Effect of mesoscale eddies on diazotroph community structure and nitrogen fixation rates in the South China Sea

Nitrogen fixation in mesoscale eddies of the North Pacific Subtropical Gyre: patterns and mechanisms

“…Our observation of higher rates in anticyclones than cyclones on two out of three cruises agrees with previous reports from the North Pacific (Fong et al 2008;Wilson et al 2017), Indian Ocean, Eastern South Pacific, and South China Sea (Holl et al 2007;Löscher et al 2016;Liu et al 2020). However, the opposite trend was observed during the 2016 cruise, with the highest NFRs in the cyclonic eddy.…”

“…There are several mechanisms through which eddies may influence N2 fixation. N-depletion in the surface waters of anticyclones, driven by isopycnal downwelling, has been suggested to explain previous observations of high NFRs in anticyclonic eddies (Wilson et al 2017;Liu et al 2020). Eddies may also affect other bottom-up factors that control NFRs (i.e.…”

Bridging the Spatiotemporal Gap in Diazotroph Activity and Diversity With High-Resolution Measurements

“…Such tolerance makes UCYN-A a good candidate to benefit from the biogeochemical conditions provided by eddy pumping in cyclonic eddies. However, abundant UCYN-A have been reported in anticyclones but not cyclones (Fong et al, 2008;Robidart et al, 2014;Liu et al, 2020). Perhaps driven by Ekman pumping of deep nutrientrich waters in anticyclonic eddies (Anderson et al, 2011), as proposed to explain higher abundances of Trichodesmium in cyclonic eddies in the North Atlantic (Olson et al, 2015).…”

“…(5) Submesoscale (1-10 km, days) structures such as filaments, and mesoscale (10-200 km, several weeks) structures including fronts and eddies (here collectively referred to as "fine scales," see section "How Fine Scale Processes May Affect Diazotrophs" of the main text). Studies seeking to link fine scales to diazotrophy have conducted discrete sampling along the central edge of mesoscale eddies at stations situated tens of km apart at best (e.g., Fong et al, 2008;Zhang et al, 2011;Löscher et al, 2016;Liu et al, 2020). In these studies, the distribution (Continued) FIGURE 1 | Continued and activity of diazotrophs has been approached with traditional methods for the characterization of the diazotroph community (i.e., microscopy, quantitative PCR, 15 N 2 stable isotope incubations).…”

“…The unicellular diazotroph Crocosphaera watsonii can be found as small (2-3.5 µm) and large (4-5 µm) cell types (Bench et al, 2016). While the oligotrophic conditions that promote Trichodesmium in anticyclonic eddies similarly contribute to the proliferation of C. watsonii (Fong et al, 2008;Liu et al, 2020), physical accumulation at high density frontal areas is less likely for non-floating unicellular diazotrophs. The large cell type may be more prone to accumulation aided by the excretion of extracellular polymeric substances and formation of aggregates, a characteristic not described for the small cell type (Webb et al, 2009;Sohm et al, 2011) and implied previously to explain their distinct vertical distributions at the edge of an anticyclone (Bench et al, 2016).…”