Nickel stable isotope data for meteorites, terrestrial basalts and sediments (A) and cultures of Archaea (B) plotted on the same vertical scale. For the meteorite data in A, filled symbols are for stones and open symbols for irons. For the continental sediments filled symbols are for loess, and open symbols are for 2 samples of River Nile sediments. The average of all of the data in A is 0.2‰ and the gray band denotes 1 standard deviation either side of this average. The first 3 microorganisms in B are methanogens; the fourth on the right (P. calidifontis) is a heterotrophic archaea. Only data for experiments where reservoir effects are insignificant or small are plotted. Filled symbols represent residual media and open symbols Ni extracted from cultured cells. All data are plotted relative to a starting medium of 0‰ (center of the gray band), and the width of the gray band denotes 1 standard deviation of the abiotic data in A either side of the value for the medium.

Results of the Ni uptake experiment conducted with the methanogen M. acetivorans. (A) Individual bottles of growth were sacrificed for each residual medium sample collected for isotope analysis (filled diamonds) and headspace gas measurement (open squares) made over the 5-day period. δ⁶⁰Ni values and the amount of CH₄ evolved over the growth period, as a function of headspace pressure, are plotted on the y-axes vs. time on the x axis. The CH₄ value on day 1 (≈0.5 bar) is the amount added to the bottles at the beginning of the experiment. As would be predicted, a positive trend is observed for cell growth, with increasing amounts of CH₄ being produced as growth proceeds. Correlating with the production of CH₄ is the increasingly heavier isotopic values in the same residual media samples. (B) Total Ni amounts measured in the residual media are plotted on the bottom axis against δ⁶⁰Ni values on the left-hand axis. Cells (filled circle) were harvested for isotopic analysis only on day 5 of the experiment. A correlating effect is seen with the isotope values and Ni amounts; i.e., the residual medium becomes increasingly heavier and depleted as the cells grow with preferential removal of the lighter Ni isotope.

Nickel stable isotope data for all methanogen cultures for which the total Ni mass balance is also available (but omitting one extreme value for M. jannaschii cells, which plots at −1.4‰; see Fig. 1B), plotted as a function of the fraction of total starting Ni remaining in the medium at the end of the experiment. All Ni isotope data for residual media (filled symbols) and cells (open symbols) are plotted relative to the starting medium for the respective experiments. Symbols for different species as in Fig. 1 except that the nonmethanogen is not plotted and the inverted triangles represent data for the M. acetivorans Ni uptake experiment. Also plotted are model closed system Rayleigh fractionation curves for residual media (solid lines) and accumulated cells (dashed lines) assuming fractionation factors for Ni uptake into cells of 0.9990 and 0.9993.