Elucidating the role of biogenic and authigenic phases in marine cycling of nickel with paired dissolved and particulate nickel isotopes

Nickel (Ni) is a bio-essential trace metal for marine phytoplankton, serving as a cofactor in key enzymes. Its nutrient-like distribution and the mirrored distribution of δ 60 Ni in seawater suggest that δ 60 Ni could serve as a tracer of biological Ni cycling. However, particulate Ni (pNi) remains understudied despite its important role. Here, we present three coupled dissolved and particulate δ 60 Ni profiles from distinct biogeochemical regimes across the Subtropical Front in the Australian sector of the Southern Ocean. In surface waters, pNi is isotopically lighter than co-located dissolved Ni, and particulate Ni/P in the mixed layer falls within the range observed for phytoplankton, reflecting an association with organic matter. These results support previous studies suggesting that phytoplankton preferentially remove isotopically light Ni. In the subtropics, higher surface productivity corresponds to elevated mixed layer pNi concentrations, below which pNi declines, associated with the regeneration of organic matter. However, particulate δ 60 Ni shows limited variation, implying minimal isotope fractionation during this process. At subantarctic stations, pNi concentrations remain relatively constant, while particulate δ 60 Ni decreases with depth in the upper 300 m. Moreover, δ 60 Ni offsets between the dissolved and particulate phases are smaller than for the subtropical counterpart, likely associated with different phytoplankton assemblages. At greater depth, particulate Mn/P increases, and Ni/P ratios exceed those of phytoplankton. This suggests a shift in pNi association from organic-rich to Mn-rich phases, with minimal change in total particulate δ 60 Ni. These findings suggest a more complex internal cycling of Ni in the oceans than one driven solely by biological processes.