Newell Silvia E, Pritchard Kaitlyn R, Foster Sarah Q, Fulweiler Robinson W
Department of Earth and Environment, Boston University, Boston, MA, USA; Department of Earth and Environmental Sciences, Wright State University, Dayton, OH, USA.
Department of Marine and Environmental Sciences, Northeastern University , Boston, MA , USA.
PeerJ. 2016 Jan 25;4:e1615. doi: 10.7717/peerj.1615. eCollection 2016.
Primary production in coastal waters is generally nitrogen (N) limited with denitrification outpacing nitrogen fixation (N2-fixation). However, recent work suggests that we have potentially underestimated the importance of heterotrophic sediment N2-fixation in marine ecosystems. We used clone libraries to examine transcript diversity of nifH (a gene associated with N2-fixation) in sediments at three sites in a temperate New England estuary (Waquoit Bay, Massachusetts, USA) and compared our results to net sediment N2 fluxes previously measured at these sites. We observed nifH expression at all sites, including a site heavily impacted by anthropogenic N. At this N impacted site, we also observed mean net sediment N2-fixation, linking the geochemical rate measurement with nifH expression. This same site also had the lowest diversity (non-parametric Shannon = 2.75). At the two other sites, we also detected nifH transcripts, however, the mean N2 flux indicated net denitrification. These results suggest that N2-fixation and denitrification co-occur in these sediments. Of the unique sequences in this study, 67% were most closely related to uncultured bacteria from various marine environments, 17% to Cluster III, 15% to Cluster I, and only 1% to Cluster II. These data add to the growing body of literature that sediment heterotrophic N2-fixation, even under high inorganic nitrogen concentrations, may be an important yet overlooked source of N in coastal systems.
沿海水域的初级生产通常受氮（N）限制，反硝化作用超过固氮作用（N2固定）。然而，最近的研究表明，我们可能低估了海洋生态系统中异养沉积物N2固定的重要性。我们使用克隆文库来检测美国马萨诸塞州韦夸特湾一个温带新英格兰河口三个地点沉积物中nifH（与N2固定相关的基因）的转录多样性，并将我们的结果与之前在这些地点测量的沉积物净N2通量进行比较。我们在所有地点都观察到了nifH表达，包括一个受人为氮严重影响的地点。在这个受氮影响的地点，我们还观察到了平均沉积物净N2固定，将地球化学速率测量与nifH表达联系起来。这个地点的多样性也最低（非参数香农指数=2.75）。在其他两个地点，我们也检测到了nifH转录本，然而，平均N2通量表明存在净反硝化作用。这些结果表明，这些沉积物中同时存在N2固定和反硝化作用。在本研究的独特序列中，67%与来自各种海洋环境的未培养细菌关系最为密切，17%与第三类群相关，15%与第一类群相关，只有1%与第二类群相关。这些数据进一步丰富了文献，表明沉积物异养N2固定，即使在高无机氮浓度下，也可能是沿海系统中一个重要但被忽视的氮源。
