Dwellers in the Deep: Biological Consequences of Dark Oxygen

Abstract: The striking recent putative detection of "dark oxygen" (dark O$_2$) sources on the abyssal ocean floor in the Pacific at $\sim 4$ km depth raises the intriguing scenario that complex (i.e., animal-like) life could exist in underwater environments sans oxygenic photosynthesis. In this work, we thus explore the possible (astro)biological implications of this discovery. From the available data, we roughly estimate the concentration of dissolved O$_2$ and the corresponding O$_2$ partial pressure, as well as the flux of O$_2$ production, associated with dark oxygen sources. Based on these values, we infer that organisms limited by internal diffusion may reach maximal sizes of $\sim 0.1-1$ mm in habitats with dark O$_2$, while those with circulatory systems might achieve sizes of $\sim 0.1-10$ cm. Optimistically, the estimated dark oxygen flux can potentially support biomass densities up to $\sim 3-30$ g m$^{-2}$, perhaps surpassing typical reported densities at similar depths in global deep-sea surveys. Finally, we outline how oceanic settings with dark O$_2$ may facilitate the origin(s) of life via the emergence of electrotrophy. Our findings indicate that complex life fueled by dark oxygen is plausibly capable of inhabiting submarine environments devoid of photosynthesis on Earth, conceivably extending likewise to extraterrestrial locations such as icy worlds with subsurface oceans (e.g., Enceladus and Europa), which are likely common throughout the Universe.

• The paper quantifies dark oxygen production with dissolved O₂ levels from 15.8 to 633.8 μM, demonstrating conditions capable of sustaining animal-like organisms.
• The paper shows that complex life may achieve sizes of 0.1–1 mm by diffusion and up to 10 cm via circulatory systems, with biomass densities between 3 and 30 g·m⁻².
• The paper proposes that polymetallic nodule-induced electrolysis could facilitate electrotrophy and abiogenesis, expanding potential habitats on Earth and icy moons.

Biological Consequences of Dark Oxygen: Exploring the Potential for Complex Life

The paper entitled "Dwellers in the Deep: Biological Consequences of Dark Oxygen" investigates the intriguing possibility that complex life might exist in marine environments devoid of oxygenic photosynthesis due to the recent detection of so-called "dark oxygen" sources on the abyssal seafloor of the Pacific Ocean. This study presents a comprehensive exploration of the implications of dark oxygen on the potential habitability of deep-sea environments and possibly, extraterrestrial locations with subsurface oceans.

Main Findings

The authors seek to estimate the concentration, partial pressure, and flux of dissolved oxygen generated through dark oxygen processes. Their analyses outline several key points:

1. Oxygen Concentration and Pressure Estimates: The study estimates that the concentration of dissolved O₂ produced by dark oxygen sources ranges between 15.8 to 633.8 μM, with a corresponding partial pressure of 7.9 to 318 mbar (3.7% to 150% of present atmospheric level). Such concentrations are potentially capable of sustaining animal-like organisms.
2. Potential for Complex Life: Organisms that obtain oxygen through diffusion could reach maximal sizes of approximately 0.1 to 1 mm, while those utilizing circulatory systems might achieve sizes of up to 10 cm. The researchers assert the potential viability of biomass densities reaching 3 to 30 g·m⁻², which, under optimal conditions, might exceed densities typically observed at similar depths based on global deep-sea surveys.
3. Abiogenesis and Electrotrophy: The paper explores the possible roles of polymetallic nodules in seawater electrolysis, positing that the nodules' surface potential difference could facilitate the electrochemical processes necessary for abiogenesis. The potential existence of mechanisms akin to electrotrophy is proposed, where electron transfer plays a pivotal role in prebiotic chemistry.

Implications and Speculations

The implications of this research are twofold, covering both theoretical and practical dimensions:

• Theoretical Implications: The existence of dark oxygen opens new avenues for the theories surrounding the origin of complex life in environments previously deemed inhospitable due to the absence of photosynthesis-driven oxygen production. It adds a new dimension to the understanding of the necessity and sufficiency of oxygen for complex multicellular organisms, potentially reshaping our understanding of the minimal requirements for the emergence and sustenance of life.
• Practical Implications: The discovery of such oxygen production mechanisms expands the potential habitability of extraterrestrial environments, particularly those with subsurface oceans like Europa and Enceladus. It necessitates a reevaluation of life detection strategies to include searching for life forms reliant on unconventional oxygen sources.

Future Directions

The paper outlines the need for further experimental investigation into the mechanisms of dark oxygen production and the exploration of polymetallic nodules' potential in promoting life. This includes assessing their role in not only sustaining existing life forms but also facilitating life's origins. The study emphasizes the importance of empirical research to validate the theoretical postulations and the adaptation of these findings to extraterrestrial exploration efforts.

In conclusion, "Dwellers in the Deep: Biological Consequences of Dark Oxygen" posits a profound hypothesis on the adaptability and diversity of life in oxygen-scarce environments. It provides a foundation for reassessing the constraints of life on Earth and beyond, further motivating exploration into life's resilience and ability to thrive in underexplored niches of the cosmos.