Biosignatures and microbial fossils in endolithic microbial communities colonizing Ca-sulfate crusts in the Atacama Desert

Abstract

Since the description of microbial communities colonizing Ca-sulfate crusts in the Atacama Desert, there has been much interest in the mechanisms that could lead to the formation and preservation of biosignatures or microbial fossils of these communities. A key to understanding physico-chemical processes of taphonomy and early diagenesis is to examine microfossils in their natural environment. In this study, we characterize organomineral traces and microbial fossils found around microbial communities present in these Ca-sulfate crusts. Through scanning electron microscopy, microanalytical (EDS) and Raman spectroscopy techniques, calcium carbonate precipitates were detected around remnants of cryptoendolithic algae beneath the crust surface. As what seems to be the final step in the organomineralization of these cryptoendolithic communities, we also observed alga cell remains permineralized by Mg-Si-rich minerals inside gypsum crystals. Additionally, Mg-Si bearing minerals formed a web-like structure within the hypoendolithic cyanobacterial habitat via permineralization of extracellular polymeric substances. Our observations indicate that despite the extremely hyperarid environment, microenvironmental conditions may be appropriate for the formation of biosignatures and microbial fossils of extinct endolithic microbial communities. A model of the possible organomineralization processes involved is presented.