Abstract
Recent studies regarding the origins of life and Mars-Earth meteorite transfer simulations suggest that biological informational polymers, such as nucleic acids (DNA and RNA), have the potential to provide unambiguous evidence of life on Mars. To this end, we are developing a metagenomics-based life-detection instrument which integrates nucleic acid extraction and nanopore sequencing: the Search for Extra-Terrestrial Genomes (SETG). Our goal is to isolate and sequence nucleic acids from extant or preserved life on Mars in order to determine if a particular genetic sequence (1) is distantly related to life on Earth, indicating a shared ancestry due to lithological exchange, or (2) is unrelated to life on Earth, suggesting a convergent origins of life on Mars. In this study, we validate prior work on nucleic acid extraction from cells deposited in Mars analog soils down to microbial concentrations (i.e., 104 cells in 50 mg of soil) observed in the driest and coldest regions on Earth. In addition, we report low-input nanopore sequencing results from 2 pg of purified Bacillus subtilis spore DNA simulating ideal extraction yields equivalent to 1 ppb life-detection sensitivity. We achieve this by employing carrier sequencing, a method of sequencing sub-nanogram DNA in the background of a genomic carrier. After filtering of carrier, low-quality, and low-complexity reads we detected 5 B. subtilis reads, 18 contamination reads (including Homo sapiens), and 6 high-quality noise reads believed to be sequencing artifacts.
Highlights
Major strides in understanding the origins of life and meteorite transfer simulations support the notion that life on Mars, if it ever existed, may share a common genesis or perhaps share a common ancestry with life on Earth
Water extractions of spore DNA remained consistent throughout the dilution series
Our results indicate that a combination of desalting and competitive binding is a viable approach for achieving extraction yields equivalent to 1 ppb life detection (i.e., 2 pg of DNA from 104 cells in 50 mg of soil)
Summary
Major strides in understanding the origins of life and meteorite transfer simulations support the notion that life on Mars, if it ever existed, may share a common genesis or perhaps share a common ancestry with life on Earth. Analogous prebiotic environments ( Johnson et al, 2008; Morris et al, 2010; Stoker et al, 2010; Grotzinger et al, 2014; Ranjan and Sasselov, 2017; Ranjan et al, 2017), molecular feedstocks (e.g., hydrogen cyanide) (Brack and Pillinger, 1998; Parker et al, 2011; Adcock et al, 2013), and plausible abiotic reactive pathways predicted on Earth and applicable on Mars may have resulted in parallel origin events in accordance with the RNA-world hypothesis (Powner et al, 2009, 2010; McKay, 2010; Ritson and Sutherland, 2012; Benner and Kim, 2015; Patel et al, 2015; Stairs et al, 2017) This hypothesis suggests that past or present martian life may have utilized known building blocks (e.g., nucleic acids, sugars, amino acids) and closely resembled life as we know it.
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