Abstract
We present a comparison of the performance of four miniature portable Raman spectrometers for the discrimination of carotenoids in samples of carotene-producing microorganisms. Two spectrometers using a green laser allowing to obtain Resonance Raman (or pre-Resonance Raman) signals, one instrument with a 785 nm laser, and a recently developed Portable Sequentially Shifted Excitation Raman spectrometer (PSSERS) were used for identifying major pigments of different halophilic (genera Halobacterium, Halorubrum, Haloarcula, Salinibacter, Ectothiorhodospira, Dunaliella) and non-halophilic microorganisms (Micrococcus luteus, Corynebacterium glutamicum). Using all the tested instruments including the PSSERS, strong carotenoids signals corresponding to the stretching vibrations in the polyene chain and in-plane rocking modes of the attached CH3 groups were found at the correct positions. Raman spectra of carotenoids can be obtained from different types of microbiological samples (wet pellets, lyophilized culture biomass and pigment extracts in organic solvents), and can be collected fast and without time-consuming procedures.
Highlights
Raman spectra of microorganisms can tell us about the presence of key biomarkers playing different roles for energy recovery and microorganisms survival
The presence of polyenic chains is the reason of the enhancement of the signal through the Resonance Raman effect (Merlin, 1985; Marshall et al, 2007)
Raman spectroscopy has an excellent sensitivity for detecting polyenes, including carotenoids, because of the characteristic strong spectral signatures of the conjugated C–C and C=C functional modes
Summary
Raman spectra of microorganisms can tell us about the presence of key biomarkers playing different roles for energy recovery and microorganisms survival. Detecting carotenoids or even their discrimination through Resonance Raman spectroscopy using green excitation is widely used (Marshall et al, 2007; Jehlicka et al, 2014a). Raman microspectrometry may be advantageous to learn about colonization of more or less extreme environments such as the dry Atacama or the Mojave Desert (Vítek et al, 2012) or biofilms on limestones in Europe (Storme et al, 2015). Raman spectra of carotenoids of different taxonomic groups of cyanobacteria, Archaea or algae were recorded from specimens grown in laboratory cultures (Jehlicka et al, 2014a; Kumar et al, 2015; Stoeckel et al, 2015). Other pigments were investigated as well using Raman spectroscopy. Examples are prodigiosin from Serratia marcescens (Jehlicka et al, 2016), violacein
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