M Microbial life has adapted to various individual extreme conditions; yet, organisms simultaneously adapted to very low pH, high salt and high temperature are unknown. We combined environmental 16S/18S rRNAgene metabarcoding, cultural approaches, fluorescence-activated cell sorting, scanning electron microscopy and chemical analyses to study samples along such unique polyextreme gradients in the Dallol-Danakil area (Ethiopia). We identify two physicochemical barriers to life in the presence of surface liquid water defined by: i) high chaotropicity-low water activity in Mg 2+ /Ca 2+-dominated brines and ii) hyperacidity-salt combinations (pH~0/ NaCl-dominated salt-saturation). When detected, life was dominated by highly diverse ultrasmall archaea widely distributed across phyla with and without previously known halophilic members. We hypothesize that high cytoplasmic K +-level was an original archaeal adaptation to hyperthermophily, subsequently exapted during multiple transitions to extreme halophily. W We detect active silica encrustment/fossilization of cells but also abiotic biomorphs of varied chemistry. Our work helps circumscribing habitability and calls for cautionary interpretations of morphological biosignatures on Earth and beyond. Belilla et al. 3 Microbial life has adapted to so-called extreme values of temperature, pH or salinity, but also to several polyextreme, e.g. hot acidic or salty alkaline, ecosystems 1,2. Various microbial lineages have been identified in acidic brines in the pH range 1.5-4.5, e.g. in Western Australia 3,4 and Chile 3. However, although some acidophilic archaea thrive at pH~0 (Picrophilus oshimae grows at an optimal pH of 0.7) 5 and many halophilic archaea live in hypersaline systems (>30%; NaCl-saturation conditions), organisms adapted simultaneously to very low pH (<1) and high salt, and eventually also high temperature, are not known among cultured prokaryotic species 1. Are molecular adaptations to these combinations incompatible or (hot) hyperacidic hypersaline environments simply rare and unexplored? The Dallol geothermal dome and its surroundings (Danakil Depression, Afar, Ethiopia) allow to address this question by offering unique polyextreme gradients combining high salt content (33 to >50%; either Mg 2+ /Ca 2+ or Na + (/Fe 2+/3+)-rich), high temperature (25-110°C) and low pH (≤-1.5 to 6). Dallol is an uplifted (~40 m) dome structure located in the North of the Danakil depression (~120 m below-sea-level), a 200 km-long basin within the Afar rift, at the triple junction between the Nubian, Somalian and Arabian Plates 6. Lying only 30 km north of the hypersaline, hydrothermally-influenced, Lake Assale (Karum) and the Erta Ale volcanic range, Dallol does not display volcanic outcrops but intense degassing and hydrothermalism. These activities are observed on the salt dome and the adjacent Black Mountain and Yellow Lake (Gaet'Ale) areas 6,7 (Fig. 1a-b). Gas and fluid isotopic measurements indicate that meteoritic waters, notably infiltrating from the high Ethiopian plat...

The variety of depositional facies of a Lower Jurassic carbonate platform has been investigated on the island of Mallorca along a transect comprising six stratigraphic proiles. Twenty-nine facies and sub-facies have been recognized, grouped into seven facies associations, ranging in depositional environment from supratidal/terrestrial and peritidal to outer platform. Spatial and temporal (2D) facies distribution along the transect relects the evolution of the carbonate platform with time showing diferent facies associations, from a broad peritidal platform (stage 1) to a muddy open platform (stage 2), and inally to a peritidal to outer carbonate platform (stage 3). Stage 1 (early Sinemurian to earliest late Sinemurian) corresponds to a nearly-lat peritidal-shallow subtidal epicontinental platform with facies belts that shifted far and fast over the whole study area. The evolution from stage 1 to stage 2 (late Sinemurian) represents a rapid looding of the epicontinental shallow platform, with more open-marine conditions, and the onset of diferential subsidence. During stage 3 (latest Sinemurian), peritidal and shallow-platform environments preferentially developed to the northeast (Llevant Mountains domain) with a rapid transition to middle-outer platform environments toward the northwest (Tramuntana Range domain). Stages 1 and 3 present facies associations typical of Bahamian-type carbonates, whereas stage 2 represents the demise of the Bahamian-type carbonate factory and proliferation of muddy substrates with suspension-feeders. The described platform evolution responded to the interplay between the initial extensional tectonic phases related to Early Jurassic Tethyan rifting, contemporaneous environmental perturbations, and progressive platform looding related to the Late Triassic-Early Jurassic worldwide marine transgression and associated accommodation changes.

One of the latest volcanic features of the Erta Ale range at the Afar Triangle (NE Ethiopia) has created a polyextreme hydrothermal system located at the Danakil depression on top of a protovolcano known as the dome of Dallol. The interaction of the underlying basaltic magma with the evaporitic salts of the Danakil depression has generated a unique, high-temperature (108 °C), hypersaline (NaCl supersaturated), hyperacidic (pH values from 0.1 to −1.7), oxygen-free hydrothermal site containing up to 150 g/L of iron. We find that the colorful brine pools and mineral patterns of Dallol derive from the slow oxygen diffusion and progressive oxidation of the dissolved ferrous iron, the iron-chlorine/-sulfate complexation, and the evaporation. These inorganic processes induce the precipitation of nanoscale jarosite-group minerals and iron(III)-oxyhydroxides over a vast deposition of halite displaying complex architectures. Our results suggest that life, if present under such conditions, does not play a dominant role in the geochemical cycling and mineral precipitation at Dallol as opposed to other hydrothermal sites. Dallol, a hydrothermal system controlled by iron, is a present-day laboratory for studying the precipitation and progressive oxidation of iron minerals, relevant for geochemical processes occurring at early Earth and Martian environments.

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T he Dallol protovolcanic area on the Danakil Depres s ion (Afar region, E thiopia) exhibits unique hydrothermal manifes tations in hypers aline context, yielding varied polyextreme phys icochemical conditions . P revious s tudies identified a wide archaeal divers ity in les s extreme brines but failed to identify microorganis ms thriving in either high-chaotropicity, low-wateractivity brines or hyperacidic-hypers aline Na-F e-rich brines . R ecently, we acces s ed s everal s mall lakes under intens e degas s ing activity adjacent to the R ound Mountain, wes t to the Dallol dome (Wes tern C anyon Lakes ; WC L1-5). T hey exhibited intermediate parameter combinations (pH~5, 34-41% NaC l-dominated s alts with relatively high levels of chaotropic Mg-C a s alts ) that s hould allow to better cons train life limits . T hes e lakes were overwhelmingly dominated by Archaea, encompas s ing up to 99% of prokaryotic 16S rR NA gene amplicon s equences in metabarcoding s tudies . T he majority belonged to Halobacteriota and Nanohaloarchaeota, the latter repres enting up to half of prokaryotic s equences . Optical and epifluores cence micros copy s howed active cells in natural s amples and divers e morphotypes in enrichment cultures . S canning electron micros copy coupled with energy-dis pers ive x-ray s pectros copy revealed tiny cells (200-300 nm diameter) epibiotically as s ociated with s omewhat larger cells (0.6-1 µm) but als o the pres ence of s ilica-dominated precipitates of s imilar s ize and s hape, highlighting the difficulty of dis tinguis hing microbes from mineral biomorphs in this kind of low-biomas s s ys tems .