Biogeochemical evidence of anaerobic methane oxidation and anaerobic ammonium oxidation in a stratified lake using stable isotopes
Bibliographic record
Abstract
Abstract. Nitrate pollution of freshwaters and methane emissions into the atmosphere are crucial factors in deteriorating the quality of drinking water and in contributing to global climate change. The nitrate dependent anaerobic methane oxidation and the anaerobic oxidation of ammonium (anammox) have the potential to reduce nitrogen loading of aquatic ecosystems and to reduce methane emissions to the atmosphere. Here, we report vertical concentration profiles and corresponding stable isotope compositions of CH4, NO3−, NO2− and NH4+ in the water column of a stratified lake, which suggest linkages between anaerobic oxidation of methane (AOM), denitrification, and anammox. In a water depth from 12 to 20 m, a methane-nitrate transition zone (NMTZ) was observed, where δ13C values of methane and δ15N and δ18O of dissolved nitrate markedly increased in concert with decreasing concentrations of methane and nitrate. These data patterns, together with a simple 1D diffusion model show that the non-linear methane concentration profile cannot be explained by diffusion or micro-aerobic methane oxidation, and that microbial oxidation of methane coupled with denitrification under anaerobic conditions is the most likely explanation for these data trends. In the methane zone at the bottom of the NMTZ (20 m to 22 m) δ15N of ammonium increased by 4 ‰, while ammonium concentrations decreased. In addition, a strong 15N enrichment of dissolved nitrate was observed at a water depth of 20 m, suggesting that anammox is occuring together with denitrification coupled to AOM. The conversion of nitrite to N2 and nitrate during anammox is namely associated with an inverse N isotope fractionation and may explain the observed increasing offset (Δδ15N) of 26 ‰ between δ15N values of dissolved nitrate and nitrite at a water depth of 20 m compared to the Δδ15Nnitrate-nitrite of 11 ‰ obtained in the NMTZ between a water depth of 16 m and 18 m. The geochemcical zones were found to contain significantly different microbial communities that consist of bacteria known to be involved in denitrification with AOM (Crenothrix and NC10), and anammox (Candidatus Anammoximicrobium), confirming the presence of microbial groups potentially responsible for the proposed linkages between AOM, denitrification, and anammox. This study gives insights into the yet overlooked AOM-denitrification-anammox process in stratified lakes that can regulate methane emisssions from and nitrogen concentrations in lakes.
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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 | 0.000 | 0.000 |
| 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".