Contribution of Heterotrophic Diazotrophs to N2 Fixation in a Eutrophic River: Free-Living vs. Aggregate-Associated

Abstract

Recent studies have indicated that heterotrophic diazotrophs are highly diverse and fix N2 in aquatic environments with potentially adverse conditions for diazotrophy, such as oxic and rich in total nitrogen. In this study, we compared the activity and diversity of heterotrophic diazotrophs associated with aggregates (>12 μm) to free-living cells in the eutrophic Qishon River during the winter and summer seasons. Overall, measured heterotrophic N2 fixation rates in the Qishon River ranged between 2.6–3.5 nmol N L–1 d–1. Heterotrophic N2 fixation was mainly associated with aggregates in the summer samples (74 ± 24%), whereas during the winter the bulk diazotrophic activity was mostly ascribed to the free-living fraction (90 ± 6%). In addition, immunolabeled micrographs indicated the presence of aggregate-associated heterotrophic diazotrophs in both seasons, while phototrophic diazotrophs were also captured during the winter. The richness of free-living and aggregate-associated heterotrophic diazotrophs were overall similar, yet the evenness of the later was significantly smaller, suggesting that few of the species gained advantage from particle lifestyle. The differences in the activity, micro-localization and diversity of the diazotrophic community were mostly attributed to spatiotemporal changes in the ambient C:N ratios (total organic carbon, TOC: total nitrogen) and the TOC concentrations. Taken together, our results shed new light on the contribution of heterotrophic diazotroph associated with aggregates to total heterotrophic N2 fixation in oxic, highly eutrophic aquatic environments.

Planktonic Aggregates as Hotspots for Heterotrophic Diazotrophy: The Plot Thickens

“… The association of NCDs with planktonic aggregates. (A) Enlarged confocal images of diazotrophs associated with aggregates after staining (red) the polysaccharide matrix by concanavalin A ( Geisler et al, 2022 ). Enlarged images within the aggregate were captured to differentiate between three distinct channels (from left to right): total bacteria (stained by DAPI, blue); total cyanobacteria by autofluorescence of the phycoerythrin pigment (orange/white), and diazotrophs by immunolabeling nitrogenase enzyme.…”

Divergent drivers of the spatial variabilities in CO2, CH4, N2O, and N2 concentrations along the Rhine river and the Mittelland canal in Germany

“…al., 2014; McCutchan et al, 2003; Ritz et al, 2018; Wang et al, 2018).In contrast to the canal ecosystem, longitudinal N 2 O hotspots downstream of the Rhine river were linked to processes other than denitri cation. High N 2 xation rates by autotrophic and heterotrophic diazotrophs, which supply fresh and bioavailable N, have been previously reported in freshwater environments(Geisler et al, 2022;Riemann et al, 2022). Within rivers, several studies have linked measures of N 2 undersaturation to signi cant rates of N 2 xation by diazotrophs(Wang et al, 2021;Wu et al, 2013…”

Estuarine mangrove niches select cultivable heterotrophic diazotrophs with diverse metabolic potentials—a prospective cross-dialog for functional diazotrophy

“…At the rhizospheric level, plant root exudates (including sugars, organic acids, and mucilage) or overall soil organic C content, can act as C sources for heterotrophic FLNF (Smercina et al, 2019). BNF by heterotrophic FLNF was positively associated with total organic C content (Smercina et al, 2019;Geisler et al, 2022), but non-diazotrophs may outcompete these heterotrophic FLNF in C-and N-replete niches (Sun et al, 2020). Also, the relative abundance of the nifH gene (which encodes the iron protein subunit of nitrogenase) in native niches was negatively associated with total N or total organic C content in the niche, indicating loss of the diazotrophic function in niches with high C and N availability (Sun et al, 2020).…”

“…and Azotobacter vinlandii ( Bombar et al, 2016 ). Marine diazotrophs also form large aggregates of cells (>1 mm in diameter) as an innovative strategy to create low-O 2 microhabitats near the center of the aggregates ( Bombar et al, 2016 ; Geisler et al, 2022 ). These aggregates are typified by high labile C (originating from the hydrolysis of the aggregates’ polysaccharide matrix) and N limitation ( Geisler et al, 2022 ).…”