Life limits at the polyextreme gradients of the Dallol geothermal field

Microbial life has adapted to various individual extreme conditions; yet, organisms simultaneously adapted to very low pH, high salt and high temperature are unknown. I will present evidence from combined environmental 16S/18S rRNA-gene metabarcoding, cultural approaches, fluorescence-activated cell sorting, scanning electron microscopy and chemical analyses supporting the existence of physicochemical barriers preventing active life 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 Mg2+/Ca2+-dominated brines and ii) hyperacidity-salt combinations. We detect extremely diverse microbial life in the less polyextreme sites at the Dallol dome and Danakil depression surroundings. Microbial diversity was dominated by highly diverse ultrasmall archaea widely distributed across phyla with and without previously known hyperhalophilic members. We detect active silica encrustment/fossilization of cells but also abiotic biomorphs of varied chemistry, raising warnings for the interpretation of morphological biosignatures on Earth and beyond. In this framework, I will comment on recent claims about the occurrence of active life (one member of the Nanohaloarchaeota) in the hyperacidic and hypersaline Dallol ponds. I will show evidence suggesting that that finding corresponds to contamination from the salt pan area combined with misinterpretation of abiotic biomorphs as fossilized microbes. Our results point to the absence of life from some polyextreme sites in the presence of liquid surface water on Earth and should help to circumscribe habitability.