College of Marine Life Sciences, Ocean University of China, Qingdao, Shandong, PR China.
College of Biological and Environmental Engineering, Binzhou University, Binzhou, Shandong, PR China.
Aquat Toxicol. 2021 Nov;240:105965. doi: 10.1016/j.aquatox.2021.105965. Epub 2021 Sep 10.
Coastal eutrophication has resulted in the rapid loss and deterioration of seagrass beds worldwide. The high concentration of ammonium in eutrophic aquatic environments has been invoked as the main cause. In this study, leaves and roots of the seagrass Zostera marina were treated with simulated eutrophic seawater with elevated ammonium concentrations. The tolerance to ammonium stress and mechanism of nitrogen metabolism detoxification in different tissues were investigated. The results showed that high ammonium stress significantly affected the growth of leaves and had a negative effect on photosynthesis. The root activity of Z. marina was not inhibited at ammonium concentrations of ≤100 mg/L, indicating that the roots exhibited tolerance to ammonium stress. Increasing ammonium concentrations led to a higher increase of ammonium and free amino acid (FAA) contents in leaves than in roots. However, nitrogen storage decreased in Z. marina leaves after high ammonium treatments. The enzyme activity and gene expression of glutamine synthetase (GS) in roots were significantly higher than in the leaves even under ammonium stress. Meanwhile, ammonium stress increased the enzyme activities and gene expression of glutamate synthase (GOGAT) and glutamate dehydrogenase (GDH) in roots, which suggested that the roots had a strong ability to assimilate ammonium under ammonium stress. In contrast, although the GOGAT and GDH activity and gene expression in the leaves were initially increased, they significantly decreased when the ammonium concentration exceeded 100 mg/L. These results indicated that the concentration of 100 mg/L might be a threshold marking a transition from tolerance to toxicity for the leaves. Our study demonstrates that Z. marina leaves could be prone to higher damage than roots because the mechanism of ammonium assimilation in leaves is more susceptible to ammonium toxicity.
沿海富营养化导致了世界各地海草床的迅速丧失和退化。富营养化水生环境中高浓度的铵被认为是主要原因。在这项研究中，用模拟富营养化海水处理了海草鳗草的叶片和根系，以研究其对铵胁迫的耐受性以及氮代谢解毒的机制。结果表明，高铵胁迫显著影响叶片的生长，对光合作用产生负面影响。在铵浓度≤100mg/L 时，鳗草根系的活性没有受到抑制，表明根系对铵胁迫具有耐受性。随着铵浓度的增加，叶片中铵和游离氨基酸（FAA）的含量增加幅度高于根系。然而，氮的储存量在鳗草叶片中却随着高铵处理而减少。即使在铵胁迫下，根系中的谷氨酰胺合成酶（GS）的酶活性和基因表达也明显高于叶片。同时，铵胁迫增加了根系中谷氨酸合酶（GOGAT）和谷氨酸脱氢酶（GDH）的酶活性和基因表达，这表明在铵胁迫下，根系具有很强的同化铵的能力。相比之下，尽管叶片中的 GOGAT 和 GDH 活性和基因表达最初增加，但当铵浓度超过 100mg/L 时，它们显著下降。这些结果表明，100mg/L 的浓度可能是叶片从耐受到毒性转变的一个阈值。我们的研究表明，鳗草叶片比根系更容易受到更高的伤害，因为叶片中铵同化的机制更容易受到铵毒性的影响。
