Soil samples from the hyper-arid region in the Atacama Desert in Southern Peru (La Joya Desert) were analyzed for total and labile organic carbon (TOC and LOC), phospholipid fatty acids (PLFA), quantitative real time polymerase chain reaction (qRT-PCR), 4′,6-diamidino-2-phenylindole (DAPI)-fluorescent microscopy, culturable microorganisms, and oxidant activity, to understand the relationship between the presence of organic matter and microorganisms in these types of soils. TOC content levels were similar to the labile pool of carbon suggesting the absence of recalcitrant carbon in these soils. LOC ranged between 2 to 60 µg/g of soil. PLFA analysis indicated a maximum of 2.3 × 10⁵ cell equivalents/g. Culturing of soil extracts yielded 1.1 × 10²–3.7 × 10³ CFU/g. qRT-PCR showed between 1.0 × 10² and 8 × 10³ cells/g; and DAPI fluorescent staining indicated bacteria counts up to 5 × 10⁴ cells/g. Arid and semiarid samples (controls) showed values between 10⁷ and 10¹¹ cells/g with all of the methods used. Importantly, the concentration of microorganisms in hyper-arid soils did not show any correlation with the organic carbon content; however, there was a significant dependence on the oxidant activity present in these soil samples evaluated as the capacity to decompose sodium formate in 10 hours. We suggest that the analysis of oxidant activity could be a useful indicator of the microbial habitability in hyper-arid soils, obviating the need to measure water activity over time. This approach could be useful in astrobiological studies on other worlds.

Oxygen isotope compositions of tooth enamel increments in theropod dinosaurs are investigated as potential proxies of climate seasonality. Six teeth of large carnivorous theropods collected from four Cretaceous formations deposited under contrasted climates have been sampled. These teeth have been analyzed for the oxygen isotope compositions of their apatite phosphate (δ¹⁸O_p) through incremental sampling performed along the major growth axis. Significant fluctuations in oxygen isotope compositions along the growth axis of each tooth are observed and interpreted as reflecting seasonality in ingested local surface water δ¹⁸O_w values. Fluctuations in δ¹⁸O_p values of theropod teeth from the Aptian of Thailand and Cenomanian of Morocco vary similarly to meteoric water δ¹⁸O_mw values occurring today in sub-tropical regions subjected to large seasonal amounts of precipitations. A dinosaur tooth recovered from the more inland and mid-latitude Nemegt Formation of Mongolia shows a seasonal pattern similar to present-day cold temperate and continental climate. Finally, the high latitude and coastal Kakanaut Formation (Russia) experienced strongly dampened seasonal variations, most likely due to the influence of warm Pacific oceanic currents. Such conditions occur today in high latitude regions submitted to marine influence. These results further highlight the potential of using the oxygen isotope compositions of large theropod teeth to reconstruct past seasonal variations of terrestrial climates. Increased knowledge of past seasonality may help to better understand the complex interactions between climate and the dynamics of land biodiversity in terms of ecological adaptations, biogeography and the evolutionary history of organisms.