Phytoplankton response to increased nickel in the context of ocean alkalinity enhancement

Abstract. Ocean alkalinity enhancement (OAE) is considered one of the most promising approaches to actively remove carbon dioxide (CO2) from the atmosphere by accelerating the natural process of rock weathering. This approach involves introducing alkaline substances sourced from natural mineral deposits, such as olivine, basalt, and carbonates or obtained from industrial waste products such as steel slag, into seawater and dispersing them over coastal areas. Some of these natural and industrial substances contain trace metals, which would be released into the oceans along with the alkalinity enhancement. The trace metals could serve as micronutrients for marine organisms at low concentrations but could potentially become toxic at high concentrations, adversely affecting marine biota. To comprehensively assess the feasibility of OAE, it is crucial to understand how the phytoplankton, which forms the base of marine food webs, responds to ocean alkalinization and associated trace metal perturbations. As one of the most abundant metals in OAE source materials, understanding the impacts of nickel (Ni) on the phytoplankton is critical for OAE assessment. In this study, we investigated the influence of nickel (Ni) on three representative phytoplankton species over a gradient of nine Ni concentrations (from 0 to 100 µmol L−1 with 12 µmol L−1 synthetic organic ligand). The impacts of elevated Ni varied among the tested phytoplankton species. The coccolithophore Emiliania huxleyi and the dinoflagellate Amphidinium carterae exhibited a growth rate inhibition of about 30 % and 20 %, respectively, at the highest Ni concentrations. The half maximal inhibitory concentration (IC50, at which the growth rate is inhibited by 50 %) of both species exceeded the tested range of Ni. This suggests that both species were only mildly affected by the elevated Ni concentrations. In contrast, the diatom Thalassiosira weissflogii displayed a considerably higher sensitivity to Ni, with a 60 % growth rate inhibition at the highest Ni concentration and an IC50 value of 63.9 µmol L−1. In conclusion, the variability in phytoplankton sensitivity to Ni exposure suggests that for OAE applications with Ni-rich materials caution is required and critical toxic thresholds for Ni must be avoided.