Influence of Ni-related enzymes on the Ni cycle in the Southern Ocean: insights from isotopes and metagenomics

Abstract. Nickel (Ni) is an essential micronutrient for marine microorganisms, being involved in enzymes controlling the nitrogen cycle and metabolic responses to oxidative stress. In this study, we examine the covariation between the abundance of Ni-related enzymes and Ni isotope fractionation. To do so, dissolved Ni concentrations and isotope compositions are presented together with metagenomics on samples from the Antarctic Circumnavigation Expedition. Overall, results reveal lower Ni concentrations and higher δ⁶⁰Ni values in surface waters north of the Sub-Antarctic Front compared to southerly stations. One exception is seen near the high-latitude Mertz Glacier, where the systematics between Ni and δ⁶⁰Ni better resemble those of low-latitude stations. Relative abundances of urease and Ni-SOD in metagenomes are found to correlate with δ⁶⁰Ni, potentially suggesting preferential biological uptake of Ni by the organisms using these enzymes. We find a particularly high abundance of urease in diatoms and alphaproteobacteria near the Mertz Glacier, matching the surprisingly high δ⁶⁰Ni. We thus hypothesise that urea could serve as a nitrogen source for microbial organisms in the late stage of polynya diatom blooms, perhaps causing the observed Ni drawdown and isotope fractionation. This study represents an initial exploration of the influence of biological processes on Ni and δ⁶⁰Ni distributions. It constitutes a first step towards the further analyses (e.g., culture experiments and metatranscriptomics) needed to determine which exact processes lead to the δ⁶⁰Ni biogeochemical divide observed between low-latitude and high-latitude waters.