Messer Lauren F, Brown Mark V, Van Ruth Paul D, Doubell Mark, Seymour Justin R
Climate Change Cluster, University of Technology Sydney, Sydney, New South Wales, Australia.
Centre for Microbiome Research, School of Biomedical Sciences, Queensland University of Technology, Brisbane, QLD, Australia.
PeerJ. 2021 Mar 1;9:e10809. doi: 10.7717/peerj.10809. eCollection 2021.
Biological dinitrogen (N) fixation is one mechanism by which specific microorganisms (diazotrophs) can ameliorate nitrogen (N) limitation. Historically, rates of N fixation were believed to be limited outside of the low nutrient tropical and subtropical open ocean; however, emerging evidence suggests that N fixation is also a significant process within temperate coastal waters. Using a combination of amplicon sequencing, targeting the nitrogenase reductase gene (), quantitative PCR, and N stable isotope tracer experiments, we investigated spatial patterns of diazotroph assemblage structure and N fixation rates within the temperate coastal waters of southern Australia during Austral autumn and summer. Relative to previous studies in open ocean environments, including tropical northern Australia, and tropical and temperate estuaries, our results indicate that high rates of N fixation (10-64 nmol L d) can occur within the large inverse estuary Spencer Gulf, while comparatively low rates of N fixation (2 nmol L d) were observed in the adjacent continental shelf waters. Across the dataset, low concentrations of NO/NO were significantly correlated with the highest N fixation rates, suggesting that N fixation could be an important source of new N in the region as dissolved inorganic N concentrations are typically limiting. Overall, the underlying diazotrophic community was dominated by sequences from Cluster 1 unicellular cyanobacteria of the UCYN-A clade, as well as non-cyanobacterial diazotrophs related to , and Cluster 3 sulfate-reducing deltaproteobacteria. Diazotroph community composition was significantly influenced by salinity and SiO concentrations, reflecting the transition from UCYN-A-dominated assemblages in the continental shelf waters, to Cluster 3-dominated assemblages in the hypersaline waters of the inverse estuary. Diverse, transitional diazotrophic communities, comprised of a mixture of UCYN-A and putative heterotrophic bacteria, were observed at the mouth and southern edge of Spencer Gulf, where the highest N fixation rates were observed. In contrast to observations in other environments, no seasonal patterns in N fixation rates and diazotroph community structure were apparent. Collectively, our findings are consistent with the emerging view that N fixation within temperate coastal waters is a previously overlooked dynamic and potentially important component of the marine N cycle.
生物固氮是特定微生物（固氮菌）缓解氮限制的一种机制。从历史上看，人们认为在低营养的热带和亚热带公海之外，固氮速率受到限制；然而，新出现的证据表明，固氮在温带沿海水域也是一个重要过程。我们结合使用针对固氮酶还原酶基因的扩增子测序、定量PCR和氮稳定同位素示踪实验，研究了澳大利亚南部温带沿海水域在秋季和夏季期间固氮菌群落结构和固氮速率的空间格局。相对于之前在包括澳大利亚北部热带地区以及热带和温带河口在内的公海环境中的研究，我们的结果表明，在大型反向河口斯宾塞湾内可出现较高的固氮速率（10 - 64 nmol L⁻¹ d⁻¹），而在相邻的大陆架水域中观察到的固氮速率相对较低（2 nmol L⁻¹ d⁻¹）。在整个数据集中，低浓度的NO₃⁻/NO₂⁻与最高的固氮速率显著相关，这表明由于溶解无机氮浓度通常具有限制性，固氮可能是该区域新氮的重要来源。总体而言，潜在的固氮群落主要由UCYN - A进化枝的第1类单细胞蓝细菌序列、与Arthrospira相关的非蓝细菌固氮菌以及第3类硫酸盐还原δ变形菌组成。固氮菌群落组成受盐度和SiO₃²⁻浓度的显著影响，反映了从大陆架水域中以UCYN - A为主的群落向反向河口高盐水域中以第3类为主的群落的转变。在斯宾塞湾的河口和南部边缘观察到由UCYN - A和假定的异养细菌混合组成的多样的过渡性固氮菌群落，这些地方的固氮速率最高。与其他环境中的观察结果不同，固氮速率和固氮菌群落结构没有明显的季节性模式。总体而言，我们的研究结果与新出现的观点一致，即温带沿海水域的固氮是海洋氮循环中一个此前被忽视的动态且可能重要的组成部分。
