Stromatolites from extreme environment as tools for astrobiological exploration, study case of Lake Ashenge, East African Rift system

Stromatolites are ideal ecosystems of microbe–sediment synergy and, therefore, represent a fundamental resource for the recognition of biosignatures and investigation of astrobiological targets with regard to environment and life interactions. Even though they are one of the oldest records of life on Earth, at present, modern occurrences of stromatolites are restricted to extreme environments, such as hypersaline lakes. This work aims to characterize stromatolite features that can be used as biosignatures, seeking to understand their morphogenetic and preservation processes. Samples were collected in Lake Ashenge, a hyperalkaline high-altitude lake located in the Tigray Region of Ethiopia.

Two distinct microfacies were observed in the Lake Ashenge stromatolites: a) columnar stromatolites; and b) bulbous stromatolites. Both microfacies feature of slightly laminated microfabric composed of intercalated thicker layers of microsparite and thinner layers of calcite micrite within which exceptionally well-preserved microbial fossils including filamentous and coccoidal cyanobacteria-dominated colonies were recognized. Columnar stromatolite layers show mostly parallel or interlaced vertical arrangements of filamentous microbes which suggest different directions of water flow, demonstrating that the spatial arrangements of microbes reflect the dynamics of the environment. Bulbous structures are composed either of colonies of shrub-like filaments along their entire length and height grading into a vertical parallel arrangement at the top. This type of arrangement suggests self-sustainability of the microbial community, improving the resistance of the structures, in order to endure physical processes in the environment such as currents and wave action or predation. All samples show extracellular polymeric substances (EPS) produced by microbes normally associated with filamentous microbes. In this context, EPS may serve to support and stabilise the biofilm and contribute to the formation of columnar and bulbous structures.

Mars 2020 Perseverance rover recorded carbonate occurrences in Martian paleoenvironments from Jezero Crater which have been compared with microbialites and tufas with the potential to preserve biosignatures similar to those found in Lake Ashenge samples. Therefore, although stromatolite morphology itself configures a macroscopic biosignature which may be detected in Mars rover images, the two scales of observation (millimetres- centimetres vs micrometric) identified biosignatures in the stromatolites of Lake Ashenge could also reflect different conditions of the dynamic environment during their development. In this regard, Lake Ashenge illustrates an important modern environment for astrobiological studies and has potential analogy to ancient lacustrine environments on Mars.