SEM Investigation of Microbialites from Cenote Azul, Mexico, and Implications for the Search for Biosignatures on Mars

Abstract

A biosignature is any phenomenon for which a biological process is a known explanation, and can therefore be used as evidence for past or present life on a planet. On Mars, the geological record has provided evidence for an environment similar to an Archean Earth’s in the planet’s distant past. Consequently, Earth-analogue structures such as microbialites, which have been found on Earth since the Archean period, may be a key target for biosignature detection. Microbialites are organosedimentary structures that form due to the activities of microorganisms in mineral-saturated aqueous environments. To better understand the microbe-mineral interactions that cause them to form, the surface characteristics of microbialites from Cenote Azul, Mexico, were examined using Scanning Electron Microscopy (SEM). Microbialite samples were collected from a range of depths and prepared for SEM by fixing in 2.5% glutaraldehyde, mounting on aluminum stubs, and sputter-coating in gold. The outer surfaces of samples were imaged at various resolutions using a JEOL 6610LV SEM. Both qualitative and quantitative observations in diatom count, surface area colonized by microbial mats, and phospholipid fatty acid analysis show decreasing abundance of microorganisms and organics on the outer surfaces of microbialites with increasing depth. This may be due to the decreasing light availability and changing environmental conditions with depth, which have been observed in Cenote Azul. Characterizing the surfaces of microbialites such as those from Cenote Azul will provide valuable information in evaluating how microbialites form and grow on Earth. This knowledge can be extended to analogue systems such as Mars to gauge whether microbialite-like structures could be formed and preserved on other planets. Ultimately, this will aid in future biosignature detection studies and help refine the search for life in our solar system and beyond.