Bacterial influence on the formation of hematite: implications for Martian dormant life

Previous exploration missions have revealed Mars as a potential candidate for the existence of extraterrestrial life. If life could have existed beneath the Martian subsurface, biosignatures would have been preserved in iron-rich minerals. Prior investigations of terrestrial biosignatures and metabolic processes of geological analogues would be beneficial for identifying past metabolic processes on Mars, particularly morphological and chemical signatures indicative of past life, where biological components could potentially be denatured following continued exposure to extreme conditions. The objective of the research was to find potential implications for Martian subsurface life by characterizing morphological, mineralogical and microbial signatures of hematite deposits, both hematite rock and related soil samples, collected from Highland Complex of Sri Lanka. Rock samples examined through scanning electron microscopy-energy dispersive X-ray (SEM-EDX) spectroscopy. Analysis showed globular and spherical growth layers nucleated by bacteria. EDX results showed a higher iron to oxygen ratio in nuclei colonies compared to growth layers, which indicated a compositional variation due to microbial interaction. X-ray diffraction analysis of the hematite samples revealed variations in chemical composition along the vertical soil profile, with the top surface soil layer being particularly enriched with Fe2O3, suggesting internal dissolution of hematite through weathering. Furthermore, inductively coupled plasma-mass spectrometry analyses carried out on both rock and soil samples showed a possible indication of microbially induced mineral-weathering, particularly release of trapped trace metals in the parent rock. Microbial diversity analysis using 16S rRNA gene sequencing revealed that the rock sample was dominated by Actinobacteria and Proteobacteria, specifically, members of iron-metabolizing bacterial genera, including Mycobacterium, Arthrobacter, Amycolatopsis, Nocardia and Pedomicrobium. These results suggest that morphological and biogeochemical clues derived from studying the role of bacterial activity in hematite weathering and precipitation processes can be implemented as potential comparative tools to interpret similar processes that could have occurred on early Mars.