Detecting Raman spectra of pigments from gypsum endoliths: is this a useful training for Martian missions?

Raman spectroscopy is an excellent tool for the detection and discrimination of biomolecules e.g. pigments. The highest relevance of Raman spectroscopy in astrobiology has been well-established. The miniaturized, dedicated Raman spectrometers are currently a part of the experimental payload within rovers in the frame of Martian missions Mars 2020 (NASA) and ExoMars (ESA). Finding biomolecules in the rocky environments (for example, detecting carotenoids) would be an amazing outcome of Mars missions. On Earth, semi-translucent or translucent minerals such as gypsum are ideal habitats for endoliths, especially phototrophs. The mineral environment participates in protecting them against harsh superficial environments. Cyanobacteria colonize gypsum, halite, ignimbrite in the Atacama extremely dry zones with high UV flux. Pigments of endoliths encountered there and detected using Raman spectroscopy include mainly UV-screening pigments such as carotenoids and scytonemin. Selenitic gypsum outcrops of Messinian age commonly harbour cyanobacteria and algae also in less harsh environments: in Sicily (annual precipitation 400-600 mm) [1], Poland (600 mm) and Northern Israel (400 mm). Details on the distribution of pigments (including scytonemin) from several gypsum sites in southern Sicily and eastern Poland are presented [2]. Raman investigations using 780, 532 and 445 nm lasers show more detail on the distribution of UV-screening pigments in the dark zones. These dark zones are colonized dominantly by cyanobacteria, mostly by black-bluish Gloeocapsa compacta and yellow-brown Nostoc sp. Raman analysis allows to discriminate between these cyanobacterial taxa. Raman bands of scytonemin at 1593, 1552, 1438 and 1173 cm^-1 were detected in colonies of Nostoc sp. Gloeocapsin, a pigment specific for Gloeocapsa sp, shows characteristic Raman bands similar to anthraquinone-based parietin of lichens: at 1665, 1575, 1378, 1310 and 465 cm^-1 [2]. Both pigments can be used as biomarkers in geobiological and astrobiological studies. Other photosynthetic and protective pigments were also detected: carotenoids, chlorophylls and phycobiliproteins. Deciphering the presence of biomolecules (including pigments) using Raman spectroscopy helps to understand endoliths. Deploying miniature Raman spectrometers at terrestrial Mars analogue localities as well as in depth investigations of colonised gypsum through laboratory microspectrometric investigations is of the highest relevance for the current Martian missions.

References

[1] J. Jehlička, A. Culka, J. Mareš, (2019) Raman spectroscopic screening of cyanobacterial chasmoliths from crystalline gypsum—The Messinian crisis sediments from Southern Sicily, Spectrochimica Acta - Part A: Molecular and Biomolecular Spectroscopy 51, 1802-1812. [2] K. Němečková, A. Culka, I. Němec, H.G.M. Edwards, J. Mareš, J. Jehlička (2021) Raman spectroscopic search for scytonemin and gloeocapsin in endolithic colonisations in large gypsum crystals. Journal of  Raman Spectroscopy 52, 2633-2647.