Microbial Iron Geochemistry of a Pristine Glaciofluvial Groundwater System
Bibliographic record
Abstract
The biogeochemical cycling of iron by microorganisms has a strong impact on groundwater resources due to their influence on aqueous geochemistry, including contaminant mobility. This thesis expands on knowledge of groundwater-surface transitions through the characterization of an iron cycling biofilm dominated groundwater spring, and presents the first in situ investigation of microbial iron cycling in a pristine, glaciofluvial aquifer that has not been used as a groundwater resource, anthropogenically contaminated, or stimulated to enhance microbial activity. Metagenomic evidence from the groundwater spring with a sharp reduction-oxidation gradient demonstrated the presence of iron biofilm forming microbes in the groundwater system being studied, as well as showing a diversity of nitrogen and sulfur cycling microorganisms. Changes in redox potential along the length of the stream corresponded with pronounced shifts in the relative abundance of bacteria known to mediate the biogeochemical cycling of iron, sulfur, and nitrogen. Five years of geochemical surveying showed a spatiotemporally persistent zone of elevated iron concentrations in an aquifer which had previously been shown to have potential for microbial iron cycling through genomic characterization. Based on the analysis of terminal electron acceptor concentrations in the groundwater, the aquifer was found to be graded into zones of O2 reduction, NO3- reduction, Fe3+ reduction, oxygen-dependent Fe2+ oxidation, and nitrate-dependent Fe2+ oxidation. Hydrogeological and geophysical evidence suggested that groundwater flow conditions differed in the vicinity of the elevated iron zone, which may have contributed to conditions conducive to the growth of a subsurface microbial community. Environmental electron scanning microscopy showed bacterial morphologies similar to known iron cycling bacteria were present in the elevated iron zone, but not outside of it. The findings of this thesis are broadly applicable to other aquifers of glacial and glaciofluvial origin, where reducing conditions will arise through non-aerobic forms of microbial respiration, and observed patterns of redox zonation will be affected by the mixing of groundwater along three dimensional flow paths. Taken together, this thesis underscores the importance of iterative interdisciplinary collaboration, including hydrogeology, geochemistry, near-surface geophysics, and microbiology, as a means of building the most complete possible representations of groundwater resources.
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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 | 0.001 | 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".