Abstract
We investigated dinitrogen (N2) 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 N2 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 N2 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 N2 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.
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Data Availability
The raw fastq files have been deposited to GenBank under accession number PRJNA756869. Raw IRMS data together with calculations of N2 fixation rates, primary production and minimum quantifiable rates are available on figshare (https://figshare.com/articles/dataset/_/13285757).
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Acknowledgements
We thank the Captain, crew, and scientists of the R/V Sarmiento de Gamboa, the staff of the UTM (CSIC). We also thank Moana Meyer for assistance with biomass filtrations. We greatly appreciate the work of X. A. Álvarez-Salgado on organic and inorganic nutrient analyses. We thank two anonymous reviewers and the handling editor for their thorough and constructive review of the manuscript.
Funding
This work is a contribution to project FLUXES (CTM2015-69392-C3-R), funded by the Spanish “Plan Nacional de I + D” to JA. SH and LR were funded by the Danish Council for Independent Research (6108-00013) to LR. JA was also supported by project TRIATLAS (AMD-817578-5) financed by the European Commission (H2020).
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SH, MB, JA, and LR designed the study. SH conducted the work and performed the data analysis. ERS performed the qPCR analysis. SH, MB, and LR wrote the manuscript. All authors edited the manuscript.
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Hallstrøm, S., Benavides, M., Salamon, E.R. et al. Activity and distribution of diazotrophic communities across the Cape Verde Frontal Zone in the Northeast Atlantic Ocean. Biogeochemistry 160, 49–67 (2022). https://doi.org/10.1007/s10533-022-00940-w
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DOI: https://doi.org/10.1007/s10533-022-00940-w