Evaluation of pore-fracture microstructure of gypsum rock fragments using micro-CT.

This study utilized X-ray micro-computed tomography (micro-CT) to investigate weathered gypsum rocks which can or do serve as a rock substrate for endolithic organisms, focusing on their internal pore-fracture microstructure, estimating porosity, and quantitative comparison between various samples. Examining sections and reconstructed 3D models provides a more detailed insight into the overall structural conditions within rock fragments and the interconnectivity in pore networks, surpassing the limitations of analyzing individual 2D images. Results revealed diverse gypsum forms, cavities, fractures, and secondary features influenced by weathering. Using deep learning segmentation based on the U-Net models within the Dragonfly software enabled to identify and visualize the porous systems and determinate void space which was used to calculate porosity. This approach allowed to describe what type of microstructures and cavities is responsible for the porous spaces in different gypsum samples. A set of quantitative analysis of the detected void and modeled networks provided a needed information about the development of the pore system, connectivity, and pore size distribution. Comparison with mercury intrusion porosimetry showed that both methods consider different populations of pores. In our case, micro-CT typically detects larger pores (> 10 µm) which is related to the effective resolution of the scanned images. Still, micro-CT demonstrated to be an efficient tool in examining the internal microstructures of weathered gypsum rocks, with promising implications particularly in geobiology and microbiology for the characterization of lithic habitats.

Gypsum endolithic phototrophs under moderate climate (Southern Sicily): their diversity and pigment composition.

In this study, we used microscopic, spectroscopic, and molecular analysis to characterize endolithic colonization in gypsum (selenites and white crystalline gypsum) from several sites in Sicily. Our results showed that the dominant microorganisms in these environments are cyanobacteria, including: Chroococcidiopsis sp., Gloeocapsopsis pleurocapsoides, Gloeocapsa compacta, and Nostoc sp., as well as orange pigmented green microalgae from the Stephanospherinia clade. Single cell and filament sequencing coupled with 16S rRNA amplicon metagenomic profiling provided new insights into the phylogenetic and taxonomic diversity of the endolithic cyanobacteria. These organisms form differently pigmented zones within the gypsum. Our metagenomic profiling also showed differences in the taxonomic composition of endoliths in different gypsum varieties. Raman spectroscopy revealed that carotenoids were the most common pigments present in the samples. Other pigments such as gloeocapsin and scytonemin were also detected in the near-surface areas, suggesting that they play a significant role in the biology of endoliths in this environment. These pigments can be used as biomarkers for basic taxonomic identification, especially in case of cyanobacteria. The findings of this study provide new insights into the diversity and distribution of phototrophic microorganisms and their pigments in gypsum in Southern Sicily. Furthemore, this study highlights the complex nature of endolithic ecosystems and the effects of gypsum varieties on these communities, providing additional information on the general bioreceptivity of these environments.

Evaluation of handheld and portable Raman spectrometers with different laser excitation wavelengths for the detection and characterization of organic minerals.

Organic minerals occur rather rarely in some types of peat bogs, sedimentary geological environments, and hydrothermal veins. Commonly, calcium oxalates are produced by several plants, terpenoids are often associated with conifers. Because of the organic precursor, these minerals, from the smallest group of the mineralogical system, are sometimes considered as biomarkers. Potential detection of these compounds has high relevance in the fields of exobiology or geobiology. Here we show the potential of four portable Raman spectrometers, using different excitation wavelengths and technologies (operating at 532, 785, and 1064nm together with an advanced spectrometer using the sequentially shifted excitation (SSE) technology), for the rapid and non-destructive identification of these phases. For the organic minerals investigated here, the most intense Raman bands are generally detected at the expected wavenumber positions ±1-4cm-1 in the region 100-2000cm-1 in the spectra obtained from all spectrometers. Additionally, two spectrometers (the 532nm instrument and the SSE) are capable of detecting Raman bands in the higher wavenumber shift region of 2000-3500cm-1, allowing the more detailed characterization and differentiation of the related phases. From this work, and on the basis of the experimental data obtained, it is clear that the longer laser excitation wavelengths are more preferable for organic minerals identification due to the better mitigation of fluorescence emission. In contrast, the Raman spectrometer equipped with the shortest excitation wavelength (532nm) gives a significantly higher spectral resolution and a more detailed discrimination of the Raman bands, provided that the conditions of general lower level of fluorescence emission are met. The results presented in the current study complement the knowledge on minerals and biomarkers of relevance for Martian environments which have been measured with mobile Raman spectrometers. The outcome creates a solid base towards the use of lightweight mobile Raman systems that can be used outdoors and on terrestrial outcrops. Moreover, these results and conclusions are of use for the further development of dedicated spectrometers destined for the instrumental suites on planetary rovers, in the frame of the forthcoming exobiology focused missions to Mars to be launched by NASA and ESA.

Detection of carotenoids of halophilic prokaryotes in solid inclusions inside laboratory-grown chloride and sulfate crystals using a portable Raman spectrometer: applications for Mars exploration.

Inclusions in evaporitic minerals sometimes contain remnants of microorganisms or biomarkers, which can be considered as traces of life. Raman spectroscopy with resonance enhancement is one of the best analytical methods to search for such biomarkers in places of interest for astrobiology, including the surface and near subsurface of planet Mars. Portable Raman spectrometers are used as training tools for detection of biomarkers. Investigations of the limits and challenges of detecting biomolecules in crystals using Raman spectroscopy is important because natural occurrences often involve mineral assemblages as well as their fluid and solid inclusions. A portable Raman spectrometer with 532 nm excitation was used for detection of carotenoid biomarkers: salinixanthin of Salinibacter ruber (Bacteroidetes) and α-bacterioruberin of Halorubrum sodomense (Halobacteria) in laboratory-grown artificial inclusions in compound crystals of several chlorides and sulfates, simulating entrapment of microorganisms in evaporitic minerals. Crystals of halite (NaCl), sylvite (KCl), arcanite (K2SO4) and tschermigite ((NH4)Al(SO4)2·12H2O) were grown from synthetic solutions that contained microorganisms. A second crystalline layer of NaCl or K2SO4 was grown subsequently so that primary crystals containing microorganisms are considered as solid inclusions. A portable Raman spectrometer with resonance enabling excitation detected signals of both carotenoid pigments. Correct positions of diagnostic Raman bands corresponding to the specific carotenoids were recorded.