Abstract
This dissertation investigates the phenomenon of dark oxygen production in aphotic marine environments, challenging traditional models that exclusively attribute biogenic oxygen generation to phototrophic processes. Evidence from geochemical analysis, molecular biology, and in situ sensor technology reveals that microbial communities in deep-sea ecosystems are capable of producing molecular oxygen through non-photosynthetic pathways. Focusing on microbial nitric oxide dismutation and perchlorate respiration, this study characterizes the enzymatic mechanisms, quantifies oxygen yields, and evaluates the ecological and geochemical significance of dark oxygen. The implications extend to Earth’s oxygen cycle, early microbial evolution, and the potential for life in extraterrestrial subsurface oceans.
Chapter 1: Introduction
Oxygen is a cornerstone of aerobic life on Earth, with its genesis historically linked to oxygenic photosynthesis. However, recent discoveries of oxygen presence in anoxic and aphotic oceanic zones necessitate a reevaluation of our understanding of biogenic oxygen sources. This chapter outlines the scope of the study, defines dark oxygen, and presents the central research questions: What are the biological mechanisms underlying dark oxygen production? What organisms are responsible? How does this process affect deep-sea ecosystems and the global oxygen budget?
Chapter 2: Literature Review
This chapter surveys foundational and contemporary literature on dark oxygen. It reviews historical oxygen theory, early anomalies in deep ocean oxygen measurements, and emerging hypotheses involving microbial nitric oxide dismutation, chlorate and perchlorate respiration, and abiotic radiolysis. Special focus is placed on pioneering studies by Ettwig et al. (2010) and Thamdrup et al. (2012), who first identified biological pathways for oxygen generation in the absence of light.
Chapter 3: Methodology
3.1 Study Sites
Fieldwork was conducted at three aphotic marine locations: the Black Sea chemocline, hydrothermal vent fields on the East Pacific Rise, and methane-rich sediments from the Hikurangi Margin.
3.2 Sample Collection and Preservation
Sediments and water columns were sampled using Niskin bottles and ROV-deployed core samplers, under strict anoxic conditions.
3.3 Metagenomic and Metatranscriptomic Analysis
DNA and RNA extraction followed by sequencing enabled identification of genes and transcripts associated with oxygen-generating pathways.
3.4 Chemical Analysis and In Situ Measurements
High-sensitivity optode sensors and mass spectrometry quantified nanomolar oxygen concentrations. Isotope tracing with 18O-labeled nitrate and chlorate distinguished biogenic from contaminant oxygen sources.
Chapter 4: Results
4.1 Detection of Molecular Oxygen in Anoxic Zones
Consistent detection of trace oxygen (0.5–2.0 µM) in anoxic sediments and water columns indicates in situ production.
4.2 Genetic and Functional Evidence
High prevalence of nod (nitric oxide dismutase), pcrA (perchlorate reductase), and associated genes was confirmed across multiple sites.
4.3 Isotopic Tracing
18O-labeling verified that oxygen originated from microbial dismutation of nitrate and chlorate, ruling out surface contamination.
Chapter 5: Discussion
5.1 Metabolic Versatility and Microbial Ecology
Dark oxygen production suggests a previously underappreciated metabolic versatility in deep-sea microbial communities, particularly within the NC10 and Chlorobi phyla.
5.2 Implications for the Deep Ocean Oxygen Cycle
While quantitatively minor compared to photosynthesis, dark oxygen could sustain localized aerobic metabolisms, influencing biogeochemical cycling.
5.3 Evolutionary and Astrobiological Significance
These findings imply that oxygenic metabolism could precede photosynthesis and be viable in extraterrestrial environments lacking light but rich in oxidants.
Chapter 6: Conclusion
This study provides compelling evidence that microbial communities in aphotic marine environments can produce molecular oxygen via non-photosynthetic means. The identification of key genes and metabolic pathways, coupled with field-based validation, highlights the ecological relevance of dark oxygen. These findings warrant a reassessment of oxygen evolution models and expand the scope of habitability in both terrestrial and extraterrestrial contexts.
References
- Ettwig, K. F., et al. (2010). Nitrite-driven anaerobic methane oxidation by oxygenic bacteria. Nature, 464, 543–548.
- Thamdrup, B., et al. (2012). Microbial production of oxygen in the dark. Science, 337(6098), 1063–1066.
- Canfield, D. E., et al. (2010). Oxygen dynamics in the deep ocean. Nature Geoscience, 3, 563–568.
- Oren, A. (2011). Microbial life at high salt concentrations: phylogenetic and metabolic diversity. Saline Systems, 7(1), 4.
- Jetten, M. S. M. (2012). The microbial nitrogen cycle. Environmental Microbiology, 14(12), 2903–2921.