Abstract
Increasing evidence supports that eukaryotes originated from the symbiosis of an Asgard archaeon, the alphaproteobacterial ancestor of mitochondria, and possibly additional bacterial contributions. Eukaryogenesis likely 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 hypothesis that Asgard archaea engaged primarily in syntrophic interactions with sulfate-reducers under early-Earth-like conditions.
Acknowledgements
We thank Olivier Grunewald for the organization of the field trip, Luigi Cantamessa and his Tigrayan/Ethiopian team for the in situ logistics, Jacques Barthélémy, Abdul Ahmed Aliyu and the Afar authorities for local assistance, and the Afar police for providing security. We thank Jodie Belilla and Ana I. López-Archilla for, respectively, help with sampling and hydrochemical analyses, and Timmy Flowers (Tropic Air Kenya) for efficiently piloting the helicopter. The field trip organization (O.G.) received support from the Mamont Foundation. This work was supported by the Gordon and Betty Moore Foundation (P.L.-G., https://doi.org/10.37807/GBMF9739), the Agence National de la Recherche (P.L.-G., ABiSYM ANR-23-CE02-0016-01; DM, DArchFolds, ANR-22-CE02-0012), and the ERC-AdG grants Plast-Evol (D.M., 787904) and SYMBEK (P.L.-G., 101141745). The equipment used to perform the microelectrode measurements was funded by Université Paris-Saclay (AAP 2023 “Innovation pédagogique” and Department of Biology).
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Gutiérrez-Preciado, A., Struillou, A., Liang, L. et al. Redox distribution of Asgard archaea and co-occurring taxa in microbial mats from an early Proterozoic ecosystem analog.
npj Biofilms Microbiomes (2026). https://doi.org/10.1038/s41522-026-01148-8
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DOI: https://doi.org/10.1038/s41522-026-01148-8
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