Raman Spectroscopic Analysis of Cyanobacteria on Mars Analogue Material

Kurzfassung

Establishing whether life ever existed, or is still active on Mars today, is one of the outstanding scientific questions of our time. In the scope of the ExoMars mission Raman measurements will be performed with the RLS Spectrometer to identify organic compounds and mineral products as indicators of biological activity1. Raman spectra of cyanobacteria (Nostoc commune) on iron (III) oxide and on mineral mixtures assigned to early acidic and late basic Mars have been measured using two excitation wavelengths. The reason why cyanobacteria were chosen is due to an assumption that photosynthesizing cyanobacteria or the anoxigenic photosynthesizing microorganisms as direct ancestors of cyanobacteria appeared for the first time on Early Earth 3.8 to 3.5 Billion years ago. During this time Mars is supposed to have a more temperate climate with a wet phase, where water was supposed to be liquid on the surface and life might have evolved under similar conditions as on Earth. This means that the potential was given on both of the terrestrial planets that ancestors of the cyanobacteria or the cyanobacteria themselves might have evolved in this short time period because of the quiet similar conditions. Some well known pigments of the photosynthesis apparatus of cyanobacteria such as carotene are detectable by Raman spectroscopy and it is of interest to determine these specific pigments even in Mars analogue mineral mixtures. In this context we have investigated if and how the Raman spectrum of the bacteria is influenced by the Raman signal of the mineral background. Raman measurements were performed on pure cyanobacteria to find the spectra that characterize them. The spectrum of beta-carotene is identified to describe the presence of cyanobacteria, which is in good agreement with literature2,3,4. The strong fluorescence of the cyanobacteria above 620 nm favours an excitation wavelength of 532 nm over 633 nm to avoid the fluorescence disturbing the spectrum between 500 cm-1 and 2000 cm-1. Scanning a sample with a mineral/bacteria mixture provides the capability to distinguish between mineral areas with and without bacteria. To get optimal spectra of Fe2O3, mineral mixtures of early acidic and late basic Mars and the cyanobacteria the laser power needs to be adjusted for each of them separately. 1 Rull Pérez F, Martinez-Frias J. Spectroscopy Europe 2006 18: 18–21. 2 Edwards HGM, Villar SEJV, Parnell Jj, Cockell CS, Lee P. The Analyst 2005; 130: 917. 3 Vitek P, Jehlicka J, Edwards HGM. Anal Bioanal Chem 2009; 393:1967 4 Edwards HGM, Wynn-Williams DD, Villar SEJV. J. Raman Spectroscopy 2004; 35: 470.