Tracing elevational changes in microbial life and organic carbon sources in soils of the Atacama Desert. atacama

The Atacama Desert frequently serves as model system for tracing life under extremely dry conditions. We hypothesized that traces of life in the Atacama Desert follow distinct micro- and macro-scale gradients such as soil depth and elevation, respectively. Different depth intervals of surface soils (0-1, 1-5, and 5-10 cm) were sampled at five sites along an elevational transect near the Quebrada Aroma, spanning from the hyperarid core of the desert towards the arid Western Andean Precordillera (1.300 to 2.700 m a.s.l.), and from one additional site in the hyperarid core near Yungay. We determined the contents of major elements, pedogenic minerals and oxides, organic carbon(OC), and its δ13C and δ15N isotopic composition. The presence of living microorganisms was assessed by cultivation and bacterial community composition was analyzed based on 16S rRNA amplicon analysis. Additional information about past and present plant and microbial life was obtained from lipid biomarker analysis. We did not detect consistent micro-scale distributions of these proxies within the soils. However, concentrations of OC and of long-chain, plant wax-derived n-alkanes increased in soils along the aridity gradient towards the wetter sites, indicating the presence of past life at places presently not covered by any vegetation. Likewise, bacterial abundance decreased as hyperaridity increased and the microbial community composition changed along the transect. The distributional patterns of phospholipid fatty acids (PLFAs) suggested a larger bacterial diversity at the higher, more humid sites compared to the drier ones. Archaeal isoprenoid glycerol dialkyl glycerol tetraethers (isoGDGTs) and bacterial branched (br)GDGTs did not follow a clear elevational trend and were absent at the driest site. These results suggest that several microorganisms are able to survive extreme dryness in the desert soil. Thus, plant-derived and microbiological markers follow the macro-scaled elevation and hyperaridity gradient. Bacteria are still culturable even at the driest sites, while the detected biomolecules also indicate past life. The detection of past plant life in nowadays apparently lifeless regions suggests that conditions for life were less hostile in former times.