Silica has the highest demonstrated potential of any phase to preserve microfossils on Earth and therefore may host potential biosignatures on Mars. We detected hydrated silica in Jezero crater, the landing site of NASA's Mars 2020 rover mission, by applying Dynamic Aperture Factor Analysis/Target Transformation (DAFA/TT) to images from the Compact Reconnaissance Imaging Spectrometer for Mars (CRISM). Hydrated silica detections with DAFA/TT were verified using commonly accepted CRISM analysis methods. The morphology of geologic units associated with silica were characterized with high resolution imaging. Several hypotheses are presented for the formation environment of hydrated silica. All are testable via in-situ investigation. We assess the likelihood of silica to preserve biosignatures in these different scenarios based on habitability considerations and biosignature preservation in Earth analog environments and materials. Also reported are possible detections of hydrated silica in the Nili Fossae basement and olivine-rich units, as well as Al-phyllosilicate within Jezero crater.

Taylor, R.S., DeMaster, D.J., Burdige, D.J., 2020. Assessing the distribution of labile organic carbon from diverse depositional environments on the West Antarctic Peninsula shelf. Deep Sea Research Part I: Oceanographic Research Papers 156, 103166.

http://www.sciencedirect.com/science/article/pii/S0967063719303127 The seabed distributions of labile organic carbon (LOC), i.e., recently produced organic matter from marine plankton, were studied in a variety of depositional environments on the West Antarctic Peninsula continental 14 210 234 shelf using the naturally occurring radioisotopes, C, Pb and Th. The effects of bioturbation on the LOC 234 pool were assessed on short-term (seasonal, using Th within the upper, 2 - 5 cm) and long-term (decadal, xs 210 using Pb from 3-22 cm) timescales to generate LOC degradation coefficients (k) and mean residence times xs 234 (MRT or τ -LOC) as a function of depth within the sediment column. Based on Th distributions, mixing 2 2 coefficients (D ) ranged from 2 cm /y to 36 cm /y, which resulted in LOC MRT values that varied from 0.8y to b 210 2 2 9.8y. Based on Pb distributions, D values at depth ranged from 0.2 cm /y to 1.8 cm /y, resulting in MRT b values that varied from 32y to 4900y. Along the West Antarctic Peninsula, seabed inventories of LOC varied 2 between 17-70 mg/cm . Bulk LOC becomes less reactive and ages with time during resuspension and transport across the shelf and as LOC is transported deeper within the sediment column by infaunal feeding activities.

Teixeira, L.R., Cordas, C.M., Fonseca, M.P., Duke, N.E.C., Pokkuluri, P.R., Salgueiro, C.A., 2020. Modulation of the redox potential and electron/proton transfer mechanisms in the outer membrane cytochrome OmcF from Geobacter sulfurreducens . Frontiers in Microbiology 10, 2941. doi: 10.3389/fmicb.2019. 02941.

https://www.frontiersin.org/article/10.3389/fmicb.2019.02941 Geobacter sulfurreducens The monoheme outer membrane cytochrome F (OmcF) from plays an important role in Fe(III) reduction and electric current production. The electrochemical characterization of this cytochrome has shown that its redox potential is modulated by the solution pH (redox-Bohr effect) endowing the protein with the necessary properties to couple electron and proton transfer in the physiological range. The analysis of the OmcF structures in the reduced and oxidized states showed that with the exception of the side chain of histidine 47 47 (His ), all other residues with protonatable side chains are distant from the heme iron and, therefore, are 47 unlikely to affect the redox potential of the protein. The protonatable site at the imidazole ring of His is in the close proximity to the heme and, therefore, this residue was suggested as the redox-Bohr center. In the present 47 work, we tested this hypothesis by replacing the His with non-protonatable residues (isoleucine - OmcFH47I and phenylalanine - OmcFH47F). The structure of the mutant OmcFH47I was determined by X-ray crystallography to 1.13 Å resolution and showed only minimal changes at the site of the mutation. Both mutants 15 were N-labeled and their overall folding was confirmed to be the same as the wild-type by NMR spectroscopy. The pH dependence of the redox potential of the mutants was measured by cyclic voltammetry. Compared to the wild-type protein, the magnitude of the redox-Bohr effect in the mutants was smaller, but not 47 fully abolished, confirming the role of His on the pH modulation of OmcF's redox potential. However, the pH effect on the heme substituents' NMR chemical shifts suggested that the heme propionate P also contributes to 13 the overall redox-Bohr effect in OmcF. In physiological terms, the contribution of two independent acid - base centers to the observed redox-Bohr effect confers OmcF a higher versatility to environmental changes by coupling electron/proton transfer within a wider pH range.