Cai Yangyang, Cao Yingjie, Tang Changyuan
School of Environmental Sciences and Engineering, Sun Yat-sen University, Guangzhou, China.
Guangdong Provincial Key Laboratory of Environmental Pollution Control and Remediation Technology, Sun Yat-sen University, Guangzhou, China.
Front Microbiol. 2019 Sep 25;10:2202. doi: 10.3389/fmicb.2019.02202. eCollection 2019.
Knowledge about the primary factor controlling stable isotope ratios of particulate nitrogen (δN) and total dissolved nitrogen (δN) in a subtropical reservoir can improve the understanding of regional and global nitrogen cycles. Taking Lianhe Reservoir as a representative subtropical reservoir, we studied the spatial and temporal distributions of δN andδN and their relationships with the surrounding physicochemical factors and phytoplankton. The results showed that variations in δN and δN followed seasonal thermal cycles. The values of δN were inversely proportional to those of δN. PCA showed that phytoplankton cell density and pH were the primary drivers of the variation of δN (45.2%). The primary factors influencing δN were Chl and phytoplankton cell density, which both indicated phytoplankton biomass. We also determined that the dominant species was during the thermal stratification period and during the mixing period. Laboratory experiments showed that δN values in both (from 19.5 to 14.6‰) and (from 19.4 to 16.0 ‰) media decreased significantly as the algal cells grew. Furthermore, the δN values increased from 4.9 to 7.9‰ and from 4.7 to 6.9‰ in and media, respectively, when the δN values decreased. These experimental results were consistent with field investigation results and indicated that variations in δN and δN were mainly controlled by phytoplankton cell density, especially the cell density of the dominant species, in both the thermal stratification and mixing periods. The results also suggested that cell density, not phytoplankton species, was the key factor regulating the distribution of nitrogen stable isotopes. These results together indicated that phytoplankton cell density is the primary factor in the regulation of nitrogen stable isotope composition and that its influence is greater than that of other physical and chemical factors. This study provided detailed information supporting the primary role of phytoplankton in the nitrogen geochemical cycle and improved the understanding of biochemical processes in natural subtropical reservoirs.
了解控制亚热带水库中颗粒态氮（δN）和总溶解态氮（δN）稳定同位素比值的主要因素，有助于增进对区域和全球氮循环的理解。以联合水库作为亚热带水库的代表，我们研究了δN和δN的时空分布及其与周围物理化学因子和浮游植物的关系。结果表明，δN和δN的变化遵循季节性热循环。δN的值与δN的值呈反比。主成分分析表明，浮游植物细胞密度和pH值是δN变化的主要驱动因素（45.2%）。影响δN的主要因素是叶绿素和浮游植物细胞密度，二者均表明了浮游植物生物量。我们还确定了热分层期的优势种为 ，混合期的优势种为 。实验室实验表明，随着藻类细胞生长， （从19.5‰降至14.6‰）和 （从19.4‰降至16.0‰）培养基中的δN值均显著下降。此外，当δN值下降时， 培养基和 培养基中的δN值分别从4.9‰升至7.9‰和从4.7‰升至6.9‰。这些实验结果与实地调查结果一致，表明在热分层期和混合期，δN和δN的变化主要受浮游植物细胞密度控制，尤其是优势种的细胞密度。结果还表明，细胞密度而非浮游植物种类是调节氮稳定同位素分布的关键因素。这些结果共同表明，浮游植物细胞密度是调节氮稳定同位素组成的主要因素，其影响大于其他物理和化学因素。本研究提供了详细信息，支持浮游植物在氮地球化学循环中的主要作用，增进了对亚热带天然水库中生物化学过程的理解。
