Detecting microbial pigments from gypsum using Raman spectroscopy: from field prospection to laboratory studies

Terrestrial detection of biomarkers in various mineral matrices using Raman spectrometers including field deploying of miniature instrumentation in Mars-analogue sites can be seen as a training for next Martian missions. In fact, both the European Space Agency (Exomars) and North American Space Agency (Mars 2020) robotic rovers will include Raman spectrometers. Feasibility of detecting biomarkers of extremophilic cyanobacteria and algae (pigments, osmotic solutes and lipids) using Raman microspectrometry was reviewed previously. Here the idea is to show - firstly how portable Raman instrumentation permits to detect carotenoids fast and onsite under field conditions. Secondly, laboratory microspectrometric investigations allow to obtain more detailed information about spatial distribution of pigments originating from microorganisms.

Macrocrystalline gypsum layers and aggregates are well-known from Tertiary series in Sicily and Eastern Poland. In Southern Sicily gypsum sediments accumulated during Messinian crisis (Late Miocene) are outcroping and were investigated near Scala dei Turchi, Torre Salsa and Siculiana Marina. Polish Tertiary (Badenian, Middle Miocene) examples of gypsum colonisations of decimetre long outcropping crystals were studied near Chotel Czerwony, Skorocice and Chwalowice. Miniature portable Raman spectrometers equipped with green lasers allowing recording of resonance Raman signals of carotenoids are evaluated here. Possibilities of collecting spectra of carotenoids under non-resonant conditions using a portable sequentially shifted Raman spectrometer (785 and 853nm lasers) are shown as well. Observed shifts of positions of Raman features of carotenoids between gypsum samples (and sites) are discussed and critically evaluated. In addition, acquired data are compared to data obtained through laboratory Raman microspectrometric investigations. Selected zones of microbial colonisations of few types of gypsum are described from the point of view of the presence of algae and cyanobacteria. Pigments are detected through conventional Raman microspectrometric measurements. Carotenoids were documented in major part of samples (common Raman bands at around 1525, 1157, and 1004 cm⁻¹). Additionally, Raman spectra of other pigments were recorded in several zones using near infrared excitation (785 nm): chlorophyll (1151, 1327, 1287, 1184, 917, and 745 cm⁻¹), scytonemin (1593, 1152, 1438, and 1173 cm⁻¹) and phycobiliproteins (1633, 1584, 1371, 1236, 813, and 667 cm⁻¹).

Portable instrumentation permits detection of carotenoids in gypsum fast and onsite under field conditions. Raman microspectrometric investigations of colonisations allow to gather detailed information about pigment distribution in micrometric zones of gypsum samples.