Using Ni isotopes as biosignatures

Nickel (Ni) is a key element in metabolic pathways of methanogenic Archaea due to its use in Ni­enzymes. Ni is strongly fractionated during its absorption by methanogens. Δ​Ni values (δ​Ni​cell​­δ​ Ni​environment​) as low as ­2‰ have been reported in methanogenic cultures. However, Ni sorption to particles, e.g. clays and Fe oxides, and organic ligands can yield similar fractionation patterns. The usefulness of Ni as a biomarker has been proposed and debated. Cell culture experiments confirm general negativeΔ​Ni values on the order of 1 to 2‰. In some cases cells do not display the well known negative Δ​Ni signature, most likely due to simultaneous abiotic fractionation by organic ligands. Experiments suggest increasing fractionation with increased Ni concentrations in growth media. To assess Ni as a viable biomarker for methanogens, we provide new Ni isotope data from rocks and fluids from an outcrop of the Tekirova ophiolite, Turkey, a system undergoing serpentinization with active CH​4 and fluid seeps. Methanogen presence and hence a partially biogenic origin of the CH​4​are confirmed. In one locality, methanogen presence can be linked to a negative excursion in δ​C and a significantly lower δ​Ni value. There is no mineralogical explanation for the negativeδ​Ni excursion, hence we suggest that the fractionation of Ni may be a true Ni biosignature. The viability of Ni as a biomarker for extraterrestrial or early terrestrial life is doubtful, due to the large number of Ni fractionating processes and our limited understanding of the Ni budget in e.g. the oceans and the biosphere. Future research should focus on revealing abiotic fractionation in relevant processes, such as serpentinization and the formation of BIFs and on in­situ analysis of Ni isotopes of microfossils.