Experimental investigations of the preservation/degradation of microbial signatures in the presence of clay minerals under Martian subsurface conditions

The astrobiological exploration of Mars is ongoing. Ancient Martian terrains covered by clay minerals are seen as the most promising targets to search for biosignatures as clay minerals are believed to have a high potential for biopreservation. Here, we experimentally investigate the biopreservation potential of saponite, a Mg-rich smectite, relying on series of experiments conducted with mixtures of E. coli cells and saponite exposed to thermal conditions (up to 200 degrees C) for different durations (up to 100 days) under a Martian atmosphere (CO2) in the presence of water, thereby simulating episodes of fluid circulation typical of those periodically occurring on Mars. Residues were characterized using elemental analyses, mid-infrared (Mid-IR) spectroscopy and X-ray diffraction (XRD) to document the chemistry of the residual organic materials as well as the nature and crystallinity of the residual mineral materials. Results show that saponite may delay the chemical degradation of some organic materials by selectively trapping N-rich organic compounds within its interlayer space. However, such trapping of N-rich organic compounds does not completely protect them from degradation with increasing temperature and experimental duration, as shown by the N/C atomic ratios, & delta;13C values, Mid-IR spectra and XRD patterns of the residues of experiments conducted at 200 degrees C or at 150 degrees C for 100 days. This suggests that saponite is not that efficient in protecting biogenic organic compounds from thermal degradation over long periods of time. Altogether, the present study provides information on what can be found (and thus pinpointing what should be searched for) in the ancient Martian geological record.