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Abstract Dinitrogen (N₂) fixation by diazotrophs supports ocean productivity. Diazotrophs include photoautotrophic cyanobacteria, non-cyanobacterial diazotrophs (NCDs) and the recently discovered N2-fixing haptophyte. While NCDs are ubiquitous in the ocean, their ecology and metabolism remain largely unknown. Unlike cyanobacterial diazotrophs and the haptophyte, NCDs are primarily heterotrophic and depend on dissolved organic matter (DOM) for carbon and energy. However, conventional DOM amendment incubations do not allow discerning how different diazotrophs use DOM molecules, limiting our knowledge on DOM-diazotroph interactions. To identify diazotrophs using DOM, we amended North Pacific microbial communities with 13C-labeled DOM from phytoplankton cultures that was molecularly characterized, revealing the dominance of nitrogen-rich compounds. After DOM additions, we observed a community shift from cyanobacterial diazotrophs like Crocosphaera and Trichodesmium to NCDs at stations where the N2-fixing haptophyte abundance was relatively low. Through DNA stable isotope probing and gene sequencing, we identified diverse diazotrophs capable of taking up DOM. Our findings highlight unexpected DOM uptake by the haptophyte’s nitroplast, changes in community structure, and previously unrecognized osmotrophic behavior in NCDs, shaped by local biogeochemical conditions.
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Fronts divide diazotroph communities in the Southern Indian Ocean
Abstract Dinitrogen (N2) fixation represents a key source of reactive nitrogen in marine ecosystems. While the process has been rather well-explored in low latitudes of the Atlantic and Pacific Oceans, other higher latitude regions and particularly the Indian Ocean have been chronically overlooked. Here, we characterize N2 fixation and diazotroph community composition across nutrient and trace metals gradients spanning the multifrontal system separating the oligotrophic waters of the Indian Ocean subtropical gyre from the high nutrient low chlorophyll waters of the Southern Ocean. We found a sharp contrasting distribution of diazotroph groups across the frontal system. Notably, cyanobacterial diazotrophs dominated north of fronts, driving high N2 fixation rates (up to 13.96 nmol N l−1 d−1) with notable peaks near the South African coast. South of the fronts non-cyanobacterial diazotrophs prevailed without significant N2 fixation activity being detected. Our results provide new crucial insights into high latitude diazotrophy in the Indian Ocean, which should contribute to improved climate model parameterization and enhanced constraints on global net primary productivity projections.
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- Award ID(s):
- 2140395
- PAR ID:
- 10648524
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- FEMS Microbiology Ecology
- Volume:
- 100
- Issue:
- 8
- ISSN:
- 1574-6941
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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Accepted Manuscript
- Journal Article:
- https://doi.org/10.1093/femsec/fiae095
