Factors shaping nickel bioavailability and isotopic signatures in marine phytoplankton
Description
This work aims to understand the Ni isotope fractionation during marine phytoplankton uptake and its underpinning mechanisms. We measured Ni/P ratios and Ni isotope compositions in three species of marine phytoplankton cultured in the lab, i.e., dinoflagellate P. minimum, diatom T. weissflogii and cyanobacterium Synechococcus sp., for whole cell analyses. In addition, we measured Ni/P ratios and Ni isotope compositions in three subcellular fractions, e.g., protein (P), cell debris (CD) and metal-rich granules (MRG), of dinoflagellate P. minimum.The elemental analyses were carried out using a PerkinElmer ICP-QQQ, while Ni isootpe composition were determined using a Nu Plasma high resolution MC-ICP-MS. The potential interference of 58Fe on 58Ni was corrected by simultaneous measurement of 57Fe (Gall et al., 2012). The normalized argon index (NAI) technique (Fietzke and Frische, 2016) was used to quantify the thermal condition of plasma and minimize the possible interferences on Ni isotopes (Yu et al., 2020). The data reduction method for the deconvolution of the double-spike was performed online (Siebert et al., 2001). We found that three species of phytoplankton were all enriched in isotopically heavy Ni from the culture media across a range of Ni availability, with species-dependent magnitudes of fractionation. In subcellular analyses, the largest fractionation was observed in CD fraction, followed by P fraction and then MRG fraction. This work demonstrates the difference of Ni isotope between culture experiments and modern ocean fractionation during marine phytoplankton uptake, suggesting new framework accommodating fractionation processes.