State Key Laboratory of Marine Environmental Science and College of Ocean and Earth Sciences, Xiamen Universitygrid.12955.3a, Xiamen, China.
Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island, USA.
mSystems. 2022 Aug 30;7(4):e0053822. doi: 10.1128/msystems.00538-22. Epub 2022 Jul 11.
The dominant marine filamentous N fixer, , conducts photosynthesis and N fixation during the daytime. Because N fixation is sensitive to O, some previous studies suggested that spatial segregation of N fixation and photosynthesis is essential in . However, this hypothesis conflicts with some observations where all the cells contain both photosystems and the N-fixing enzyme nitrogenase. Here, we construct a systematic model simulating metabolism, showing that the hypothetical spatial segregation is probably useless in increasing the growth and N fixation, unless substances can efficiently transfer among cells with low loss to the environment. The model suggests that accumulates fixed carbon in the morning and uses that in respiratory protection to reduce intracellular O during the mid-daytime, when photosynthesis is downregulated, allowing the occurrence of N fixation. A cell membrane barrier against O and alternative non-O evolving electron transfer also contribute to maintaining low intracellular O. Our study provides a mechanism enabling N fixation despite the presence of photosynthesis across . The filamentous is a globally prominent marine nitrogen fixer. A long-standing paradox is that the nitrogen-fixing enzyme nitrogenase is sensitive to oxygen, but conducts both nitrogen fixation and oxygen-evolving photosynthesis during the daytime. Previous studies using immunoassays reported that nitrogenase was limited in some specialized cells (termed diazocytes), suggesting the necessity of spatial segregation of nitrogen fixation and photosynthesis. However, attempts using other methods failed to find diazocytes in , causing controversy on the existence of the spatial segregation. Here, our physiological model shows that can maintain low intracellular O in mid-daytime and achieve feasible nitrogen fixation and growth rates even without the spatial segregation, while the hypothetical spatial segregation might not be useful if substantial loss of substances to the environment occurs when they transfer among the cells. Our study then suggests a possible mechanism by which can survive without the spatial segregation.
优势海洋丝状固氮生物 能够在白天进行光合作用和固氮。由于固氮对氧敏感，一些先前的研究表明，固氮和光合作用在 中的空间分离是必不可少的。然而，这一假设与一些观察结果相冲突，即所有细胞都同时含有光合作用系统和固氮酶氮酶。在这里，我们构建了一个系统模型来模拟 的代谢，表明假设的空间分离可能对增加 的生长和固氮没有用处，除非物质可以有效地在细胞之间转移，同时损失很少到环境中。该模型表明， 在早晨积累固定碳，并在中午光合作用下调时，利用该固定碳来进行呼吸保护，以减少细胞内的氧，从而允许固氮发生。细胞膜对氧的屏障和替代的非 O 进化电子传递也有助于维持低细胞内氧。我们的研究提供了一种机制，使 即使在存在光合作用的情况下也能进行固氮。 是一种全球性的重要海洋氮固定生物。长期以来的一个悖论是，固氮酶对氧敏感，但 在白天既能进行固氮又能进行产氧光合作用。以前使用免疫测定法的研究报告称，氮酶在一些专门的 细胞（称为固氮细胞）中受到限制，这表明固氮和光合作用的空间分离是必要的。然而，使用其他方法的尝试未能在 中发现固氮细胞，这导致了对空间分离存在的争议。在这里，我们的生理模型表明， 即使在没有空间分离的情况下，也可以在中午保持低细胞内氧，并实现可行的固氮和生长速率，而假设的空间分离如果在物质转移到 细胞之间时发生大量损失到环境中，可能没有用处。我们的研究随后提出了一种可能的机制，使 能够在没有空间分离的情况下生存。
