a, HF input fluids from both Marcellus and Utica shales contain substrates that sustain microbial metabolism. Parentheses indicate metabolites detected in one shale. b, Microorganisms in shales adapt to high salinities by producing and using osmoprotectants such as GB (red circles), which can be released into fluids by viral lysis. c, Marinobacter and Halomonadaceae have the potential to aerobically oxidize hydrocarbons and respire sugars using nitrate and oxygen as electron acceptors. d, Candidatus Frackibacter and Halanaerobium ferment GB, yielding trimethylamine, which supports methanogenesis by Methanohalophilus and Methanolobus (blue box). Methylamines and methanol in the input fluids can also support methanogenesis (yellow box).