Activity and distribution of diazotrophic communities across the Cape Verde Frontal Zone in the Northeast Atlantic Ocean

We investigated dinitrogen (N-2) fixation activity and diazotroph community composition across the Cape Verde Frontal Zone (CVFZ), from photic epipelagic waters (0-200 m) to aphotic meso- (200-1000 m) and bathypelagic (> 1000 m) waters. The highest N-2 fixation rates of 4.1 +/- 2.2 nmol N l(- 1) day(- 1) and 7.8 +/- 2.3 nmol N l(- 1) day(- 1) were in epipelagic waters south of the front. We detected aphotic N-2 fixation in 5 out of 32 samples, primarily south of the front, and sporadically down to 3,000 m, with rates ranging from 0.03 +/- 0.01 nmol N l(- 1) day(- 1) to 0.07 +/- 0.01 nmol N l(- 1) day(- 1). Cyanobacteria dominated the diazotroph community and nitrogenase gene (nifH) expression profiles in surface waters and, surprisingly, in aphotic waters. The detection of cyanobacterial nifH genes by DNA sequencing and quantitative PCR in the aphotic zone, together with nifH expression in meso- and bathypelagic waters, indicates a downward flux of metabolically active cyanobacteria, and points to a contribution to the observed aphotic N-2 fixation rates. In the photic zone, UCYN-A dominated north of the front, whereas Trichodesmium was mainly found in the southern region. However, our results also show that cross-frontal advection of cyanobacterial diazotrophs can occur via intrusions of surface water. Salinity, temperature, and mixed layer depth were the main determinants of the diazotroph composition and distribution of the key cyanobacteria. Thus, the front appeared to act as a dynamic barrier controlling the distribution of cyanobacterial diazotrophs.

Automated summary

This study investigated the distribution of nitrogen fixation activity and diazotroph community across the Cape Verde Frontal Zone. The results showed variations in nitrogen fixation rates and community composition, with cyanobacteria dominating in both surface and aphotic waters. The findings also suggested the potential downward flux of metabolically active cyanobacteria and cross-frontal advection of cyanobacterial diazotrophs.