<i>Azotobacter vinelandii</i> N <sub>2</sub> fixation increases in co-culture with the PGPR <i>Bacillus subtilis</i> in a nitrogen concentration-dependent manner
Bibliographic record
Abstract
ABSTRACT Biological nitrogen fixation (BNF) is an essential source of new nitrogen (N) for terrestrial ecosystems. The abiotic factors regulating BNF have been extensively studied in various ecosystems and laboratory settings. Despite this, our understanding of the impact of neighboring bacteria on N 2 fixer activity remains limited. Here, we explored this question using a co-culture of the two model species: the free-living diazotroph Azotobacter vinelandii and the non-fixing plant growth-promoting rhizobacteria Bacillus subtilis . We observed that the interaction between the two bacteria was modulated by N availability. Under N-replete conditions, B. subtilis outcompeted A. vinelandii in the co-culture. Under N-limiting conditions, BNF activity by A. vinelandii was enhanced in the presence of B. subtilis . Reciprocally, the presence of A. vinelandii repressed sporulation by B. subtilis and supported its growth likely through N transfer. N inputs by A. vinelandii were doubled in the presence of B. subtilis compared to the monoculture, primarily due to the retention of a robust N 2 fixation activity in the stationary phase. A proteomic analysis revealed that A. vinelandii N metabolism, particularly the molybdenum nitrogenase isoform protein levels (NifK and NifD), was upregulated during the stationary growth phase in the presence of B. subtilis . This study revealed that N stress drives bacterial interactions and activity in a two-species community, especially in the stationary phase. IMPORTANCE Reducing inputs of chemical N fertilizers is essential to develop a more sustainable agriculture. The stimulation of biological nitrogen fixation by N2 fixers in multispecies cultures, here the plant growth-promoting rhizobacteria Azotobacter vinelandii and Bacillus subtilis , opens opportunities for the formulation of biofertilizers consortia. While most research on N2 fixation historically focussed on the exponential growth phase of microorganisms, we observed that Bacillus subtilis stimulated Azotobacter vinelandii N2 fixation mostly during the stationary phase. This result highlights that more research on the factors controlling N2 fixation repression during the stationary growth phase, especially bacteria-bacteria interactions, is eagerly needed.
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How this classification was reachedexpand
Full frame machine prediction
Teacher imitationNot calibrated prevalence, not ground truth. Human validation pending. The Gemma side is a direct model label for every work in the frame, read from the title-only record. The Codex side is a classifier learned from the 10,348 direct Codex labels and calibrated to design-weighted sample rates; fields without enough sample support carry no Codex call. Candidate is the union of the two sides; consensus is their intersection. These outputs are machine_predicted_unvalidated and are not human labels.
Distilled classifier scores by category (both heads)
| Category | Codex | Gemma |
|---|---|---|
| Metaresearch | 0.000 | 0.000 |
| Meta-epidemiology (narrow) | 0.001 | 0.000 |
| Meta-epidemiology (broad) | 0.001 | 0.000 |
| Bibliometrics | 0.000 | 0.000 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.001 | 0.000 |
| Open science | 0.000 | 0.001 |
| Research integrity | 0.000 | 0.001 |
| Insufficient payload (model declined to judge) | 0.001 | 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 itClassification
machine, unvalidatedMachine predicted; a candidate call from one source (direct Gemma or distilled Codex), not a consensus.
How this classification was reached, model by model and score by score, is at the end of the page under "How this classification was reached".