Department of Civil and Environmental Engineering, The University of Utah, 110 S Central Campus Drive, Salt Lake City, UT, 84112, USA.
Wasatch Front Water Quality Council, Salt Lake City, UT, USA.
Chemosphere. 2022 Sep;302:134784. doi: 10.1016/j.chemosphere.2022.134784. Epub 2022 Apr 30.
Nitrogen (N) cycling is an essential process in lake systems and N-fixation is an important component of it. Recent studies have also found that nitrate reduction through heterotrophic denitrification in lake systems did not prevent harmful cyanobacterial blooms, but instead, may have favored the dominance of N-fixing cyanobacteria. The overall objective of this study was to estimate nitrogen fixation rates and the expressions of associated nitrogenase (nif gene) functional gene at several sites at different occasions in freshwater Utah Lake. For comparison purposes, one time sampling was also conducted in the brackish Farmington Bay of Great Salt Lake (GSL). The microbial ecology of the top 20-cm of surface water was investigated to assess the dominant cyanobacterial communities and N-related metabolisms. Our study revealed that Dolichospermum and Nodularia were potential N-fixers for Utah Lake and brackish Farmington Bay, respectively. The in situ N-fixation rates were 0-0.73 nmol N hrL for Utah Lake and 0-0.85 nmol N hrL for Farmington Bay, and these rates positively correlated with the abundance and expressions of the nif gene. In addition, nitrate reduction was measured in sediment (0.002-0.094 mg N VSS hr). Significantly positive correlations were found among amoA, nirS and nirK abundance (R = 0.56-0.87, p < 0.05, Spearman) in both lakes. An exception was the lower nirK gene abundance detected at one site in Farmington Bay where high ammonium retentions were also detected. Based on a mass balance approach, we concluded that the amount of inorganic N loss through denitrification still exceeded the N input by N-fixation, much like in most lakes, rivers, and marine ecosystems. This indicates that N cycling processes such as denitrification mediated by heterotrophic bacteria contributes to N-export from the lakes resulting in N limitations.
氮循环是湖泊系统中的一个重要过程，而固氮作用是其重要组成部分。最近的研究还发现，湖泊系统中通过异养反硝化作用还原硝酸盐并不能阻止有害蓝藻的爆发，反而可能有利于固氮蓝藻的优势生长。本研究的总体目标是在不同时间点的犹他湖的几个地点估算固氮速率和相关固氮酶（nif 基因）功能基因的表达。为了进行比较，还在大盐湖的半咸水法明顿湾进行了一次采样。研究了表层水的前 20 厘米的微生物生态学，以评估优势蓝藻群落和与 N 相关的代谢。我们的研究表明，多瘤鱼腥藻和节旋藻分别是犹他湖和法明顿湾的潜在固氮生物。犹他湖的原位固氮速率为 0-0.73 nmol N hrL，法明顿湾的固氮速率为 0-0.85 nmol N hrL，这些速率与 nif 基因的丰度和表达呈正相关。此外，还测量了沉积物中的硝酸盐还原（0.002-0.094 mg N VSS hr）。在两个湖泊中，amoA、nirS 和 nirK 的丰度均呈显著正相关（R=0.56-0.87，p<0.05，Spearman）。法明顿湾的一个地点的 nirK 基因丰度较低是个例外，该地点还检测到高氨氮保留。基于质量平衡方法，我们得出结论，通过异养细菌介导的反硝化作用导致的无机 N 损失量仍然超过了固氮作用的 N 输入量，这与大多数湖泊、河流和海洋生态系统非常相似。这表明，反硝化等氮循环过程通过异养细菌的作用，促进了湖泊中的 N 输出，导致了 N 的限制。
