Abundance and distribution of major groups of diazotrophic cyanobacteria and their potential contribution to N₂ fixation in the tropical Atlantic Ocean

Abstract

The abundances of six N₂-fixing cyanobacterial phylotypes were profiled at 22 stations across the tropical Atlantic Ocean during June 2006, and used to model the contribution of the diazotrophs to N₂ fixation. Diazotroph abundances were measured by targeting the nifH gene of Trichodesmium, unicellular groups A, B, C (UCYN-A, UCYN-B and UCYN-C), and diatom-cyanobiont symbioses Hemiaulus-Richelia, Rhizosolenia-Richelia and Chaetoceros-Calothrix. West to east gradients in temperature, salinity and nutrients [NO₃⁻ + NO₂⁻, PO₄³⁻, Si(OH)₄] showed the influence of the Amazon River plume and its effect on the distributions of the diazotrophs. Trichodesmium accounted for more than 93% of all nifH genes detected, dominated the warmer waters of the western Atlantic, and was the only diazotroph detected at the equatorial upwelling station. UCYN-A was the next most abundant (> 5% of all nifH genes) and dominated the cooler waters of the eastern Atlantic near the Cape Verde Islands. UCYN-C was found at a single depth (200 m) of high salinity and low temperature and nutrients, whereas UCYN-B cells were widespread but in very low abundance (6.1 × 10¹ ± 4.6 × 10² gene copies l⁻¹). The diatom-cyanobionts were observed primarily in the western Atlantic within or near the high Si(OH)₄ input of the Amazon River plume. Overall, highest diazotroph abundances were observed at the surface and declined with depth, except for some subsurface peaks in Trichodesmium, UCYN-B and UCYN-A. Modelled contributions of Trichodesmium, UCYN-B and UCYN-A to total N₂ fixation suggested that Trichodesmium had the largest input, except for the potential of UCYN-A at the Cape Verde Islands.

Global distribution and vertical patterns of a prymnesiophyte–cyanobacteria obligate symbiosis

“…Our study reinforces the presence of B. bigelowii in symbiosis with UCYN-A in open ocean stations, consistent with the finding of nifH copies of the UCYN-A2 phylotype in the open sea (Thompson et al, 2014). In the ocean, cyanobacterial diazotrophs generally show a consistent pattern in depth with higher abundances in the upper euphotic zone (Goebel et al, 2010;Moisander et al, 2010). Recurring summer blooms of these diazotrophs are reported in the North Pacific subtropical gyre (Dore et al, 2008) usually related to shallow mixing conditions that ensure the solar energy needed for nitrogen fixation.…”

“…The patches of symbiosis observed in this study in the eastern North Atlantic, the western coast of Australia or even near Hawaii could be explained by deposition events, as high dissolved Fe concentrations have been reported in these areas (Brown et al, 2005;Langlois et al, 2012). Patchiness seems to be a common feature of diazotrophic populations (Church et al, 2009;Goebel et al, 2010), and UCYN-A is regarded as very dynamic over small spatio-temporal scales (Robidart et al, 2014).…”

Analogous nutrient limitations in unicellular diazotrophs and Prochlorococcus in the South Pacific Ocean

“…For C. watsonii, the estimated maximum net growth rate was 0.61 d À1 , while previous laboratory experiments with C. watsonii isolates reported maximum growth rates of 0.46-0.49 d À1 (Tuit et al, 2004;Goebel et al, 2010). A maximum growth rate of 0.77 d À1 was approximated theoretically for the uncultivated UCYN-A, based on cell size (Goebel et al, 2010), while the maximum net growth rate in our experiment was 0.19 d À1 .…”

“…A maximum growth rate of 0.77 d À1 was approximated theoretically for the uncultivated UCYN-A, based on cell size (Goebel et al, 2010), while the maximum net growth rate in our experiment was 0.19 d À1 . Growth rate of g-24774A11, 0.52 d À1 , falls in this range.…”

High levels of heterogeneity in diazotroph diversity and activity within a putative hotspot for marine nitrogen fixation

“…Despite its shelf sea nature, ATS diazotroph communities typically resembled those of similar latitudes, such as the tropical Atlantic Ocean, where Trichodesmium dominates (Langlois et al, 2005;Foster et al, 2009;Goebel et al, 2010). However, heterotrophic groups were also a feature of ATS communities, and we observed a shift from γ-to δ-proteobacterial phylotypes between spring and winter.…”