Phototrophic Community in Gypsum Crust from the Atacama Desert Studied by Raman Spectroscopy and Microscopic Imaging

The hyperarid core of the Atacama Desert represents one of the driest places on Earth with an exceptional occurrence of microbial life coping with extreme environmental stress factors. The gypsum crusts have already been found to harbor diverse communities in this area. Here, we present a Raman spectroscopic study, complemented by correlative microscopic imaging using SEM-BSE and fluorescence microscopy, of the endolithic microbial communities inside the Ca-sulphate crusts dominated by phototrophic microorganisms. Differences of pigment composition within different zones follow the cyanobacterial and algal colonization and also reveal the degradation of phycobiliproteins within the decayed biomass of cyanobacteria. Carotenoids of at least three different types were recognized, differing in dependence on the particular phylum of phototrophic microorganisms. Moreover, calcium oxalate monohydrate - whewellite - was found to be associated with the algae and hyphal associations living in the lower regions of the crust. The 785nm excitation wavelength employed here was found to be the correct source for studying pigment composition as well as for the detection of the oxalate. A comparison of these results with those using 514.5nm laser excitation which is widely adopted for the detection of carotenoids due to the resonance Raman effect is made and discussed.