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.

Aix Marseille Univ Université de Toulon CNRS IRD MIO Marseille France

Department of Biological Oceanography Leibniz Institute for Baltic Sea Research Rostock Germany

Department of Plankton and Microbial Ecology Leibniz Institute of Freshwater Ecology and Inland Fisheries Stechlin Germany

Institute for Biochemistry and Biology Potsdam University Potsdam Germany

Institute of Microbiology CAS Centre ALGATECH Třeboň Czechia

Israel Oceanographic and Limnological Research Haifa Israel

Marine Biology Section University of Copenhagen Helsingør Denmark

Max Planck Institute for Marine Microbiology Bremen Germany

Ocean Science Centre Memorial University of Newfoundland St John's NL Canada

The Jacob Blaustein Institutes for Desert Research Zuckerberg Institute for Water Research Ben Gurion University of the Negev Be'er Sheva Israel

Turing Center for Living Systems Aix Marseille University Marseille France

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