Raman spectroscopy for future planetary exploration: photodegradation, self-absorption and quantification of carotenoids in microorganisms and mineral matrices

Carotenoids are among the key biomarkers in the search for life on other planets, and non-destructive Raman spectroscopy on future rover missions is a potential sensitive detection method, especially under resonant conditions. In this research, reflectance spectra of minerals and microorganisms were measured using ultraviolet/visible diffuse reflectance spectroscopy in order to evaluate potential resonance Raman conditions and the possible degree of sample damage during laser irradiation. We report a photodegradation and semi-quantitative Raman study of β-carotene and the carotenoid-containing extremophile Deinococcus radiodurans mixed with calcite at excitation wavelengths of 440 nm, 532 nm and 785 nm. A different type of carotenoid was detected in a culture of Chroococcidiopsis. Carotenoids embedded in bacterial membranes were found to be less sensitive to photodegradation than in a mineral matrix. Corrections for self-absorption effects were performed using the 1085 cm-1 peak of calcite as an internal standard. Carotenoid-type signals from 1 mg·g-1 D. radiodurans in calcite could be detected, corresponding to about 5 μg·g-1 β-carotene in calcite (≈5% cell weight). This research emphasises the potential suitability of Raman spectroscopy in the detection of organic biomarkers in future planetary exploration.