Planktonic Aggregates as Hotspots for Heterotrophic Diazotrophy: The Plot Thickens

Abstract

Biological dinitrogen (N2) fixation is performed solely by specialized bacteria and archaea termed diazotrophs, introducing new reactive nitrogen into aquatic environments. Conventionally, phototrophic cyanobacteria are considered the major diazotrophs in aquatic environments. However, accumulating evidence indicates that diverse non-cyanobacterial diazotrophs (NCDs) inhabit a wide range of aquatic ecosystems, including temperate and polar latitudes, coastal environments and the deep ocean. NCDs are thus suspected to impact global nitrogen cycling decisively, yet their ecological and quantitative importance remain unknown. Here we review recent molecular and biogeochemical evidence demonstrating that pelagic NCDs inhabit and thrive especially on aggregates in diverse aquatic ecosystems. Aggregates are characterized by reduced-oxygen microzones, high C:N ratio (above Redfield) and high availability of labile carbon as compared to the ambient water. We argue that planktonic aggregates are important loci for energetically-expensive N2 fixation by NCDs and propose a conceptual framework for aggregate-associated N2 fixation. Future studies on aggregate-associated diazotrophy, using novel methodological approaches, are encouraged to address the ecological relevance of NCDs for nitrogen cycling in aquatic environments.

Dark Diazotrophy during the Late Summer in Surface Waters of Chile Bay, West Antarctic Peninsula

“…However, in this study, we did not detect any nif H gene OTUs related to photoautotrophs, such as cyanobacteria and the UCYN-A symbiont, as detected in other polar regions [11,97]. The high availability of organic carbon provided by eukaryotic photosynthesis and the dark chemoautotrophic activity observed in these waters of Chile Bay, which is reinforced by the possible occurrence of diazotrophic chemoautotrophs and heterotrophs, further support the cosmopolitan distribution of these organisms in the ocean [28,73,89], which now includes Western Antarctica. Chemoautotrophy, an important process for dark carbon fixation in Chile Bay, has previously been detected by active genes related to ribulose-1,5bisphosphate carboxylase/oxygenase (RuBisCO) (the Calvin cycle) [39]; however, only <5.6% of all best-hit related sequences in our phylogenetic reconstruction of the nif H gene were identified as chemoautotrophs with potential as N 2 fixers; that is, a coupling of both processes.…”

“…However, more effort is needed to evaluate the real effect of phosphorus and other elements, such as iron (which was not evaluated in the present study), on diazotrophic organisms, like that seen for Trichodesmium cultures from the central Atlantic Ocean [107]. In addition, to better understand the functioning of these diazotrophic heterotrophs and chemoautotrophs found here in the WAP, laboratory studies on the physiological adaptations to avoid oxygen-induced nitrogenase inhibition [108][109][110] are now necessary, as are studies on the existence of potential microenvironments in sinking particles [73,85].…”

“…These diazotrophs have recently been found worldwide in hundreds of metagenomes from the Tara Ocean project [28], and in the Eastern Antarctic Ocean [11] and the Central Arctic Ocean [13], confirming their presence in polar regions. In addition, diverse and active heterotrophic and chemoautotrophic diazotrophs have been found in sinking particles [72], planktonic aggregates [73], aphotic waters [22,74,75], oxygen minimum zones [76][77][78][79] and NH 4 + -rich sulfidic-anoxic waters of the Baltic Sea [80], where these nif H sequences were mainly related to anaerobic bacteria [11,22]. The nif H sequences related to the Cluster I nitrogenase found in Chile Bay also indicate the potential presence in these Antarctic waters of anaerobic heterotrophic diazotrophs associated with anoxygenic bacteria, such as Desulfuromonas, as well as chemoautotrophs associated with methanotrophy and methylotrophy (Supplementary Materials, Table S1).…”

“…Due to the high-energy requirements of the N 2 fixation reaction, it has been suggested that dissolved organic matter stimulates heterotrophic diazotrophs in aphotic environments [74,75,81], as well as in coastal waters [82,83]. Furthermore, the interior of carbon-rich particles has been described as a suitable microenvironment for chemosynthetic and heterotrophic N 2 fixation [72,73,84,85]. This scenario could also explain the presence of nif H sequences closely affiliated with soil and marine sediments in the shallow waters of Chile Bay (maximum depth between 80-230 m), where water mixing conditions have been reported [53], which could enhance the suspension of particulate matter from bottom sediment.…”

Quantification of Aquatic Unicellular Diazotrophs by Immunolabeled Flow Cytometry

“…Studies have indicated that aquatic diazotrophs include autotrophic (Zehr, 2011), heterotrophic (Bombar et al, 2016; Riemann et al, 2010) and mixotrophic (Benavides et al, 2020; Feng et al, 2010) metabolism to maintain the energetic requirements of the nitrogenase enzyme. Aquatic diazotrophs can be found in benthic mats, organized in long filamentous chains (up to few hundreds of micrometers), free living planktonic cells, or associated with aggregates (Bergman et al, 2013; Bertics et al, 2010; Riemann et al, 2022).…”