Direct carbon monoxide fixation via the bacterial and archaeal Wood–Ljungdahl pathways
Bibliographic record
Abstract
Abstract The Wood–Ljungdahl (WL) pathway, which is widely distributed in both archaea and bacteria, is an ancient carbon fixation pathway from CO 2 . CO 2 fixation proceeds via two branches of pathway: progressive reduction to the methyl group in the methyl branch and one-step reduction to CO in the carbonyl branch. In the final step of the pathway, the methyl group, CO, and CoA are combined into a carbon monoxide dehydrogenase (CODH)/acetyl-CoA synthase (ACS) complex to form acetyl-CoA. Here, we show direct CO fixation to the carbonyl group of acetyl-CoA in both archaeal and bacterial WL pathways under hydrogenogenic growth conditions using 13 C tracer-based metabolomics. A combination of metabolomics and proteomics suggested that the hydrogenotrophically grown Thermodesulfatator indicus and Archaeoglobus sp. strain MCR cells, directly fixed CO using free-form ACS in the carbonyl branch with relatively low CO availability. In contrast, carboxydotrophically grown Archaeoglobus cells utilize the CODH/ACS complex for CO 2 fixation rather than CO fixation. Direct CO fixation by free-form ACS is more advantageous for conserving reduced ferredoxin compared with the thermodynamically challenged CO 2 reduction by CODH. These findings provide further insight into the origin and evolution of the most ancient inorganic carbon fixation pathway and geochemical cycles on early Earth.
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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 | 0.001 | 0.001 |
| 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".