Diverse community of rhizobia-diatom symbioses fixes nitrogen in the South Pacific gyre
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Bibliographic record
Abstract
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.
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Full frame distilled prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. Learned from the 10,348 direct Codex labels and 10,348 direct Gemma labels. Candidate is the union of thresholded teacher heads; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels or direct frontier model labels.
Codex and Gemma teacher scores by category
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.001 | 0.000 |
| Meta-epidemiology (narrow) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.002 | 0.000 |
| Research integrity | 0.000 | 0.000 |
| Insufficient payload (model declined to judge) | 0.000 | 0.000 |
Machine scores (provisional)
The two teacher heads of the student model, read on this work. A score orders the frame for review; it never asserts a category, and the validation status ships verbatim with every row.
Baseline scores from an immature model (maturity gate not passed, 7 training rounds). Scores rank; they never assert a category.
score_only:v0-immature-baseline · verbatim from the scoring run: score_only means the number may rank works, and no category label ships from it