On the Relationship Between Hydrogen Saturation in the Tropical Atlantic Ocean and Nitrogen Fixation by the Symbiotic Diazotroph UCYN‐A

Abstract

Dissolved hydrogen measurements were made at high resolution in surface waters along a tropical north Atlantic transect between Guadeloupe and Cape Verde in 2015 (Meteor 116). Parallel water samples acquired to assess the relative abundance of the nifH gene from several types of diazotrophs, indicated that Trichodesmium and UCYN‐A were dominant in this region. We show that a high degree of correlation exists between the hydrogen saturations and UCYN‐A nifH abundance, and a weak correlation with Trichodesmium. The findings suggest that nitrogen fixation by UCYN‐A is a major contributor to hydrogen supersaturations in this region of the ocean. The ratio of hydrogen released to nitrogen fixed has not been determined for this symbiont, but the indications are that it may be high in comparison with the small number of diazotrophs for which the ratio has been measured in laboratory cultures. We speculate that this would be consistent with the diazotroph being an exosymbiont on its haptophyte host. Our high resolution measurements of hydrogen concentrations are capable of illustrating the time and space scales of inferred activity of diazotrophs in near real‐time in a way that cannot be achieved by biological sampling and rate measurements requiring incubations with 15N2. Direct measurement of high resolution spatial variability would be relatively challenging through collection and analysis of biological samples by qPCR, and extremely challenging by 15N‐uptake techniques, neither of which methods yields real‐time data. Nonetheless, determination of fixation rates still firmly depends on the established procedure of incubations in the presence of 15N2.

Latitudinal constraints on the abundance and activity of the cyanobacterium UCYN‐A and other marine diazotrophs in the North Pacific

“…) and the absence of such strong relationships for other diazotrophic groups (Table ) suggest that UCYN‐A was the primary diazotroph driving the observed community NFRs in the NPSG. A similar relationship between UCYN‐A abundance and hydrogen production (a proxy for N 2 fixation) has been described in the tropical North Atlantic (Moore et al ), suggesting a major contribution of UCYN‐A to community N 2 fixation in that region. Additionally, during the 2017 cruise, we directly measured cell‐specific NFRs of UCYN‐A symbioses in five NPSG stations (Fig.…”

H2 accumulation and N2 fixation variation by Ni limitation in Cyanothece

“…a suggest that Ni availability can be an important factor causing the huge variations of H 2 concentrations observed in oceanic surface water. Although both H 2 concentrations and N 2 fixation rates varied considerably in the tropical and subtropical oceans (Moore et al , ; Chen et al ), field data show that total dissolved Ni concentrations are relatively comparable in the surface water (Bruland et al ; Mackey et al ; Wang et al ; Schlitzer et al ), around 2 nmol L −1 . Unlike Fe and Cu which are strongly chelated by organic ligands in the surface water, studies showed that a significant portion (40–80%) of dissolved Ni in the surface water is labile (Achterberg and van den Berg ; Saito et al ; Boiteau et al ), which was operationally determined by the exchangeable dissolved Ni with 12‐h 20‐ μ mol L −1 dimethylglyoxime treatment.…”

“…In the North Pacific, Moore et al () also showed that net N 2 fixation rates exhibited a strong positive correlation with H 2 concentrations ( r = 0.96), which ranged from 0.30 to 12.6 nmol L −1 and were higher than its corresponding seawater equilibrium concentrations under H 2 atmospheric partial pressure. In the Atlantic Ocean, it was reported that H 2 concentrations were positively correlated with the abundances of Candidatus Atelocyanobacterium thalassa ( r = 0.77), a dominant unicellular diazotrophic cyanobacterium (Moore et al ). Laboratory culture studies also showed that prevalent marine diazotrophic cyanobacteria, including Trichodesmium , Crocosphaera , and Cyanothece , produced significant amount of H 2 during N 2 fixation process (Punshon and Moore ; Wilson et al , b , ).…”

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

“…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). Underway N 2 fixation rate measurements hold great promise in providing submesoscale resolved data (e.g., Moore et al, 2018;Tang et al, 2019), but to date such approaches have not been investigated in depth with parallel physical oceanography parameters. 6Regional studies often stem from direct N 2 fixation rates or diazotroph abundance measurements obtained from a single oceanographic cruise or a combination of several datasets extrapolated over a given region (Luo et al, 2012;Tang et al, 2019).…”

“…However, uptake hydrogenase enzymes are also present in diazotrophs to re-assimilate H 2 and recover energy, altering the ratio of N 2 reduced to H 2 released, which in practice varies between 0.2 and 13 (Wilson et al, 2010b;Wilson et al, submitted). This method yields rates every ∼20 min, and can be used in diel cycle studies (Wilson et al, 2010a,b) or on highfrequency measurements over oceanographic cruise transects (Moore et al, 2014(Moore et al, , 2018.…”