Diatoms and Cold Water: A Novel Hypothesis Regarding Nitrate Uptake and Reduction

硅藻和冷水：一个关于硝酸盐吸收和还原的新假设硅藻华在温带、上升流和极地地区很常见，当上升流、混合或径流等环境条件增加硝酸盐进入系统的通量时，最常发生在冷水中。通常表明，硅藻不表现出对铵的偏好，这是在其他浮游植物中通常观察到的，但是为什么硝酸盐被不成比例地吸收的生理基础还没有得到很好的理解。根据已发表的数据和初步实验，提出了一种假设，即硅藻在寒冷、富含硝酸盐的水中茁壮成长，这是它们的碳和氮代谢之间相互作用的结果。具体来说，本研究将试图确定，硝酸盐可以被硅藻在高浓度下积累，可能作为过量电子通量的汇，这种通量是由于光合作用的光反应（对温度不敏感）中电子的产生与暗反应（即碳固定，对温度敏感）中电子的消耗之间的不平衡而偶尔产生的。参与硝酸还原酶、硝酸还原酶和碳固定酶、核酮糖-二磷酸-羧化酶的不同温度条件也起作用。硝酸还原酶似乎具有更低的最适温度，使其在冷水中成为更有利的电子受体。因此，至少在-8-20℃的温度范围内，硝酸盐的吸收速率与温度呈负相关。这项工作的意义在于，从生理学上解释了为什么当硝酸盐含量高、水温低时，硅藻会大量繁殖。硝酸盐吸收与温度之间的负相关关系有力地证明了在新生产模型中加入温度函数的必要性。最后，如果硝酸盐是能量耗散的一种机制，那么氮将以亚硝酸盐或有机氮的形式从这些细胞中释放出来。本研究将试图确定为维持细胞氮需求而减少的硝酸盐的比例，以及通过细胞中的其他调节机制减少了多少。如果硝酸盐的很大一部分只是作为一种能量耗散机制被减少，那么仅仅基于硝酸盐吸收的新产量估计可能会有严重偏差。

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.