Carotenoid analysis of Halophilic Archaea by Resonance Raman spectroscopy

Abstract

Recently, halite and sulfate evaporate rocks have been discovered on Mars by the NASA rovers, Spirit and Opportunity. It is reasonable to propose that halophilic microorganisms could have potentially flourished in these settings. If so, biomolecules found in microorganisms adapted to high salinity and basic pH environments on Earth may be reliable biomarkers for detecting life on Mars. Therefore, we investigated the potential of Resonance Raman (RR) spectroscopy to detect biomarkers derived from microorganisms adapted to hypersaline environments. RR spectra were acquired using 488.0 and 514.5 nm excitation from a variety of halophilic archaea, including Halobacterium salinarum NRC-1, Halococcus morrhuae, and Natrinema pallidum. It was clearly demonstrated that RR spectra enhance the chromophore carotenoid molecules in the cell membrane with respect to the various protein and lipid cellular components. RR spectra acquired from all halophilic archaea investigated contained major features at approximately 1000, 1152, and 1505 cm−1. The bands at 1505 cm−1 and 1152 cm−1 are due to in-phase C=C (ν1 ) and C–C stretching ( ν2 ) vibrations of the polyene chain in carotenoids. Additionally, in-plane rocking modes of CH3 groups attached to the polyene chain coupled with C–C bonds occur in the 1000 cm−1 region. We also investigated the RR spectral differences between bacterioruberin and bacteriorhodopsin as another potential biomarker for hypersaline environments. By comparison, the RR spectrum acquired from bacteriorhodopsin is much more complex and contains modes that can be divided into four groups: the C=C stretches (1600–1500 cm−1), the CCH in-plane rocks (1400–1250 cm−1), the C–C stretches (1250–1100 cm−1), and the hydrogen out-of-plane wags (1000–700 cm−1). RR spectroscopy was shown to be a useful tool for the analysis and remote in situ detection of carotenoids from halophilic archaea without the need for large sample sizes and complicated extractions, which are required by analytical techniques such as high performance liquid chromatography and mass spectrometry.