Microbial methanogenesis and acetogenesis are ancient carbon-cycling pathways that depend on nickel (Ni). The bioavailability of Ni can be assessed using its stable isotopes. While Ni isotope fractionation during methanogenesis has been demonstrated experimentally, its variability and controlling factors remain unclear. This study investigated the effects of metabolic pathway, temperature (growth rate), and Ni concentration on Ni isotope fractionation, using culture experiments with a metabolically versatile methanogen ( Methanosarcina barkeri ) and acetogen ( Moorella thermoacetica ). Regarding the effect of metabolic pathway, methanogen cells grown on methanol, acetate, or H 2 /CO 2 were consistently enriched in light Ni isotopes relative to the medium, with an average Δ 60 Ni cells − final medium value of −1.58‰. A similar fractionation was observed in the acetogen grown on methanol (−1.29‰), indicating that substantial fractionation is not specific to methanogenesis. The similar Ni isotope fractionation observed across methanogenic and methanol-based acetogenic pathways appears to result mainly from a kinetic isotope effect associated with the dissociation of organically complexed Ni during transport. In contrast, acetogenesis from glucose showed a smaller fractionation (−0.36‰), reflecting low Ni demand. Shifting focus to temperature and growth rate, culturing the methanogen at lower temperatures led to reduced methane production rates and a minor decrease in Ni isotope fractionation. The Δ 60 Ni cells − final medium value negatively correlated with cellular Ni uptake rate, indicating a reduced kinetic isotope effect under slow uptake. Turning to Ni concentration, lower initial Ni concentrations led to heavier Ni isotope composition in cells and medium, consistent with Rayleigh-type fractionation from enhanced cellular Ni uptake. However, fractionation markedly decreased to ∼−0.20‰ at 0.1 μM Ni, likely reflecting passive uptake of Ni organic complexes without ligand dissociation. Our findings highlight that Ni isotope fractionation during methanogenesis and acetogenesis varies with growth conditions and Ni speciation, providing a framework for using Ni isotope signatures to assess Ni bioavailability and microbial activity in anoxic environments across geological time.
Miyajima et al. (Fri,) studied this question.