Luis Cáceres

Transitory microbial habitat in the hyperarid Atacama Desert

Significance It has remained an unresolved question whether microorganisms recovered from the most arid environments on Earth are thriving under such extreme conditions or are just dead or dying vestiges of viable cells fortuitously deposited by atmospheric processes. Based on multiple lines of evidence, we show that indigenous microbial communities are present and temporally active even in the hyperarid soils of the Atacama Desert (Chile). Following extremely rare precipitation events in the driest parts of this desert, where rainfall often occurs only once per decade, we were able to detect episodic incidences of biological activity. Our findings expand the range of hyperarid environments temporarily habitable for terrestrial life, which by extension also applies to other planetary bodies like Mars. Traces of life are nearly ubiquitous on Earth. However, a central unresolved question is whether these traces always indicate an active microbial community or whether, in extreme environments, such as hyperarid deserts, they instead reflect just dormant or dead cells. Although microbial biomass and diversity decrease with increasing aridity in the Atacama Desert, we provide multiple lines of evidence for the presence of an at times metabolically active, microbial community in one of the driest places on Earth. We base this observation on four major lines of evidence: (i) a physico-chemical characterization of the soil habitability after an exceptional rain event, (ii) identified biomolecules indicative of potentially active cells [e.g., presence of ATP, phospholipid fatty acids (PLFAs), metabolites, and enzymatic activity], (iii) measurements of in situ replication rates of genomes of uncultivated bacteria reconstructed from selected samples, and (iv) microbial community patterns specific to soil parameters and depths. We infer that the microbial populations have undergone selection and adaptation in response to their specific soil microenvironment and in particular to the degree of aridity. Collectively, our results highlight that even the hyperarid Atacama Desert can provide a habitable environment for microorganisms that allows them to become metabolically active following an episodic increase in moisture and that once it decreases, so does the activity of the microbiota. These results have implications for the prospect of life on other planets such as Mars, which has transitioned from an earlier wetter environment to today’s extreme hyperaridity.

Corrosion Kinetics Studies of AISI 1020 Carbon Steel from Dissolved Oxygen Consumption Measurements in Aqueous Sodium Chloride Solutions

Abstract A study was made of the kinetic behavior of AISI 1020 (UNS G10200) carbon steel corrosion as carbon steel shavings were immersed in distilled water and aqueous 0.02-M and 0.1-M sodium chlo...

Mars-Like Soils in the Yungay Area, the Driest Core of the Atacama Desert in Northern Chile

The data obtained from the Viking lander’s analyses of soils on Mars were unexpected. First, was the finding that when soil samples were exposed to water vapor in the gas exchange experiment (GE) there was rapid release of molecular oxygen, at levels of 70-770 nmole g-1 (Oyama and Berdahl, 1977). The next puzzling result was that organic material in the labeled release experiment (LR) was consumed as would be expected if life would have been present (Levin and Straat, 1977). Lastly, there were no organic materials at levels of part-per-billion (ppb), as measured by pyrolysis-gas chromatography-mass spectrometry (pyr-GC-MS) detected (Biemann et al., 1977); which were in apparent contradiction with the presence of life as detected by the LR experiment. The reactivity of the martian soil is currently believed to result from the presence of one or more inorganic oxidants (e.g., superoxides, peroxides, or peroxynitrates) at the part-per-million (ppm) level. The absence of organics in the soils results from their oxidation by such oxidants and/or direct ultraviolet radiation damage (McKay et al., 1998).

Corrosion of bare carbon steel as a passive sensor to assess moisture availability for biological activity in Atacama Desert soils

Here we consider that the corrosion of polished bared metal coupons can be used as a passive sensor to detect or identify the lower limit of water availability suitable for biological activity in Atacama Desert soils or solid substrates. For this purpose, carbon steel coupons were deposited at selected sites along a west–east transect and removed at predetermined times for morphological inspection. The advantage of this procedure is that the attributes of the oxide layer (corrosion extent, morphology and oxide phases) can be considered as a fingerprint of the atmospheric moisture history at a given time interval. Two types of coupons were used, long rectangular shaped ones that were half-buried in a vertical position, and square shaped ones that were deposited on the soil surface. The morphological attributes observed by SEM inspection were found to correlate to the so-called humectation time which is determined from local meteorological parameters. The main finding was that the decreasing trend of atmospheric moisture along the transect was closely related to corrosion behaviour and water soil penetration. For instance, at the coastal site oxide phases formed on the coupon surface rapidly evolve into well-crystallized species, while at the driest inland site Lomas Bayas only amorphous oxide was observed on the coupons.