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

Abstract

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 (≈0.5% cell weight). This research emphasizes the potential suitability of Raman spectroscopy in the detection of organic biomarkers in future planetary exploration. Copyright © 2015 John Wiley & Sons, Ltd.