The relative contributions of unicellular and filamentous diazotrophs to N2 fixation in the South China Sea and the upstream Kuroshio

Nitrogen Fixation by Trichodesmium and unicellular diazotrophs in the northern South China Sea and the Kuroshio in summer

“…The growth of unicellular cyanobacteria in this station could be limited by iron resources due to the competition from non-diazotrophs. The maximum whole-water N 2 fixation rate was observed in the 10% light depths in our study, and the result was different from the previous study that the maximum N 2 fixation was in the surface layer and decreasing with depth in the lower euphotic zone 31 . Notably, the main factor resulted in this situation was the N 2 fixation rate by >10 μm fractions in the 10% light depths were larger than other depths extraordinarily.…”

“…were traditionally perceived to be the most abundant cyanobacteria in the marine, and it has been demonstrated to contribute the vast majority of N 2 fixation rates and primary production to the ocean 3,42 . However, increasing studies have shown that unicellular diazotrophs often appear to be of equal or greater abundance to the other known N 2 fixers due to the revolution use of polymerase chain reaction (PCR) techniques 7,31 . A DIC budget analysis at the SEATS in the SCS basin demonstrated that unicellular diazotrophs contributed enormously to the N 2 fixation, which were thus an indispensable part of the N cycle study in the SCS 43 .…”

“…Chen et al . 31 observed that unicellular diazotrophs contributed 65% and 50% of the total N 2 fixation in the SCS and the Kuroshio, respectively 31 . According to the results of size fractionation experiments in the present study, unicellular diazotrophs were important N 2 fixers in the surface of the nSCS and the Kuroshio.…”

“…In the present study, however, the correlation between size-fractioned N 2 fixation and temperature was not significant in both the nSCS and the Kuroshio. This might be attributed to the unusually strong wind and highly turbulent surface during summer cruise 31 . Diazotrophs, especially Trichodesmium , usually thrives in relatively stable environment.…”

High Rates of N2 Fixation in Temperate, Western North Atlantic Coastal Waters Expand the Realm of Marine Diazotrophy

“…We now know that marine diazotrophs are diverse and include unicellular cyanobacteria as well as noncyanobacterial diazotrophs that occupy a wider range of marine habitats than previously thought (Berthelot et al, 2017;Bombar et al, 2016;Messer et al, 2016;Moisander et al, 2010;Rees et al, 2009;Zehr & Turner, 2001), but we know little about their physiological capabilities and environmental controls on their biogeography. High N 2 fixation rates have now been measured during summer in temperate northwestern Atlantic and Pacific coastal waters when dissolved N concentrations in surface water were seasonally depleted (Mulholland et al, 2012;Shiozaki et al, 2015), in temperate and tropical coastal systems (Cassar et al, 2018;Chen et al, 2014;Grosse et al, 2010;Larsson et al, 2001;Moisander et al, 2010;Mulholland et al, 2012;Rees et al, 2009;Shiozaki et al, 2015;Voss et al, 2006;Zhang et al, 2012), in nutrient-rich coastal upwelling systems (Voss et al, 2004;Wen et al, 2017), and in coastal Arctic Seas (Blais et al, 2012;Harding et al, 2018;Sipler et al, 2017), broadening the latitudinal range and the diversity of habitats supporting diazotrophy. Based on their phylogenetic affiliations and the lack of quantifiable N 2 fixation rates, the high nifH gene diversity and presence of diverse bacterial diazotroph groups in coastal systems were attributed to microorganisms being transported there from terrestrial systems or sediments, rather than to autochthonous populations of active planktonic diazotrophs (Jenkins et al, 2004;Zehr et al, 2003).…”