TY  - JOUR
A1  - Riemann, Lasse
A1  - Rahav, Eyal
A1  - Passow, Uta
A1  - Grossart, Hans-Peter
A1  - de Beer, Dirk
A1  - Klawonn, Isabell
A1  - Eichner, Meri
A1  - Benavides, Mar
A1  - Bar-Zeev, Edo
T1  - Planktonic aggregates as hotspots for heterotrophic diazotrophy: the plot thickens
JF  - Frontiers in microbiology
N2  - Biological dinitrogen (N-2) 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 N-2 fixation by NCDs and propose a conceptual framework for aggregate-associated N-2 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.
KW  - aggregates
KW  - nitrogen fixation
KW  - heterotrophic bacteria
KW  - marine
KW  - aquatic
KW  - NCDs
Y1  - 2022
U6  - https://doi.org/10.3389/fmicb.2022.875050
SN  - 1664-302X
VL  - 13
PB  - Frontiers Media
CY  - Lausanne
ER  -