Nitrogen (N) starvation in algae induces a variety of structural and metabolic changes including accumulation of triacylglycerol (TAG). Given the promising prospect of using algae as feedstock for biofuel production, accumulation of TAG upon N starvation becomes an ideal system to study TAG biosynthesis. Under nitrogen-depleted conditions, algae also accumulate compatible solutes such as sugar and certain amino acids, which is expected to elevate osmolarity in the cytoplasm. However, how osmoregulation is maintained and how it impacts on carbon metabolism, especially TAG accumulation under N starvation, are not well understood. We show here that potassium channel KCN11 localized in the contractile vacuole (CV) mediates osmoregulation during N starvation and loss of KCN11 profoundly affects cell physiology and TAG biosynthesis. KCN11 level is increased and the CV pulsation is accelerated. Loss of KCN11 induces aberrant CV cycle, inhibition of cell growth, increase of cell size, inhibition of chlorophyll loss and TAG accumulation. These effects are rescued by addition of sucrose to raise osmolarity in the culture medium, indicating that osmoregulation is required for cell adaptation to N starvation. Metabolomic analysis shows reduction of acetyl-CoA and accumulation of glyceraldehyde-3-phosphate in kcn11 mutant relative to the control under N starvation, indicating that defects in acetyl-CoA biosynthesis and some metabolic steps from glyceraldehyde-3-phosphate to TAG contribute to the decreased TAG accumulation due to loss of osmoregulation. This work provides novel insight of osmoregulation during N starvation in the control of cell physiology and metabolism especially TAG accumulation. According to these findings, we propose that osmolarity should be carefully monitored during the industrial production of biodiesel.

中文翻译：

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钾通道 KCN11 是在氮饥饿期间维持细胞渗透压所必需的，以控制莱茵衣藻中适当的细胞生理和 TAG 积累。

藻类中的氮 (N) 饥饿会导致各种结构和代谢变化，包括三酰基甘油 (TAG) 的积累。鉴于使用藻类作为生物燃料生产原料的前景广阔，在 N 饥饿时积累 TAG 成为研究 TAG 生物合成的理想系统。在缺氮条件下，藻类还会积累相容的溶质，例如糖和某些氨基酸，预计这会提高细胞质中的渗透压。然而，渗透调节是如何维持的，以及它如何影响碳代谢，尤其是氮饥饿下的 TAG 积累，尚不清楚。我们在这里展示了位于收缩液泡 (CV) 中的钾通道 KCN11 在 N 饥饿期间介导渗透压调节，并且 KCN11 的损失深刻影响细胞生理学和 TAG 生物合成。KCN11 水平升高，CV 脉动加速。KCN11 的缺失诱导异常的 CV 循环、细胞生长的抑制、细胞大小的增加、叶绿素损失的抑制和 TAG 的积累。通过添加蔗糖以提高培养基中的渗透压，可以挽救这些影响，这表明细胞适应 N 饥饿需要渗透压调节。代谢组学分析显示，相对于 N 饥饿下的对照，kcn11 突变体中乙酰辅酶A 的减少和 3-磷酸甘油醛的积累，表明乙酰辅酶 A 生物合成的缺陷和从 3-磷酸甘油醛到 TAG 的一些代谢步骤有助于由于渗透压调节的丧失，TAG 积累减少。这项工作提供了在 N 饥饿期间渗透调节在控制细胞生理和代谢尤其是 TAG 积累方面的新见解。根据这些发现，我们建议在生物柴油的工业生产过程中仔细监测渗透压。