesocosm-maintained mats revealed that Asgardarchaeota thrived in the sulfate-reduction zone, mainly co-occurring with Desulfurobacterota-Myxococcota, among others. Lokiarchaeia and Thorarchaeia preferred anoxic layers.
Within Heimdallarchaeia, Heimdallarchaeales were enriched in upper layers, correlating with oxygen-tolerant hydrogenase and sulfate-reduction genes, and Hodarchaeales, in anoxic layers, correlating with methanogenesis.
Although reactive-oxygen-species defense mechanisms were widespread, Asgardarchaeota lacked aerobic respiration. These results support the idea that Asgard archaea engaged primarily in syntrophic interactions with sulfate-reducers under early-Earth-like conditions.
Redox distribution of Asgard archaea and co-occurring taxa in microbial mats from an early Proterozoic ecosystem analog, biorxiv.org (open access)
Astrobiology, genomics, extremophile,
