Microbial community dynamics in polar hypersaline springs: viral ecology and sulfur cycling
Bibliographic record
Abstract
Astrobiology spans a range of disciplines. Experimental work in this field is essential to constraining the likelihood of life's origin, evolution and detectability. The work described in this thesis focuses on the microbial ecology of an extreme environment. Microbes that inhabit such environments demonstrate deviations on biological processes such as might be found under conditions disparate from those thought of as hospitable for life. This work focuses on two polar hypersaline springs in the Canadian High Arctic, considered extreme by their low temperature, high salinity and low oxygen. Within those springs, this work focuses on two processes: 1) microbial mortality by viruses; and 2) sulfur isotope biosignatures of microbial metabolism. The abundance of viruses from Arctic hypersaline spring water and sediment environments was investigated and the first estimates of contact rates between viruses and bacteria in sediments were modeled. The oligotrophic nature of these springs maintains some of the lowest virus-bacteria ratios and contact rates observed in natural environments to date, indicating that viruses do not play a major lytic role in these springs. Comparison of these results to reports from marine sediments identifies a viral biogeographic divide in deep-sea sediments that separates shallower bacterial communities controlled by lytic viruses from deeper ones that are not. Complementary viral dynamics experiments were performed in sediments of the same springs. This work addresses 1) the trophic hypothesis of viral influence on microbial growth and 2) the relationship between oligotrophy and lysogenic replication. These sediments maintain extremely low rates of microbial growth and viral production and a substantial fraction of viruses are extremely resistant to decay. These viruses contribute to microbial mortality, but are not the primary cause of such. A substantial fraction of microbes in these sediments appear to be lysogens, harboring inducible provirus, yet temperate viruses do not seem to account for a large fraction of the in situ population of virions. These findings support the trophic hypothesis, but offer limited support for an increasing frequency of lysogens in oligotrophic environments. These findings extend the range of geochemical conditions under which viral dynamics have been explored and suggest that viruses are able to maintain a role in microbial mortality even in extremely low biomass environments, potentially by means of resisting decay. The preserved signals of biological isotopic fractionation of sulfur in one of the two springs was described, as were the accompanying sulfate reducing microorganisms that inhabit its sediments. This work addresses the influence that environmental chemistry and genomic diversity have on the fractionation of sulfur isotopes across a small spatial scale. These sediments show little variation in sulfur fractionation, despite differences in several of the characters known to contribute to the extent of fractionation by cultured sulfate reducing isolates. To address the discrepancy between measured and modeled cell-specific sulfate reduction rates (csSRR) and isotopic fractionation; environmental data from the spring was incorporated into a thermodynamic-based model. The model indicated a likely cause of deviation from predicted fractionation values was a greater than measured csSRR, suggesting heterogeneous activity among that group. Employing the same model to address the unexpected lack of correlation between dsrB alpha diversity metrics and isotope fractionation between stations, kinetic parameters associated with the apr gene in the sulfate reduction pathway were found to have greater influence on expressed fractionation factor than the dsr gene.
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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 | 0.001 | 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".