Effects of CO2 enrichment on photosynthesis, growth, and nitrogen metabolism of the seagrass Zostera noltii

Seagrass ecosystems are expected to benefit from the global increase in CO2 in the ocean because the photosynthetic rate of these plants may be Ci-limited at the current CO2 level. As well, it is expected that lower external pH will facilitate the nitrate uptake of seagrasses if nitrate is cotransported with H+ across the membrane as in terrestrial plants. Here, we investigate the effects of CO2 enrichment on both carbon and nitrogen metabolism of the seagrass Zostera noltii in a mesocosm experiment where plants were exposed for 5 months to two experimental CO2 concentrations (360 and 700 ppm). Both the maximum photosynthetic rate (Pm) and photosynthetic efficiency (a) were higher (1.3- and 4.1-fold, respectively) in plants exposed to CO2-enriched conditions. On the other hand, no significant effects of CO2 enrichment on leaf growth rates were observed, probably due to nitrogen limitation as revealed by the low nitrogen content of leaves. The leaf ammonium uptake rate and glutamine synthetase activity were not significantly affected by increased CO2 concentrations. On the other hand, the leaf nitrate uptake rate of plants exposed to CO2-enriched conditions was fourfold lower than the uptake of plants exposed to current CO2 level, suggesting that in the seagrass Z. noltii nitrate is not cotransported with H+ as in terrestrial plants. In contrast, the activity of nitrate reductase was threefold higher in plant leaves grown at high-CO2 concentrations. Our results suggest that the global effects of CO2 on seagrass production may be spatially heterogeneous and depend on the specific nitrogen availability of each system. Under a CO2 increase scenario, the natural levels of nutrients will probably become limiting for Z. noltii. This potential limitation becomes more relevant because the expected positive effect of CO2 increase on nitrate uptake rate was not confirmed.海草生态系统有望从海洋CO₂浓度的全球性升高中获益，因为在当前CO₂水平下，这类植物的光合速率可能受胞间CO₂浓度（Ci）限制。此外，若硝酸盐如陆生植物般与H+协同跨膜转运，则更低的外界pH将促进海草的硝酸盐摄取。本研究通过中宇宙实验，探究了CO₂富集对诺氏大叶藻（Zostera noltii）碳、氮代谢的影响，实验中将海草暴露于两种CO₂浓度（360 ppm与700 ppm）环境中并持续培养5个月。结果显示，高CO₂浓度处理组海草的最大光合速率（Pm）与光合效率（α）均显著提升，分别为对照组的1.3倍与4.1倍。另一方面，CO₂富集对海草叶片生长速率无显著影响，这可能源于叶片氮含量偏低所反映的氮素限制效应。海草叶片的铵盐摄取速率与谷氨酰胺合成酶（glutamine synthetase）活性均未受CO₂浓度升高的显著影响；与之相对，高CO₂浓度处理组海草的叶片硝酸盐摄取速率仅为当前CO₂浓度组的四分之一，这表明诺氏大叶藻的硝酸盐转运并非如陆生植物般与H+协同跨膜进行。与之相反，高CO₂环境下生长的海草叶片中，硝酸还原酶（nitrate reductase）活性提升至对照组的3倍。本研究结果显示，CO₂升高对海草生产力的全球效应可能存在空间异质性，且取决于各生态系统特有的氮素可利用性；在CO₂浓度升高的情景下，自然水体的营养盐水平或将成为诺氏大叶藻的限制因子，而由于此前预期的CO₂升高对硝酸盐摄取速率的促进效应未被证实，这一潜在限制的重要性更为凸显。