Nitrates as a Potential N Supply for Microbial Ecosystems in a Hyperarid Mars Analog System

Abstract

Nitrate is common in Mars sediments owing to long-term atmospheric photolysis, oxidation, and potentially, impact shock heating. The Atacama Desert in Chile, which is the driest region on Earth and rich in nitrate deposits, is used as a Mars analog in this study to explore the potential effects of high nitrate levels on growth of extremophilic ecosystems. Seven study sites sampled across an aridity gradient in the Atacama Desert were categorized into 3 clusters—hyperarid, middle, and arid sites—as defined by essential soil physical and chemical properties. Intriguingly, the distribution of nitrate concentrations in the shallow subsurface suggests that the buildup of nitrate is not solely controlled by precipitation. Correlations of nitrate with SiO2/Al2O3 and grain sizes suggest that sedimentation rates may also be important in controlling nitrate distribution. At arid sites receiving more than 10 mm/yr precipitation, rainfall shows a stronger impact on biomass than nitrate does. However, high nitrate to organic carbon ratios are generally beneficial to N assimilation, as evidenced both by soil geochemistry and enriched culturing experiments. This study suggests that even in the absence of precipitation, nitrate levels on a more recent, hyperarid Mars could be sufficiently high to benefit potentially extant Martian microorganisms.

Unraveling biogeochemical phosphorus dynamics in hyperarid Mars‐analogue soils using stable oxygen isotopes in phosphate

“…For this study, five sites were sampled on a latitudinal gradient from 22°S to 28°S (Figure 2). This sampling strategy was selected to provide an aridity gradient from hyperarid in the north to arid in the south (Figure 3); the sites are María Elena South (MES, 1,493 m altitude) (Azua‐Bustos, Caro‐Lara, & Vicuna, 2015), Point of No Return Dos (PONR‐2, 1,493 m altitude), Yungay (1,007 m altitude), Transition Zone 4 (TZ‐4, 1,658 m altitude), and Transition Zone 5 (TZ‐5, 588 m altitude), with 0–23 mm average annual precipitation estimated by Shen et al (2019) shown in Table 1. We note that visual small plant vegetation appeared in TZ‐4 and TZ‐5, and dried grass roots appeared in their soil samples (Figure 3b).…”

“…Similarly, we recently postulated that bulk N concentrations, largely supplied as nitrate by atmospheric deposition (Ewing et al, 2007;Michalski et al, 2004), could be diluted by higher sedimentation rates and thus more limited to microorganisms (Shen et al, 2019).…”

“…In normal soil environments, resin‐extractable P is turned over on timescales from seconds to minutes, oxide‐bound P is recycled on weekly to monthly timescales, and Ca‐bound P over years to millennia (Helfenstein et al, 2020; Helfenstein et al, 2018). The least bioavailable P pool is therefore normally Ca‐bound P. In an alkaline environment like the Atacama Desert (Crits‐Christoph et al, 2013; Makhalanyane et al, 2015; Shen, Zerkle, Stueken, & Claire, 2019) or on Mars (pH 7.2–8.2; (Hecht et al, 2009; Smith et al, 2009)), where resin‐extractable P (bioavailable P) is high, it would tend to be sorbed by calcite (Reddy, Wetzel, & Kadlec, 2005) and incorporated through precipitation into Ca‐bound P mineral phases (Kreuzeder, 2011), making it less bioavailable (Oburger, Jones, & Wenzel, 2011; Urrutia et al, 2014). However, where this Ca‐bound P is a major source of P within a biological system it can be reversibly solubilized by the excretion of acids by specialized microbial communities, breaking down Ca‐P and once again making it bioavailable.…”

“…Isotopic data here show mean ± standard error. The standard deviations of MgO and Fe 2 O 3 are between 0.01 and 0.05% based on precision analyses of X-ray fluorescence standards(Shen et al, 2019)…”

Phospholipid biomarkers in Mars-analogous soils of the Atacama Desert

“…Environmental variables such as elevation, pH, electrical conductivity, salt concentrations, total organic carbon (TOC) and total organic nitrogen (TON) were reported in Shen et al (2019). δ 13 C of TOC and δ 15 N of TON were determined together with TOC and TON by Elemental Analyzer-Isotope Ratio Mass Spectrometry (EA-IRMS) (Shen et al, 2019). Concentrations of microbial P extracted using the sequential soil phosphate extraction method (Tamburini et al, 2018) were reported in Shen et al (2020b).…”