Diatoms and Cold Water: A novel hypothesis regarding nitrate uptake and reduction Blooms of diatoms are common in temperate, upwelling, and polar regions, and most often occur in cool waters when the environmental conditions such as upwelling, mixing, or runoff increase the flux of nitrate to the system. It has often been shown that diatoms do not exhibit the preference for ammonium typically observed for other phytoplankton, but the physiological basis for why nitrate is taken up disproportionately is not well understood. Based on published data and preliminary experiments, a hypothesize is proposed that diatoms thrive in cold, nitrate-rich water as a result of interactions between their carbon and nitrogen metabolisms. Specifically, this study will try to establish that nitrate, which can be accumulated at high concentrations by diatoms, may serve as a sink for excess electron flux that results episodically from an imbalance between production of electrons in the light reactions of photosynthesis (which is temperature insensitive) and the consumption of electrons in the dark reactions (i.e. carbon fixation, which is temperature sensitive). Different temperature optima for the enzymes involved in nitrate reduction, nitrate reductase and carbon fixation, ribulose-biphosphate-carboxylase, also come into play. It appears that nitrate reductase has a much lower temperature optimum, making it a more favorable electron acceptor in cold water. Consequently, the rate of nitrate uptake is negatively correlated with temperature, at least over the temperature range of -8-20'C. The significance of this work lies in the development of a physiological explanation for why diatoms bloom when nitrate levels are high and waters are cool. The negative relationship between nitrate uptake and temperature argues strongly for the incorporation of temperature functions in models of new production. Lastly, should nitrate serve as a mechanism for energy dissipation, then release of nitrogen from these cells in the form of nitrite or organic nitrogen would be expected. This study will seek to determine the fraction of nitrate that is reduced for maintenance of cellular nitrogen requirements, and how much is reduced via other regulatory mechanisms in the cells. If a significant fraction of nitrate is reduced simply as an energy dissipatory mechanism then estimates of new production based solely on nitrate uptake could be seriously biased.
