Diverse community of rhizobia-diatom symbioses fixes nitrogen in the South Pacific gyre

Abstract Nitrogen fixation is crucial for sustaining productivity in most of the open ocean. Cyanobacteria are the most prominent N2-fixers, but based on the nifH gene, a marker gene of the enzyme that fixes N2 into ammonia, non-cyanobacterial N2-fixers often predominate the N2-fixing community. Yet, the vast majority of them remain poorly characterized. In the oligotrophic South Pacific gyre, we found that most nifH gene sequences belonged to non-cyanobacterial N2-fixers that dominated the waters with measurable N2 fixation rates. Approximately two thirds of the non-cyanobacterial sequences affiliated with the group “Marine 1” which also contains the recently identified diatom symbiont Ca. Tectiglobus diatomicola, a heterotrophic bacterium belonging to the order Rhizobiales, and its closest relative, Ca. Tectiglobus profundi. Using fluorescence in situ hybridization and electron microscopy, we found that Ca. Tectiglobus-diatom symbioses were present throughout the gyre. These diatom symbioses were also present in samples devoid of nifH from Ca. T. diatomicola and Ca. T. profundi indicating that other members of the “Marine 1” group are also diatom symbionts. At least two morphologically distinct diatoms harbored Ca. Tectiglobus symbionts, revealing a so far unknown diversity in hosts for these rhizobial N2-fixers. Single-cell activity measurements showed that Ca. Tectiglobus-diatom symbioses actively fixed nitrogen and could account for up to 40% of the N2 fixation in the South Pacific gyre. Given the size of the largest oceanic biome and the abundance of Ca. Tectiglobus-related nifH genes in other ocean regions, these heterotrophic N2-fixers likely play a major role in marine nitrogen cycling.

• Publication year

  2025

• Venue

  ISME Communications

• Publication date

  2025-01-01

• Fields of study

  Biology, Medicine, Environmental Science

• Identifiers
  DOI 10.1093/ismeco/ycaf207PMID 41334095PMCID 12667272
• External record

  Open on Semantic Scholar

• Source metadata

  Semantic Scholar, PubMed

• No claims are published for this paper.

• No concepts are published for this paper.

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