Biogeochemical evidence of anaerobic methane oxidation and anaerobic ammonium oxidation in a stratified lake using stable isotopes
Bibliographic record
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 n-damo (nitrite-dependent anaerobic methane oxidation), nitrate-dependent anaerobic methane oxidation and the anaerobic oxidation of ammonium (anammox) represent two microbially mediated processes that can reduce nitrogen loading of aquatic ecosystems and associated methane emissions to the atmosphere. Here, we report vertical concentration and stable-isotope profiles of CH 4 , NO3-, NO2-, and NH4+ in the water column of Fohnsee (lake in southern Bavaria, Germany) that may indicate linkages between denitrification, anaerobic oxidation of methane (AOM), and anammox. At a water depth from 12 to 20 m, a methane–nitrate transition zone (NMTZ) was observed, where δ 13 C values of methane and δ 15 N and δ 18 O of dissolved nitrate markedly increased in concert with decreasing concentrations of methane and nitrate. These data patterns, together with the results of a simple 1-D diffusion model linked with a degradation term, show that the nonlinear methane concentration profile cannot be explained by diffusion and that microbial oxidation of methane coupled with denitrification under anaerobic conditions is the most parsimonious explanation for these data trends. In the methane zone at the bottom of the NMTZ (20 to 22 m) δ 15 N of ammonium increased by 4 ‰, while ammonium concentrations decreased. In addition, a strong 15 N enrichment of dissolved nitrate was observed at a water depth of 20 m, suggesting that anammox is occurring together with denitrification. The conversion of nitrite to N 2 and nitrate during anammox is associated with an inverse N isotope fractionation and may explain the observed increasing offset (Δ δ 15 N) of 26 ‰ between δ 15 N values of dissolved nitrate and nitrite at a water depth of 20 m compared to the Δ δ 15 N nitrate-nitrite of 11 ‰ obtained in the NMTZ at a water depth between 16 and 18 m. The associated methane concentration and stable-isotope profiles indicate that some of the denitrification may be coupled to AOM, an observation supported by an increased concentration of bacteria known to be involved in n-damo/denitrification with AOM (NC10 and Crenothrix ) and anammox (“ Candidatus Anammoximicrobium”) whose concentrations were highest in the methane and ammonium oxidation zones, respectively. This study shows the potential for a coupling of microbially mediated nitrate-dependent methane oxidation with anammox in stratified freshwater ecosystems, which may be important for affecting both methane emissions 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 | 0.001 | 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) | 0.001 | 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".