Pelagic N₂ fixation dominated by sediment diazotrophic communities in a shallow temperate estuary

Abstract

Estuaries receive substantial anthropogenic nitrogen loading and are mainly considered net nitrogen sinks. While several studies have identified diverse diazotrophic communities in estuarine sediments, the role of pelagic diazotrophs in these systems is not well understood. We investigated the links between diazotrophic community composition, nitrogenase (nifH) gene expression, N 2 fixation, and environmental conditions in Narragansett Bay (USA). Pelagic N 2 fixation rates ranged between 0.02 and 9.41 nmol N L À1 d À1 and correlated significantly with fluctuations in diazotroph community composition. These fluctuations were also correlated with temperature, salinity, and mean sea level. The dominant sequences in our pelagic samples were related to sequences previously detected in the bay's sediments and were dominated by nifH gene Clusters I and III. We interpret this as a coupling between sediment and pelagic diazotroph communities and speculate that resuspension plays an important role for pelagic N 2 fixation in shallow estuarine environments such as Narragansett Bay. For instance, the finding of active sulfate reducers in the oxygenated water illustrates that the sediment-pelagic coupling can impact nutrient cycling in shallow environments. The pelagic N 2 fixation measured during our study period showed only a minor contribution (< 1%) to the total estimated nitrogen load to Narragansett Bay. However, with intensifying nitrogen management in estuaries, the need to constrain the rates of pelagic N 2 fixation in these systems will be essential for estimating nitrogen fluxes within the bay and to the adjacent coastal ecosystems.

Planktonic Aggregates as Hotspots for Heterotrophic Diazotrophy: The Plot Thickens

“…Aquatic studies have traditionally focused on photoautotrophic cyanobacterial diazotrophs inhabiting oligotrophic and sunlit environments where energy is made available via photosynthetic carbon fixation ( Zehr, 2011 ). However, during the last decade it has become evident that non-cyanobacterial diazotrophs (NCDs; see Box 1 ) have an almost ubiquitous distribution in pelagic environments ( Farnelid et al, 2011 ; Langlois et al, 2015 ; Geisler et al, 2020 ; Hallstrøm et al, 2021 ; Messer et al, 2021 ). For instance, the presence and/or expression of the nitrogenase gene ( nifH ) by NCDs has been reported from low latitude open oceans ( Halm et al, 2011 ; Moisander et al, 2014 ; Langlois et al, 2015 ) to environments previously not regarded as suitable for N 2 fixation such as eutrophic rivers, estuaries and coastal waters ( Mulholland et al, 2012 ; Bentzon-Tilia et al, 2015 ; Geisler et al, 2020 ; Hallstrøm et al, 2021 ), the aphotic deep sea ( Rahav et al, 2013 ; Benavides et al, 2015 ), and nutrient-rich arctic waters ( Harding et al, 2018 ).…”

Eddy-driven diazotroph distribution: horizontal variability prevails over particle sinking speed

“…An exception occurred in the suspended particle fraction at station 2, where UCYN-A accounted only for 3% of the reads and Crocosphaera dominated instead (94.5% cyanobacterial, 58% overall reads), becoming the second most abundant cyanobacterial group. While Crocosphaera is typically low or unreported in the northeast compared to the southwest Atlantic [33,34], high abundances have been linked to high N 2 xation rates measured in anticyclonic eddies in the North Paci c [19,31]. In this study, 99% of the Crocosphaera sequences were found between stations 1 and 3, seemingly forming a bloom at the eastern edge of the eddy.…”

Global biogeography of N2-fixing microbes:nifHamplicon database and analytics workflow

“…Switching the trimming approach from one based on individual read quality profiles (using truncQ in Table 3) to fixed-length trimming based on overall quality profiles of the forward and reverse reads (using truncLen.fwd and truncLen.rev in Table 2) resulted in more reads being retained for some studies (Sato et al, 2021; Selden et al, 2021; Hallstrøm et al, 2022b; Gradoville et al, 2020). However, fixed-length trimming would have required the selection of trim lengths based on visual, qualitative assessments of hundreds of FASTQ quality plots which is difficult to accomplish in a systematic manner.…”