Controlling factors on the global distribution of a representative marine non-cyanobacterial diazotroph phylotype (Gamma A)

Abstract

Abstract. Non-cyanobacterial diazotrophs may be contributors to global marine N2 fixation, although the factors controlling their distribution are unclear. Here, we explored what controls the distribution of the most sampled non-cyanobacterial diazotroph phylotype, Gamma A, in the global ocean. First, we represented Gamma A abundance by its nifH quantitative polymerase chain reaction (qPCR) copies reported in the literature and analyzed its relationship to climatological biological and environmental conditions. There was a positive correlation between the Gamma A abundance and local net primary production (NPP), and the maximal observed Gamma A abundance increased with NPP and became saturated when NPP reached ∼ 400 mg C m−2 d−1. Additionally, an analysis using a multivariate generalized additive model (GAM) revealed that the Gamma A abundance increased with light intensity but decreased with increasing iron concentration. The GAM also showed a weak but significant positive relationship between Gamma A abundance and silicate concentration, as well as a substantial elevation of Gamma A abundance when the nitrate concentration was very high (≳ 10 µM). Using the GAM, these climatological factors together explained 43 % of the variance in the Gamma A abundance. Second, in addition to the climatological background, we found that Gamma A abundance was elevated in mesoscale cyclonic eddies in high-productivity (climatological NPP > 400 mg m−2 d−1) regions, implying that Gamma A can respond to mesoscale features and benefit from nutrient inputs. Overall, our results suggest that Gamma A tends to inhabit ocean environments with high productivity and low iron concentrations and therefore provide insight into the niche differentiation of Gamma A from cyanobacterial diazotrophs, which are generally most active in oligotrophic ocean regions and need a sufficient iron supply, although both groups prefer well-lit surface waters. More sampling on Gamma A and other non-cyanobacterial diazotroph phylotypes is needed to reveal the controlling mechanisms of heterotrophic N2 fixation in the ocean.

Global biogeography of N2-fixing microbes:nifHamplicon database and analytics workflow

“…Cosmopolitan distributions were evident for γ-proteobacterial (1G) and cyanobacterial diazotrophs (1B; Fig. 8a), corroborating and extending previous findings (Farnelid et al, 2011; Shao and Luo, 2022; Halm et al, 2012; Fernandez et al, 2011; Löscher et al, 2014; Cheung et al, 2016). At low to mid latitudes, γ- proteobacterial (1G) diazotrophs generally had high relative abundances and were often the dominant taxa when present.…”

“…Distinctive pigments and morphologies that enable some cyanobacterial diazotrophs to be identified by microscopy are lacking in many diazotrophs (Carpenter and Capone, 1983; Carpenter and Foster, 2002), including NCDs. Furthermore, many marine diazotrophs are uncultivated, which has required the use of cultivation-independent approaches such as PCR and quantitative PCR (qPCR) (Luo et al, 2012; Shao and Luo, 2022; Turk-Kubo et al, 2022). The nifH gene encodes the identical subunits of the Fe protein of nitrogenase, the enzyme that catalyzes the N 2 fixation reaction, and contains both highly conserved and variable regions enabling its use as a phylogenetic marker and as a proxy for N 2 -fixing potential in marine ecosystems globally (Gaby and Buckley, 2011).…”

Upper Ocean Biogeochemistry of the Oligotrophic North Pacific Subtropical Gyre: From Nutrient Sources to Carbon Export

“…For example, at the global scale, Trichodesmium may be under a weak but stable degree of top‐down control, while the degree of top‐down control on UCYNs varies substantially over space and time (H. Wang & Luo, 2022). Finally, NCDs, which are frequently found in the NPSG, appear to occupy different environmental niches from those of phototrophic cyanobacteria (Cheung et al., 2021; Shao & Luo, 2022), further hindering explanations of the observed distribution of N 2 fixation.…”

“…In contrast, Trichodesmium is typically most abundant at low latitudes and decreases northward. No clear latitudinal patterns have been observed for NCD phylotypes (Cheung et al., 2020, 2021; Church et al., 2008; Shao & Luo, 2022; Shiozaki et al., 2017). Notably, however, the distribution of the diazotrophic community shows seasonal variations (see Section 4.3), and because the field surveys were conducted in different seasons, it is likely that the spatial patterns of the diazotrophic community shown here are confounded by seasonal variability (Turk‐Kubo et al., 2023).…”

Diazotrophy in the Indian Ocean: Current understanding and future perspectives

“…S3). A recent study showed that gammaproteobacterial diazotrophs are negatively influenced by temperature during the NEM in the BoB and positively correlated with silicate, suggesting a possible association with diatoms (Shao and Luo 2022). Instead, in the BoB alpha‐ and betaproteobacterial diazotrophs are positively correlated to temperature (Wu et al 2022).…”