A novel biomarker for deep-time methanogenesis – perspectives from nickel isotope fractionation in modern microbialites

Activity of methanogenic archaea on early Earth is mainly deduced based on modern processes and supported by the carbon isotope record (δ¹³C_carb). Recently, Ni isotopes preserved in authigenic carbonate have been proposed to provide a further signature to trace methanogenesis in ancient environments because methanogens require nickel (Ni) as a metal co-factor for key enzymes to produce CH₄. Further, uptake in cultured methanogens has been shown to fractionate stable Ni isotopes (δ⁶⁰Ni) from their aqueous medium, a process that could potentially be recorded in authigenic carbonates. Therefore, Ni isotopes in microbialites could provide valuable insights into whether methanogenesis was active in ancient microbial habitats. Here, we explore this idea using combined CNi isotope analyses on individual layers of microbial carbonates from the Lagoa Salgada, a Holocene ephemeral lake in Brazil where abundant microbialites formed in the presence of methanogens. Microbial carbonates show distinct positive δ¹³C_carb (up to +20 ‰) at negative δ⁶⁰Ni_carb values (down to -1.36 ‰) that can be linked to methanogenic metabolism—in contrast to higher δ⁶⁰Ni of ambient gastropod shells (+1.72 ‰) and authigenic non-lithified sediments from the lagoon (+1.1 to +1.58 ‰). Our results show that Ni isotopes in microbial carbonates are a promising novel isotope proxy for methanogenic Ni isotope fractionation. We anticipate this study as a starting point for future research on combined metallome and microbiome evolution reflected in microbialites through time on Earth and beyond.