To test the possibility of nickel (Ni) limitation and nickel‑nitrogen (Ni-N) colimitation on phytoplankton growth, seven urea/Ni enrichment experiments were conducted in the subtropical and subarctic Pacific Ocean. Effects of additions of urea, nickel, and combination of urea and nickel were examined by monitoring the growth of the whole phytoplankton community and three different phytoplankton populations, Synechococcus, Prochlorococcus, and eukaryotes. In all the experiments in the subtropical regions, urea addition significantly increased the total chlorophyll a concentration as compared to the unamended control after two days of incubation, confirming widespread nitrogen limitation and high accessibility of the urea‑nitrogen to phytoplankton in the subtropical Pacific. In contrast, Ni addition did not affect the total chlorophyll a concentration, whether it was added alone or in combination with urea. To elucidate population-level responses to urea and/or Ni addition, cellular and population chlorophyll content and carbon content were estimated using flow cytometric parameters. Synechococcus and Prochlorococcus mainly responded to the urea enrichment. The addition of Ni alone did not show a significant effect on the chlorophyll or carbon content of any phytoplankton population in most of the experiments. Ni-N colimitation was evident only for Synechococcus, and not for Prochlorococcus or eukaryotic phytoplankton. Ni-N colimitation was evident in the urea drawdown rates only for one experiment out of the six experiments. In contrast, in the subarctic region, urea drawdown decreased with Ni addition, although the reason for this was unclear. The present study demonstrates that Ni bioavailability in the subtropical Pacific can evoke Ni-N colimitation in the pico-sized cyanobacteria Synechococcus, but it does not seem to affect phytoplankton biomass at the community level.
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