Abstract
Planktonic heterotrophic diazotrophs (N2-fixers) are widely distributed in marine and freshwater systems, yet limited information is available on their activity, especially in environments with adverse conditions for diazotrophy (e.g., N-rich and oxygenated). Here, we followed the localization and activity of heterotrophic diazotrophs in the hyper-eutrophic N-rich Qishon River—an environment previously considered to be unfavorable for diazotrophy. Our results indicate high heterotrophic N2 fixation rates (up to 6.9 nmol N L–1 d–1), which were approximately three fold higher at an upstream location (freshwater) compared to an estuary (brackish) site. Further, active heterotrophic diazotrophs were capture associated with free-floating aggregates by a newly developed immunolocalization approach. These findings provide new insights on the activity of heterotrophic diazotrophs on aggregates in environments previously considered with adverse conditions for diazotrophy. Moreover, these new insights may be applicable to other aquatic regimes worldwide with similar N-rich/oxygenated conditions that should potentially inhibit N2 fixation.
Highlights
Dissolved inorganic nitrogen (DIN) often controls primary and bacterial production in aquatic systems (Gruber and Galloway, 2008; Moore et al, 2013; Rahav et al, 2018b)
Our results suggest that particles could be loci for N2 fixation by heterotrophic bacteria in eutrophic environments traditionally considered to be unfavorable for diazotrophy
Since we used the same 15N2-enriched water in each sampling event, a comparison between the estuary and stream sites at the Qishon River reflects the measured differences between the sites.15N2 stock was added to the 1 L experimental bottles (5% of total sample volume, Rahav et al, 2015) and incubated for 48 h in the dark with DCMU to impair phototrophic diazotrophy (Rahav et al, 2015; Benavides et al, 2018) and focus on heterotrophic diazotrophs
Summary
Dissolved inorganic nitrogen (DIN) often controls primary and bacterial production in aquatic systems (Gruber and Galloway, 2008; Moore et al, 2013; Rahav et al, 2018b). In comparison to the surrounding waters, these aggregates usually contain high levels of organic and inorganic compounds, trace elements, as well as detrital matter and fecal pellets held together by a sticky scaffold comprised of proteins and polysaccharides such as transparent exopolymer particles (TEP) (Ploug and Grossart, 2000; Passow, 2002) These aggregates usually have a high C:N ratio compared to the typical ∼6.6:1 Redfield ratio, thereby inducing N-limiting conditions for different microbes, including heterotrophic diazotrophs. The high aerobic respiration by bacteria colonizing aggregates (including heterotrophic diazotrophs), combined with slow diffusion rates, may lead to reduced oxygen levels toward the aggregate’s center of ≤80% air saturation and occasionally even to anoxia conditions (Paerl and Prufert, 1987; Klawonn et al, 2015) Such oxygen-reduced micro-zones may greatly benefit diazotrophs since the nitrogenase enzyme may be irreversibly damaged by O2 (Gruber, 2008). Our results suggest that particles could be loci for N2 fixation by heterotrophic bacteria in eutrophic environments traditionally considered to be unfavorable for diazotrophy
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