Fossilized Extracellular Polymeric Substances (EPS) in An Ancient Impact-Induced Hydrothermal System

Impact-induced hydrothermal systems have provided a potential niche for thermophilic organisms throughout Earth history. Studies of various impact crater deposits indicate that the temperature range of associated hydrothermal systems often were favorable for thermophilic and hyperthermophilic communities, and these systems would have remained active for thousands of years. Our investigation of the Siljan Impact Structure, Sweden, revealed evidence of microbial remains within the low-temperature hydrothermal system generated by the Devonian impact. Hydrothermal minerals in the veins were found to enclose carbonaceous films and thread-like features characterized by a highly complex morphology similar to that of partly mineralized extracellular polymeric substances (EPS) associated with modern hyperthermophilic biofilms. The mode of occurrence, flexible structure, multiple attachment points, and chemical composition indicate that the features are fossilized remnants of microbial biofilms that were trapped within the hydrothermal system. These findings support the hypothesis that impact-induced hydrothermal systems may be favorable targets in the search for microbial biosignatures.