The limit of life at extremely low water activity: Lithium-concentration ponds in a solar saltern (Salar de Atacama, Chile)

Extremely hyper-saline ponds from an industrial lithium-concentration process in solar salterns in the Atacama Desert were studied to determine the limits of life at very low water activity. Water activity (aw) of 0.61 is the lowest aw value for growth of living beings recorded to date. Xerophilic (sometimes called osmophilic) filamentous fungi and yeasts are predominant in high-sugar foods with such low aw values. Some microorganisms are capable of growth at that water activity level. By contrast, high-salt environments are almost exclusively populated by prokaryotes, notably the Halobacteria class and some Bacteroidetes, capable of growing in saturated NaCl (aw 0.75). The lowest aw that can be achieved by the addition of NaCl is 0.75 (saturation point for NaCl). Crystallizer ponds in Li+ concentration plants reach down to water activity levels around 0.1. The aim of this study was to determine how far along the salinity gradient could life be found. Cell counts were attempted by epifluorescence microscopy and qPCR with bacterial and archaeal universal primers. Biomass for DNA extraction was obtained by an optimized protocol involving dialysis of brines previously fixed with ethanol. Prokaryotic diversity was studied by DNA extraction, PCR, qPCR and 16S rRNA amplicon sequencing in different ponds along the salinity gradient. Archaeal DNA was found in the lower salinity ponds, while bacterial DNA was found along the whole gradient. Bacterial cDNA was retrieved from ponds down to an aw of 0.2. Moreover, bacteria could be grown in enriched cultures from most ponds.