“…Aquatic studies have traditionally focused on photoautotrophic cyanobacterial diazotrophs inhabiting oligotrophic and sunlit environments where energy is made available via photosynthetic carbon fixation ( Zehr, 2011 ). However, during the last decade it has become evident that non-cyanobacterial diazotrophs (NCDs; see Box 1 ) have an almost ubiquitous distribution in pelagic environments ( Farnelid et al, 2011 ; Langlois et al, 2015 ; Geisler et al, 2020 ; Hallstrøm et al, 2021 ; Messer et al, 2021 ). For instance, the presence and/or expression of the nitrogenase gene ( nifH ) by NCDs has been reported from low latitude open oceans ( Halm et al, 2011 ; Moisander et al, 2014 ; Langlois et al, 2015 ) to environments previously not regarded as suitable for N 2 fixation such as eutrophic rivers, estuaries and coastal waters ( Mulholland et al, 2012 ; Bentzon-Tilia et al, 2015 ; Geisler et al, 2020 ; Hallstrøm et al, 2021 ), the aphotic deep sea ( Rahav et al, 2013 ; Benavides et al, 2015 ), and nutrient-rich arctic waters ( Harding et al, 2018 ).…”