Groundwater Nitrogen Fixation Is Associated with Methane and Sulfur Cycling
Bibliographic record
Abstract
Nitrogen fixation is an energy-intensive process critical for microbial survival in nitrogen-depleted environments. Groundwater, the Earth’s largest accessible freshwater reservoir, is typically oligotrophic. However, recent studies have identified productive groundwater habitats where nitrogen fixation may become important to support the productivity. This study investigated the diversity, activity, and metabolisms of diazotrophs in groundwater through 25 metagenomes and five proteomes. We identified 203 nonredundant nitrogenase iron proteins (NifH) and 140 diazotroph genomes. They were affiliated with diverse bacterial and archaeal taxa and could comprise half of the community. Methanotrophic Methylomonadaceae emerged as the most abundant and active diazotrophs in methane-rich groundwater systems. These microorganisms, along with diazotrophic methanogens, anaerobic methanotrophs, and methylotrophs, highlighted a strong linkage between nitrogen fixation and the methane cycle. In addition, sulfur-cycling diazotrophs, including chemoautotrophic and heterotrophic sulfur oxidizers and sulfate reducers, were ubiquitous and showed metabolic versatility in varying geochemical conditions. Proteomes further suggested active nitrogen fixation by both methane- and sulfur-associated diazotrophs. This study highlights nitrogen fixation as an important process in groundwater, especially in aquifers where nitrogen-depleted methane, produced by methanogens, serves as a major carbon source.
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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) | 0.000 | 0.000 |
| Meta-epidemiology (broad) | 0.000 | 0.000 |
| Bibliometrics | 0.001 | 0.001 |
| Science and technology studies | 0.000 | 0.000 |
| Scholarly communication | 0.000 | 0.000 |
| Open science | 0.000 | 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 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".