Remote Laser-Induced Breakdown Spectroscopy of Bacterial Growths in Carbonate Rocks in a Mars-like Atmosphere.

Understanding the past habitable environments of Mars increases the requirement to recognize and examine modern analogs and to evaluate the mechanisms that may preserve biosignatures in them. The phenomenon that originates and preserves possible microbial biosignatures in mineral phases is of particular interest in astrobiology. On Earth, the precipitation of carbonate matrices can be mediated by bacteria. Besides microbialites and other sedimentary structures, carbonate formations can be observed in certain karstic caves. The present work is focused on the remote laser-induced breakdown spectroscopy (LIBS) characterization of cyanobacteria, exploring the possibilities for identification and discrimination on carbonate substrates. For this purpose, the extremophile cyanobacterium Chroococcidiopsis sp. (collected from the Nerja Cave, Malaga, Spain) was analyzed under laboratory-simulated martian conditions in terms of chemical composition and gas pressure. LIBS results related to acquired molecular emission features allowed bacterial differentiation from the colonized mineral substrate. In addition, the limits of detection were estimated with a laboratory-grown culture of the cyanobacterium Microcystis aureginosa. Our results reveal LIBS's capability to detect biological traces under simulated martian conditions. Additionally, the time-resolved analysis of the biological samples demonstrates the selection of optimal temporal conditions as a critical parameter for the preferential acquisition of molecular species in organic material.

Microbial communities in carbonate precipitates from drip waters in Nerja Cave, Spain.

Research on cave microorganisms has mainly focused on the microbial communities thriving on speleothems, rocks and sediments; however, drip water bacteria and calcite precipitation has received less attention. In this study, microbial communities of carbonate precipitates from drip waters in Nerja, a show cave close to the sea in southeastern Spain, were investigated. We observed a pronounced difference in the bacterial composition of the precipitates, depending on the galleries and halls. The most abundant phylum in the precipitates of the halls close to the cave entrance was Proteobacteria, due to the low depth of this sector, the direct influence of a garden on the top soil and the infiltration of waters into the cave, as well as the abundance of members of the order Hyphomicrobiales, dispersing from plant roots, and other Betaproteobacteria and Gammaproteobacteria, common soil inhabitants. The influence of marine aerosols explained the presence of Marinobacter, Idiomarina, Thalassobaculum, Altererythrobacter and other bacteria due to the short distance from the cave to the sea. Nineteen out of forty six genera identified in the cave have been reported to precipitate carbonate and likely have a role in mineral deposition.