Nickel utilization in phytoplankton assemblages from contrasting oceanic regimes

In most oceanic environments, dissolved nickel (Ni) concentrations are drawn down in surface waters with increasing concentrations at depth, implying a role for biology in the geochemical distribution of Ni. Studies with phytoplankton isolates from the surface ocean have established the biochemical roles of Ni in the assimilation of urea and oxidative defense. To determine if these requirements are relevant in natural marine planktonic assemblages, bottle-based fertilization experiments were used to test the effects of low-level additions of Ni, urea, or both Ni and urea to surface waters at several locations offshore of Peru and California, as well as in the Gulf of California. Urea and Ni+urea additions consistently promoted phytoplankton growth relative to control and +Ni treatments, except in a coastal upwelling site and Peruvian water. No effect was observed in the upwelling site, but in Peruvian waters urea additions resulted in increased phytoplankton pigments and phosphate drawdown only when Ni was added concurrently, suggesting a biochemically dependent Ni–urea colimitation. In the Gulf of California, Ni additions without urea resulted in increased abundances of cyanobacteria, picoeukaryotes, and the corresponding pigments. As urea additions showed the overall phytoplankton community was also urea-limited, it appears that the cyanobacteria and potentially the picoeukaryotes were colimited by Ni and urea in a biochemically independent fashion. In parallel, radiotracer-based uptake experiments were used to study the kinetics and spatial variation of biological Ni assimilation. In these experiments, the added radiotracer rarely equilibrated with the natural Ni present, precluding estimates a determination of in situ Ni uptake rates and suggesting that much of the natural Ni was not bioavailable. The lack of equilibration likely did not preclude the measurement of community Ni uptake kinetics, nor the comparison of measured rates between locations. The highest VmaxKρ−1 values, which reflect a competitive advantage in Ni acquisition at low concentrations, were observed in stratified nitrogen-deplete communities, potentially linking Ni and nitrogen biogeochemistry in a manner consistent with the biochemical utilization of Ni. Overall, uptake rates were higher in the euphotic rather than non-euphotic zone communities, directly reconciling the nutrient-like depth profile of Ni. The Ni uptake rates observed at the nitrate-replete Fe-deplete Peru stations were an order of magnitude lower than the other sites. This result agrees with calculations suggesting that saturation of the cell surface with Ni and iron (Fe) transporters may limit uptake rates in low Fe waters.