This adventure started some years ago when the nature photographer Olivier Grunewald knocked at our lab door looking for scientists that might be willing to study microbial life in a particular extreme environment, the geothermal field of Dallol. Captivated by the striking beauty of its changing colourful ponds in utter contrast with the mineral asperity of the Danakil desert, he wanted to know whether the discovery of microbes thriving in such conditions might add knowledge value to the place and perhaps push the Ethiopian authorities to set up protective measures to preserve this unique ecosystem. We took the challenge and soon realized that Dallol comprised some of the most fascinating polyextreme environments of the planet.

Dallol looks like a salt volcano with intense degassing and hydrothermal activity on top. Lying on the Danakil depression along the East African Rift, the whole region undergoes intense tectonic, seismic and volcanic activity, being one of the hottest places on Earth (daily winter temperatures can easily exceed 45°C!). Dallol and its surroundings display unique combinations of physicochemical parameters, from hypersaline sites (cave reservoirs in the Dallol west salt canyons, southern Lake Assale), to the acidic hypersaline magnesium-rich brines (Black and Yellow lakes south to the dome) and the hypersaline, hyperacidic (down to negative pH!) and hot pools on Dallol’s dome. Although halophilic, acidophilic and hyperthermophilic microorganisms are well known, do microorganisms exist that are adapted simultaneously to several of those extreme conditions?

In our Nature Ecology and Evolution article (https://www.nature.com/articles/s41559-019-1005-0), we explore this possibility by collecting many samples along gradients of polyextreme conditions and studying the potential presence of life and its traces though a variety of interdisciplinary approaches, including massive sequencing of gene markers, cultivation, flow cytometry, chemical analyses of brines and scanning electron microscopy coupled to chemical imaging. Our results are surprising as they convergently and strongly suggest that life is absent from the hyperacidic and hypersaline Dallol dome ponds and the magnesium-dominated Black and Yellow Lakes, even though microbial dispersal by wind and human visitors is intense. The presence of liquid water at the surface of a planet is often used as habitability criterion. However, our study presents evidence that there are sterile places on the Earth’s surface in the presence of liquid water, partly due to the abundance of chaotropic (disordering) magnesium salts.

However, we detect life in the hypersaline systems near Dallol (cave reservoir in the Dallol salt canyons and the neighbouring salt plain) and Lake Assale or Karum. Life here is largely dominated by tiny archaea (cells 200-300 nm in diameter) that belong to typical halophilic lineages but also to an astounding diversity of archaeal phyla (including novel groups) without known halophilic members. At the same time, we identified rounded mineral nanoprecipitates in the Dallol brines that might be confounded with very small cells. Chemical mapping unambiguously show that these are mostly silica-rich biomorphs (microbe-looking amorphous mineral precipitates), highlighting how difficult it is to tell apart true biosignatures from artefacts, especially for simple morphologies and very small structures.

The detected microbial lineages span the archaeal tree and likely adapted independently to high salinity from non-halophilic ancestors. Culturing these archaea and studying their genomes and metagenomes should provide valuable information about their molecular adaptations and lifestyles.