Abstract
The next NASA-led Mars mission (Mars 2020) will carry a suite of instrumentation dedicated to investigating Martian history and the in situ detection of potential biosignatures. SHERLOC, a deep UV Raman/Fluorescence spectrometer has the ability to detect and map the distribution of many organic compounds, including the aromatic molecules that are fundamental building blocks of life on Earth, at concentrations down to 1 ppm. The mere presence of organic compounds is not a biosignature: there is widespread distribution of reduced organic molecules in the Solar System. Life utilizes a select few of these molecules creating conspicuous enrichments of specific molecules that deviate from the distribution expected from purely abiotic processes. The detection of far from equilibrium concentrations of a specific subset of organic molecules, such as those uniquely enriched by biological processes, would comprise a universal biosignature independent of specific terrestrial biochemistry. The detectability and suitability of a small subset of organic molecules to adequately describe a living system is explored using the bacterium Escherichia coli as a model organism. The DUV Raman spectra of E. coli cells are dominated by the vibrational modes of the nucleobases adenine, guanine, cytosine, and thymine, and the aromatic amino acids tyrosine, tryptophan, and phenylalanine. We demonstrate that not only does the deep ultraviolet (DUV) Raman spectrum of E. coli reflect a distinct concentration of specific organic molecules, but that a sufficient molecular complexity is required to deconvolute the cellular spectrum. Furthermore, a linear combination of the DUV resonant compounds is insufficient to fully describe the cellular spectrum. The residual in the cellular spectrum indicates that DUV Raman spectroscopy enables differentiating between the presence of biomolecules and the complex uniquely biological organization and arrangements of these molecules in living systems. This study demonstrates the ability of DUV Raman spectroscopy to interrogate a complex biological system represented in a living cell, and differentiate between organic detection and a series of Raman features that derive from the molecular complexity inherent to life constituting a biosignature.
Highlights
The search for life beyond Earth has motivated decades of planetary exploration from the first life detection experiments on board the Viking missions to Mars in 1975 (Klein et al, 1972; Soffen and Snyder, 1976; Levin and Straat, 1977) to current exploration by the Mars Science Laboratory (Summons et al, 2011; Grotzinger et al, 2012) to future missions to potentially habitable icy bodies in our Solar System (e.g., Phillips and Pappalardo, 2014)
We have confirmed that we can differentiate a cell from DNA based on its spectra and that the resulting spectra cannot be explained by the spectral contribution of aromatic amino acids (AAAs), but rather is primarily due to the intracellular pool of free nucleotides combined with the hypochromatism of nucleobases when stacked in nucleic acids
We have shown that nucleotides are of sufficient structural complexity to adequately describe cellular spectra, and that obtaining standard spectra of more complex molecules may not be necessary to identify biosignatures using Raman
Summary
The search for life beyond Earth has motivated decades of planetary exploration from the first life detection experiments on board the Viking missions to Mars in 1975 (Klein et al, 1972; Soffen and Snyder, 1976; Levin and Straat, 1977) to current exploration by the Mars Science Laboratory (Summons et al, 2011; Grotzinger et al, 2012) to future missions to potentially habitable icy bodies in our Solar System (e.g., Phillips and Pappalardo, 2014). SHERLOC has the ability to detect the Raman peaks of organic molecules in situ at concentrations below 0.1 wt% in a 100 micron area and 10-5 ww over an observed area of 5 mm × 5 mm (Abbey et al, 2017) This includes the nucleobases and amino acids known to be essential components of terrestrial life. The mere presence of organic compounds is not evidence of life – here we demonstrate a spectral threshold for biogenicity based on the Raman active components within a cell compared to the Raman spectra of the cell itself These results provide a context for the interpretation of DUV Raman spectra of organic molecules collected by SHERLOC on Mars 2020 as potential biosignatures
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