Abstract EANA2026-105

The distribution of microbial communities is strongly controlled by local nutrient availability, which frequently results from mineral weathering and erosion releasing nutrient elements, including those important to the cellular metallome. On Earth, basaltic rocks in habitable alkaline lacustrine environments are often initially colonised by ‘pioneer’ microbial communities that induce the precipitation of thin microbial carbonate ‘coatings’ over basalt on the scale of several hundred microns to millimetres. On Mars, if a biosphere ever emerged in Noachian–Hesperian alkaline lakes, similar microbial carbonate ‘coating’ precipitates may be expected to develop over mafic volcanic substrates. Identifying and characterising such phenomena in terrestrial Mars-analogue settings thus provides critical guidance for identifying potential microbe–basalt interactions in ancient Martian lakeshore sedimentary successions, which may preserve evidence of an ancient Martian biosphere. 

We report the micromorphology and biogeochemistry of post-Palaeogene pioneer microbial communities preserved within carbonate coatings on basaltic substrates at two terrestrial planetary field analogues: Lake Ashenge (Tigray, Ethiopia) and the Carri Laufquen lakes (Patagonia, Argentina). Using a correlated approach combining optical and electron microscopy, spectroscopy and in situ mass spectrometry, we show that cellular fossils of the pioneer community in both lakes are preserved within carbonate coatings directly over regions of microscale dissolution of feldspar and volcanic glass, which presumably released essential elemental nutrients, encouraging localised biomass growth that eventually formed microbial carbonate coatings over the substrate. Three-dimensional X-ray attenuation data from computed microtomography (X-ray µCT) show that the carbonates preserving the pioneer community are dense relative to the overlying stromatolites, supporting the uptake of metals into biomass within this specific community. At the micro–nanoscale, organic materials preserve carbon–nitrogen associations within amorphous and microcrystalline phases, including phyllosilicates and carbonates.

Based on our observations of these two basalt-hosted alkaline lakes on Earth, we propose an astrobiological model for the potential widespread formation and preservation of such carbonate ‘coating’ structures in lacustrine settings on early Mars. Carbonated mafic igneous rock sequences, including coatings, have already been observed—and some sampled—by the Mars 2020 Perseverance rover; these may be biosignature-bearing. Deposits in alkaline lakes on Earth therefore provide a test bed for biosignature searches in such rocks. We emphasise, however, that detecting such subtle, often micrometric, microbial biosignatures within carbonate coatings likely lies beyond the capabilities of rover instrumentation and necessitates advanced analysis in terrestrial laboratories following Mars Sample Return.