Destruction of Raman biosignatures by ionising radiation and the implications for life detection on Mars

Abstract

Raman spectroscopy has proven to be a very effective approach for the detection of microorganisms colonising hostile environments on Earth. The ExoMars rover, due for launch in 2018, will carry a Raman laser spectrometer to analyse samples of the martian subsurface collected by the probe's 2-m drill in a search for similar biosignatures. The martian surface is unprotected from the flux of cosmic rays, an ionising radiation field that will degrade organic molecules and so diminish and distort the detectable Raman signature of potential martian microbial life. This study employs Raman spectroscopy to analyse samples of two model organisms, the cyanobacterium Synechocystis sp. PCC 6803 and the extremely radiation resistant polyextremophile Deinococcus radiodurans, that have been exposed to increasing doses of ionising radiation. The three most prominent peaks in the Raman spectra are from cellular carotenoids: deinoxanthin in D. radiodurans and β-carotene in Synechocystis. The degradative effect of ionising radiation is clearly seen, with significant diminishment of carotenoid spectral peak heights after 15 kGy and complete erasure of Raman biosignatures by 150 kGy of ionising radiation. The Raman signal of carotenoid in D. radiodurans diminishes more rapidly than that of Synechocystis, believed to be due to deinoxanthin acting as a superior scavenger of radiolytically produced reactive oxygen species, and so being destroyed more quickly than the less efficient antioxidant β-carotene. This study highlights the necessity for further experimental work on the manner and rate of degradation of Raman biosignatures by ionising radiation, as this is of prime importance for the successful detection of microbial life in the martian near subsurface.

Will Raman meet bacteria on Mars? An overview of the optimal Raman spectroscopic techniques for carotenoid biomarkers detection on mineral backgrounds

“…The peak assignments of the relevant vibrational modes are given in Table 2. As expected, (Vítek et al, 2009a) 1157 (1156) (Dartnell et al, 2012) 1152 (1155) (C-C) str 1003 (1005) (C-CH 3 ) bend Wavenumbers in parentheses refer to this work. sym, symmetric; str, stretch; bend, bending.…”

Selection of Portable Spectrometers for Planetary Exploration: A Comparison of 532 nm and 785 nm Raman Spectroscopy of Reduced Carbon in Archean Cherts

“…Recent studies have indicated that the martian surface radiation environment will likely cause the degradation of carotenoids and similar biomolecules, altering the Raman spectra of any such biosignatures that may be present on Mars (Dartnell et al, 2012). Ancient deposits of reduced carbon, however, are less likely to be affected by solar UV radiation and cosmic rays, providing a potentially more robust marker of habitat and biological activity.…”

Raman Spectroscopy of Microbial Pigments

“…Carotenoids play many vital roles in a large number of prokaryotic and eukaryotic microorganisms: they participate in energy-harvesting complexes, protect cells against damage by short-wavelength visible and UV radiation, and help repair cellular damage (33). They represent a diverse group of chemicals with a conjugated polyene skeletal backbone.…”

“…They represent a diverse group of chemicals with a conjugated polyene skeletal backbone. The presence of this polyene backbone enhances the Raman signals, permitting the detection of extremely small amounts of carotenoids (33,34). The first RRS of carotenoids were reported from 1970 on (23,26).…”