Search

Soils in hyper‐arid climates, such as the Chilean Atacama Desert, show indications of past and present forms of life despite extreme water limitations. We hypothesize that fog plays a key role in sustaining life. In particular, we assume that fog water is incorporated into soil nutrient cycles, with the inland limit of fog penetration corresponding to the threshold for biological cycling of soil phosphorus (P). We collected topsoil samples (0–10 cm) from each of 54 subsites, including sites in direct adjacency (<10 cm) and in 1 m distance to plants, along an aridity gradient across the Coastal Cordillera. Satellite‐based fog detection revealed that Pacific fog penetrates up to 10 km inland, while inland sites at 10–23 km from the coast rely solely on sporadic rainfall for water supply. To assess biological P cycling we performed sequential P fractionation and determined oxygen isotope of HCl‐extractable inorganic Pδ18OHCl−Pi$$ \mathrm{P}\ \left({\updelta}^{18}{\mathrm{O}}_{\mathrm{HCl}-{\mathrm{P}}_{\mathrm{i}}}\right) $$. Total P (Pt) concentration exponentially increased from 336 mg kg−1 to a maximum of 1021 mg kg−1 in inland areas ≥10 km. With increasing distance from the coast, soil δ18OHCl−Pi$$ {\updelta}^{18}{\mathrm{O}}_{\mathrm{HCl}-{\mathrm{P}}_{\mathrm{i}}} $$ values declined exponentially from 16.6‰ to a constant 9.9‰ for locations ≥10 km inland. Biological cycling of HCl‐Pi near the coast reached a maximum of 76%–100%, which could only be explained by the fact that fog water predominately drives biological P cycling. In inland regions, with minimal rainfall (<5 mm) as single water source, only 24 ± 14% of HCl‐Pi was biologically cycled. We conclude that biological P cycling in the hyper‐arid Atacama Desert is not exclusively but mainly mediated by fog, which thus controls apatite dissolution rates and related occurrence and spread of microbial life in this extreme environment. [ABSTRACT FROM AUTHOR]

The extreme environmental conditions and lack of water on the soil surface in hyperarid deserts hamper microbial life, allowing only highly specialized microbial communities to the establish colonies and survive. Until now, the microbial communities that inhabit or have inhabited soils of hyperarid environments at greater depths have been poorly studied. We analyzed for the first time the variation in microbial communities down to a depth of 3.4 m in one of the driest places of the world, the hyperarid Yungay region in the Atacama Desert, and we related it to changes in soil physico-chemical characteristics. We found that the moisture content changed from 2 to 11% with depth and enabled the differentiation of three depth intervals: (i) surface zone A (0–60 cm), (ii) intermediate zone B (60–220 cm), and (iii) deep zone C (220–340 cm). Each zone showed further specific physicochemical and mineralogical features. Likewise, some bacterial phyla were unique in each zone, i.e., members of the taxa Deinococcota , Halobacterota , and Latescibacterota in zone A; Crenarchaeota , Fusobacteriota , and Deltaproteobacterium Sva0485 in zone B; and Fervidibacteria and Campilobacterota in zone C, which indicates taxon-specific preferences in deep soil habitats. Differences in the microbiota between the zones were rather abrupt, which is concomitant with abrupt changes in the physical-chemical parameters. Overall, moisture content, total carbon (TC), pH, and electric conductivity (EC) were most predictive of microbial richness and diversity, while total sulfur (TS) and total phosphorous (TP) contents were additionally predictive of community composition. We also found statistically significant associations between taxa and soil properties, most of which involved moisture and TC contents. Our findings show that under-explored habitats for microbial survival and existence may prevail at greater soil depths near water or within water-bearing layers, a valuable substantiation also for the ongoing search for biosignatures on other planets, such as Mars. [ABSTRACT FROM AUTHOR]

The hyper-arid soils of the Atacama frequently served as model systems for tracing life under extreme dry conditions such as on Mars. Plants rely on fog moisture (coastal zones) or on irregular rainfalls (mainly north-eastern part), which are controlled by the El Niño Southern Oscillation (ENSO) and the Pacific Decadal Oscillation (PDO) and vary with latitude and altitude. We therefore hypothesized that traces of life in the Atacama Desert are patchy, but nevertheless also follow latitudinal and altitudinal gradients. To test this hypothesis, we sampled surface soils (0-10 cm) along an altitude transect in the region of Quebrada Aroma spanning from the arid Precordillera of the Andes towards the hyper-arid core of the desert (1,300 m to 2,700 m a.s.l). We assessed the contents of organic carbon (OC), total nitrogen (N), extractable total S as well as of n-alkanes and n-fatty acids for tracing organic residues in these soils. We screened the element composition in order to derive information on potential soil formation processes along these gradients. In addition, we analyzed living microbiota using a combination of cultivation-dependent and independent approaches and we performed DNA extraction for amplification of the 16S rRNA gene fragments with subsequent analyses of PCR products by denaturing gradient gel electrophoresis. Additional information on bacterial and archaeal community composition was obtained from phospholipid fatty acids (PFLA) and glycerol dialkyl glycerol tetraethers (GDGTs). OC contents in the surface soils increased from the hyper-arid core in the desert to wetter sites at higher altitudes and presence of sparse vegetation. The abundances of aliphatic lipid biomarkers (short chain n-alkanes, n-fatty acids) increased along the altitudinal transect. Most n-alkanes and n-fatty acids at the drier, lower altitude (1,340 m) were short-chain homologues suggesting a microbial origin, whereas at higher elevation (2,721 m; marginally wetter) more long-chain compounds were present suggesting a larger contribution of plant-derived OC. Ongoing radiocarbon analysis will give further insights into OC sources (old versus young). Living cells were detectable at all sites with a consistent decrease in cell numbers as aridity increased. Lowest cell numbers below the detection limit of 5 CFUs g-1 soil were found at the driest sites. Intriguingly, we did not find pronounced changes in bacterial diversity using cell counting and simple denaturing gradient gel electrophoresis. The distributional patterns of the PLFAs suggest a significantly larger bacterial diversity at the highest, wettest altitude than at the lower, dryer sites. In addition, both archaeal isoprenoid GDGTs and bacterial branched GDGTs were detected in all samples also following the altitudinal trend with similar concentrations at sites located at 2,020 and 2,720 m altitude. These results suggest that several microorganisms are able to survive extreme dryness. The abundance of these organisms spans along an altitudinal gradient that follows the pattern of rare precipitation events, correlating also with the presence of sparse vegetation remains. After cell death, many of these biomolecules are preserved, now offering novel potentials also for tracing the evolution of past life forms from their cell wall remains. [ABSTRACT FROM AUTHOR]

• Fan ages in the Atacama dated to ∼13.6 ka and ∼56.4 ka. • The younger fan with rough surfaces was dominated by nano and fine colloids. • The older fan showed smooth surface but heterogenous size distribution of colloids. • Soil depth distributions pointed to colloidal transport in the younger fan. • Plants enhanced accumulation of nanocolloids with elevated OC and Ca content. As located in one of the oldest and driest deserts on Earth, soils in the Atacama Desert are greatly affected by atmospheric dust deposited on soil surface and the related fate of water-dispersible colloids (WDCs, <300 nm). We hypothesize that formation and content of these WDCs change with topography and age of natural soils. To highlight the processes involved, we investigated a mid-sized and gently (∼5°) sloping alluvial fan system of multi-phase evolution at 1480 m a.s.l. in the Paposo region of the hyper-arid Atacama Desert, which is considered typical for this part of the Coastal Cordillera. Sampling was done along a topographic transect in 11 pits, and assessed the distribution and composition of WDCs by means of asymmetric flow field-flow fractionation (AF4). The younger fan section (optically stimulated luminescence (OSL)-age of ∼13.6 ka) exhibited a pronounced surface roughness and steep slopes. Here, WDCs from the top soils (0–1 cm) free of plants contained nearly 54 ± 7% of medium-sized colloids (MCs, 210–300 nm) with a dominance of Si and Al. The elevated concentrations of fine colloids (FC, 24–210 nm) and particularly nanocolloids (NCs, 0.6–24 nm) was shown in levelled surface soils near shrubs with predominance of organic carbon (OC) and Ca. With higher collodial OC and Ca content in soils near shrubs, more WDC-P was formed concomitantly through increased OC-Ca-P associations. Larger variations in total WDC content were detected in the surface soils of the older fan section, which was dated to ∼56.4 ka. Here, the peaking NC had almost disappeared and thus MC dominated, probably reflecting re-aggregation and wind erosion over longer periods of time across a relatively smooth land surface. The WDCs and WDC-P peaked at 5–10 cm depth in the older fan section, as here a solid mineral/salt layer was present, while in the younger fan section the WDCs were more likely to be translocated from 'permeable' surface into deeper layers, likely reflecting leaching with occasional heavy rainfall. Overall, forms and distribution of WDCs depended on both topographic position and sediment age, thus making colloids as unique tracers of soil development processes during myriad or more years. [ABSTRACT FROM AUTHOR]

• Low δ 18O HCl-P values were constant with parent material at the driest sites. • δ 18O HCl-P values increase at less arid sites. • δ 18O HCl-P values trace past biological activity in the Atacama Desert. The Atacama Desert (Chile) is one of driest places on Earth, with a hyper-arid climate and less than 2 mm yr−1 precipitation; nevertheless, it has experienced rare periods of sporadic rainfall. These periods shortly enhanced vegetation growth and microbial activity, which must have utilized major nutrients such as phosphorus (P). However, any biological cycling of P involves an oxygen exchange with water, which should now reside in the hyperarid soils as tracer of life. In order to identify such evidences, we performed sequential P fractionation and analyzed the oxygen isotope composition of HCl-extractable phosphate (δ 18O HCl–P) in the surface soil (0–15 cm) of a climatic gradient along the rising alluvial fans of the Central Depression to the Precordillera, Chile. At the driest sites, the δ 18O HCl-P values were constant with depth and deviated from biologically-driven isotopic equilibrium. In contrast, we observed a considerable increase of δ 18O HCl-P values below the soil surface at less arid sites, where some isotope values were even within the range of full isotopic equilibrium with biologically cycled phosphate. For the latter sites, this points to most efficient biological P cycling right below the uppermost surface of the desert. Critically, the absolute concentrations of this biologically cycled P exceeded those of P potentially stored in living microbial cells by at least two orders of magnitude. Therefore, our data provides evidence that δ 18O HCl-P values trace not recent but past biological activity, making it a powerful tool for assessing the existence, pathways and evolution of life in such arid ecosystems on Earth and, thus, potentially on other planets such as Mars. [ABSTRACT FROM AUTHOR]

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 (1300 to 2700 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 gene sequencing. Additional information about past and present plant and microbial life was obtained from lipid biomarker analysis. We did not detect consistent micro-scale distributions for most 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 vegetation. Likewise, bacterial abundance and diversity decreased as hyperaridity increased and the microbial community composition changed along the transect, becoming enriched in Actinobacteria. The distributional patterns of phospholipid fatty acids (PLFAs) confirmed the 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, which can also indicate past life, did not follow a clear elevational trend and were absent at the driest site. Taken together, plant-derived and microbiological markers follow primarily the macro-scaled elevation and aridity gradient. Viable bacteria are present even at the driest sites, while 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. • Traces of past plant and microbial life detected in hyperarid Atacama Desert soils. • Higher microbial diversity and abundance at more humid sites / higher elevation. • Bacterial and plant-derived lipid biomarkers follow aridity gradient. • No consistent patterns of soil properties or biomolecules with soil depth. [ABSTRACT FROM AUTHOR]

At the land surface, the availability of water controls the evolution of life as well as the morphological development. It is currently hardly known how far both evolutionary processes interact in water-limited environments. While it is well considered that biotic processes contribute to weathering and therewith also prepare the landscape for erosion, little is known to what degree different landforms form barriers and corridors of life across geological time scales. Here, we present an example of how to tackle this complex interplay of biology and landscape evolution by introducing the Collaborative Research Centre CRC 1211 'Earth – Evolution at the dry limit'. A CRC is a German Research Foundation funding program, bringing together scientists from different disciplines to tackle jointly complex emergent research questions for up to twelve years. The CRC 1211 focuses on the arid cores of the Atacama Desert and the Namib Desert. Main objective is to disentangle how the shaping of land-surfaces by past episodes of wetter climate coevolved with the evolution of life. It is hypothesized that aridity was interrupted by punctuated wetter periods, which left fingerprints in surface processes and the radiation of the biota, with climatic thresholds beyond which biota could adapt to and influence landscape development. Specific desert landscape elements such as soil surface crusts, desiccation cracks and subsoil water potentially form underexplored refugia of life at particular sites, while old channels and alluvial fan systems potentially structure its dispersal at small to intermediate scales (< 100 m2), In addition, there are large-scale (>10,000 km2) gradients in surface processes and biodiversity, likely controlled by the intensity and duration of fog or rainfall. To substantiate these hypotheses we combine expertise from the fields of population and (phylo-) genetics, molecular biology, biogeography, ecology, soil sciences, Earth-surface sciences, geomorphology, meteorology, (paleo-) climatology, (isotope-) geochemistry and physical dating to discover and evaluate the trajectories and thresholds of the evolution and isolation of life. While presenting here our current research strategy, we invite scientists from all over the world to link their research on related questions for the joint development of a global picture on how evolution of life was triggered and regulated by land-surface processes. • Interdisciplinary research in extremely water-limited environments • Investigating coevolution of life and landscape in response to climatic change • Long-term climate records of the oldest deserts on Earth-Methodological development to date climate archives and determine dates and rates of surface processes [ABSTRACT FROM AUTHOR]

The Atacama Desert is the oldest and driest desert on earth, nevertheless traces of life have been observed in several places, accumulating residues of organic matter (OM) on the desert surface, i.e., in its soil. We asked if these OM traces of life preferentially accumulate in topsoil, or whether also enrichment of OM in deeper subsoil layers appears. We also evaluated to which degree spatial and vertical heterogeneity in the distribution of soil organic carbon (SOC) stocks depends on increasing aridity in the desert ecosystem. We sampled an east-west directed aridity transect with several replicates spanning the Atacama Desert from north to south, i.e. Codpa (18.4°S), Rio Tana (19.2°S), Quebrada Aroma (19.4°S), Yungay (24.6°S) and Paposo (25.1°S). With a nested sampling design (3≥n≤18) we addressed topsoil heterogeneity at each sampling site (n=433). For 13 of these sites soil profiles were dug to 0.8 to 3.9 m depth depending on soil thickness. This amounted to a total of 637 samples. Bulk density was taken for each site and depth of 15 cm. SOC concentrations were taken for each sample by thermally resolved dry combustion with a Soli TOC (Elementar Analysensysteme, Hanau, Germany DIN 19539GS). We found that with decreasing precipitation (~7 to <0.1 mm yr-1), SOC stocks decreased from 3.42±4.08 to 0.09±0.01 kg SOC m-2 in the 15 cm of topsoil. Hence, traces of life were ubiquitously preserved both in the arid and hyper-arid desert areas. Intriguingly, relatively large amounts of soil organic matter were also found at depth below 15cm, adding an extra of 52.9 to 95.9 % of subsoil C to the topsoil SOC stock. Furthermore, the subsoil SOC concentrations peaked between 40 and 150 cm depth in the soil, irrespectively of the presence or absence of visible vegetation. Biomarker analyses are currently performed to reveal the origin of SOC found in the various depths. We conclude that C accrual in this arid to hyper-arid system shifts from the preferential C enrichment in topsoil to that in the subsoil, therewith providing the potential for deep(er) biosphere foodwebs and demonstrating the likelyhood to need to dig into soil for discovering traces of (active) life in comparable environments, including other planets such as Mars. [ABSTRACT FROM AUTHOR]

Despite numerous studies about the change of soil biogeochemical properties by climate alternation driven by elevation, latitude, etc. in arid to hype-arid regions, little is known about soil colloids (1-1000 nm) and nanoparticles (1-100 nm) as nutrient carriers and which events are driving this in such environments. We investigated the size and composition of water dispersible colloids (WDCs) to quantify colloidal bound C and P from three depth profiles in an altitudinal transect in the Atacama Desert (Paposo region) using asymmetric field flow field fractionation (A-FFFF) coupled online to various detectors (ICP-MS, organic carbon detector (OCD) and UV). One of the profiles (A; 200 cm) was located in a semi-arid part and the two others (70 cm depth) in an arid area in the vicinity of each other (B on an abandoned alluvial fan, and C on the more active part of the fan). Three size categories of WDCs were detected based on the FFF results, including nanoparticles from 0.6 to 24 nm, fine colloids from 24 to 210 nm, and medium colloids from 210 to 500 nm. Nanoparticles and soil colloids accounted for up to 60% of the bioavailable P concentration (Olsen P) at the surface layer of profile C. Moreover, the contribution of colloidal Corg to total Corg was 5% at the surface of profile A, with a maximum of 11% at depth of 115 cm, and maximum 1% at the surface layer of both profiles B and C. The size distribution of colloids and their elemental content was significantly different between B and C especially at upper 20 cm. Our results, showing pronounced local differences in soil colloidal properties, suggest that colloidal matter (here mainly C and P) preserved in the Atacama deserts is terrestrial proxy record of both local (e.g. intermittent drainage events) and larger scale (periods of dust input) of environmental changes which have occurred in this part of South America. [ABSTRACT FROM AUTHOR]