Berube, P. M.; LeMaster, T.; Chisholm, S. W.

Prochlorococcus is a diverse and widespread cyanobacterium with significant contributions to the marine nitrogen and carbon cycles. Some Prochlorococcus reduce and divert up to 20-30% of the nitrate that they take up to external pools of nitrite. Given that nitrite is a central intermediate of the nitrogen cycle and Prochlorococcus is highly abundant in nitrogen-limited waters, our goal was to advance our understanding of nitrite cycling in the context of nitrogen limitation. Here we observe that nitrate-limited Prochlorococcus have cell-specific nitrite production rates that are approximately a magnitude higher than nitrogen-replete Prochlorococcus when challenged with a pulse of nitrate. Nitrite production rates are unchanged or depressed during light and cold shocks, suggesting that nitrate is not used as an alternative electron acceptor to mitigate the impacts of excess photochemically generated electrons. These results suggest that in regions where phytoplankton growth is limited by nitrogen, Prochlorococcus cells could be primed to transform substantial quantities of nitrate into extracellular pools of nitrite during episodic upwellings of nitrate-rich water. Given that nitrite is an important intermediate in the nitrogen cycle, these results have ramifications for our understanding of nitrogen cycling in nitrogen-limited open ocean ecosystems.