Facsaria manual

The presence of cell/particle populations above background level in Dallol samples was assessed with a flow-cytometer cell-sorter FACSAria™III (Becton Dickinson). Several DNA dyes were tested for lowest background signal in forward scatter (FSC) red (695±20 nm) and green (530±15 nm) fluorescence (Extended Data Fig. 9a) using sterile SALT-YE medium as blank. DRAQ5™ and SYTO13® (ThermoFisher) were retained and used at 5 μM final concentration to stain samples in the dark at room temperature for 1 h. Cell-Trap concentrated samples were diluted at 20% with 0.1-μm filtered and autoclaved MilliQ® water. The FACSAria™III was set at purity sort mode triggering on the forward scatter (FSC). Fluorescent target cells/particles were gated based on the FSC and red or green fluorescence (Extended Data Fig. 9b) and flow-sorted at a rate of 1-1,000 particles per second. Sorting was conducted using the FACSDiva™ software (Becton Dickinson); figures were done with the FCSExpress 6 software (De Novo Software). Sorted cells/particles were subsequently observed by scanning electron microscopy for characterization. Minimum and maximum cell abundances were estimated based on the number of sorted particles, duration of sorting and minimal (10μl min-1) and maximal (80μl min-1) flow rates of the FACSAria (Becton Dickinson FACSAria manual).