Abstract
In addition to the cyanobacterial N2-fixers (diazotrophs), there is a high nifH gene diversity of non-cyanobacterial groups present in marine environments, yet quantitative information about these groups is scarce. N2 fixation potential (nifH gene expression), diversity and distributions of the uncultivated diazotroph phylotype γ-24774A11, a putative gammaproteobacterium, were investigated in the western South Pacific Ocean. γ-24774A11 gene copies correlated positively with diazotrophic cyanobacteria, temperature, dissolved organic carbon and ambient O2 saturation, and negatively with depth, chlorophyll a and nutrients, suggesting that carbon supply, access to light or inhibitory effects of DIN may control γ-24774A11 abundances. Maximum nifH gene-copy abundance was 2 × 104 l−1, two orders of magnitude less than that for diazotrophic cyanobacteria, while the median γ-24774A11 abundance, 8 × 102 l−1, was greater than that for the UCYN-A cyanobacteria, suggesting a more homogeneous distribution in surface waters. The abundance of nifH transcripts by γ-24774A11 was greater during the night than during the day, and the transcripts generally ranged from 0–7%, but were up to 26% of all nifH transcripts at each station. The ubiquitous presence and low variability of γ-24774A11 abundances across tropical and subtropical oceans, combined with the consistent nifH expression reported in this study, suggest that γ-24774A11 could be one of the most important heterotrophic (or photoheterotrophic) diazotrophs and may need to be considered in future N budget estimates and models.
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Acknowledgements
We thank I Hewson, I Biegala, M Furnas, E Preston, K Sandman and personnel onboard R/V Kilo Moana for technical assistance, and J Montoya, C Carlson, K Johnson, A White for hydrographic, nutrient and DOC analyses. This study was supported by a first phase Gordon and Betty Moore Foundation Marine Investigator ship award (JPZ), NSF C-MORE (EF0424599) (JPZ), NSF OCE1130495 (PHM) and funds from the University of Massachusetts Dartmouth (PHM).
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Moisander, P., Serros, T., Paerl, R. et al. Gammaproteobacterial diazotrophs and nifH gene expression in surface waters of the South Pacific Ocean. ISME J 8, 1962–1973 (2014). https://doi.org/10.1038/ismej.2014.49
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DOI: https://doi.org/10.1038/ismej.2014.49
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