Ecogenomic sensor reveals controls on N2-fixing microorganisms in the North Pacific Ocean

Abstract

Nitrogen-fixing microorganisms (diazotrophs) are keystone species that reduce atmospheric dinitrogen (N 2 ) gas to fixed nitrogen (N), thereby accounting for much of N-based new production annually in the oligotrophic North Pacific. However, current approaches to study N 2 fixation provide relatively limited spatiotemporal sampling resolution; hence, little is known about the ecological controls on these microorganisms or the scales over which they change. In the present study, we used a drifting robotic gene sensor to obtain high-resolution data on the distributions and abundances of N 2 -fixing populations over small spatiotemporal scales. The resulting measurements demonstrate that concentrations of N 2 fixers can be highly variable, changing in abundance by nearly three orders of magnitude in less than 2 days and 30 km. Concurrent shipboard measurements and long-term time-series sampling uncovered a striking and previously unrecognized correlation between phosphate, which is undergoing long-term change in the region, and N 2 -fixing cyanobacterial abundances. These results underscore the value of high-resolution sampling and its applications for modeling the effects of global change.

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

“…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).…”

Temporal Variability of Trichodesmium spp. and Diatom-Diazotroph Assemblages in the North Pacific Subtropical Gyre

“…Unicellular diazotrophs were typically found to be most abundant whereas episodic increases of DDA's and Trichodesmium were noted. Similar nifH gene-based surveys in the region (Robidart et al, 2014) report highly patchy diazotroph assemblages, with Trichodesmium gene-copies varying up to three orders of magnitude over <30 km and <2 d time scales. While powerful, relationships between gene and cellular abundances remain largely unknown for naturallyoccurring marine diazotrophs.…”

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

“…N 2 fixation has been found to be a significant biogeochemical feature of this important region of the ocean (Montoya et al, 2004;Luo et al, 2014), but we observed substantial physicochemical variability, manifest in differential temperature and salinity signatures and nutrient availability between the ATS and Coral Sea, which may influence the relative importance of diazotroph activity, particularly given the highly dynamic nature of diazotroph communities (Robidart et al, 2014). By combining nifH sequencing and sizefractionated 15 N 2 rate measurements, we assessed spatial and temporal patterns in the diversity and activity of N 2 -fixing bacteria across this region with the aim of characterising the dynamics of diazotrophy within this putative global N 2 fixation hotspot.…”

“…Identifying the factors that influence the composition and activity of diazotrophs is key to understanding the relative importance of N 2 fixation on local and global scales (Zehr and Kudela, 2011;Robidart et al, 2014). N 2 fixed by diverse diazotrophic taxa may have different fates within the marine environment (Glibert and Bronk 1994;Mulholland, 2007;Foster et al, 2011;Karl et al, 2012;, and therefore characterisation of the composition of active N 2 -fixing assemblages, combined with size-fractionated N 2 fixation rates, is necessary to determine the differential contribution of newly fixed N to pelagic ecosystems.…”