Dark Oxygen Could Change Our Understanding of Habitability

The discovery of dark oxygen at an abyssal plain on the ocean floor generated a lot of interest. Could this oxygen source support life in the ocean depths? And if it can, what does that mean for places like Enceladus and Europa?

What does it mean for our notion of habitability?

Oxygen is key to complex life on Earth, where photosynthesis generates most of it. The Great Oxygenation Event (GOE), which occurred about 2.5 billion years ago, led to the development of complex life and changed Earth forever. In the GOE, the oxygen was generated by living things.

Our notions of habitability rest on a planet’s proximity to its star, and part of that is because we know that the Sun drives life on Earth by allowing water to remain liquid and providing energy for organisms. But dark oxygen on the ocean floor is strictly abiotic, meaning no life was involved in its production and sunlight isn’t involved.

In recent years, we’ve learned that other Solar System bodies, far beyond the circumstellar habitable zone, could be habitable. The icy ocean moons of Europa, Ganymede, and Enceladus may harbour vast, warm oceans under frigid caps of ice. If Earth produces dark oxygen on its ocean floors, maybe these worlds do, too.

New research examines Earth’s dark oxygen and what it might mean for biology here and on other worlds. It’s titled “Dwellers in the Deep: Biological Consequences of Dark Oxygen.” The lead author is Manasvi Lingam from the Department of Aerospace, Physics, and Space Sciences at the Florida Institute of Technology. The research is awaiting peer review.

Dark oxygen comes from metal deposits called polymetallic nodules. These nodules generate enough electricity to drive electrolysis, which splits water molecules apart and releases oxygen. The amount of oxygen is not large, but it’s there, and it’s measurable.

“The striking recent putative detection of “dark oxygen” (dark O2) sources on the abyssal ocean floor in the Pacific at ~4 km depth raises the intriguing scenario that complex (i.e., animal-like) life could exist in underwater environments sans oxygenic photosynthesis,” the authors write.

The amount of dark oxygen in the ocean is small, which limits the size of organisms. Organisms use oxygen through diffusion and circulation, and oxygen levels place restraints on the sizes of both types.

Diffusion is a simple process in which nutrients, waste, and water diffuse through a few layers of tissue. Circulation is more complex and involves a heart pumping fluid to an organism’s cells, delivering nutrients and removing waste. The amount of environmental oxygen places limits on the sizes of both types of organisms.

“The maximal sizes attainable by idealized unicellular or multicellular organisms (i.e., constrained by internal or external diffusion processes) for the estimated concentrations of dark O₂ may be ~ 0.1–1 mm.,” the authors write.

For animals with circulation systems, the upper size boundary is higher but still limited.

“In contrast, the upper-size bounds of organisms with internal circulation systems for the distribution of oxygen could range between ~ 0.1 cm to ~ 10 cm, with the latter threshold falling under the umbrella of “megafauna,” the researchers explain.

Aside from the size of individual organisms, there’s the overall biomass density. In an optimistic scenario, the researchers report that biomass density could exceed the reported density. “Under optimistic circumstances, the biomass densities might reach as high as ~ 3–30 g m^?2, in principle exceeding the reported macrofaunal densities at depths of ~ 4 km in global deep-sea surveys,” the authors write.

This work inspires a multitude of questions. We know that microorganisms in groundwater use dark oxygen. What types of microorganisms have adapted to these ocean dark oxygen environments? What about their metabolism allows them to live there? Have larger organisms adapted to these environments? Did organisms in these environments play a role in the evolution of life on Earth?

The discovery also compels us to consider its implications for astrobiology. On Earth, abyssal deep sea plains represent about 70% of the ocean floor, making them the largest ecosystem on Earth. Even with a low biomass density, the region is significant.

When considering the habitability of the ocean moons, we’re at a disadvantage. We don’t know what the sea floors look like on these bodies. In fact, despite all of the enthusiasm, we don’t even know for certain if these moons have oceans. We also don’t know if the oceans, if any of them exist, can produce polymetallic nodules that generate dark oxygen.

However, there are other ways dark oxygen can be generated without nodules. One of them is radiolysis.

Radiolysis is the breaking apart of molecules by ionizing radiation, and there’s plenty of that in the vicinity of Jupiter. Spacecraft have spotted O₂ trapped in bubbles on Europa, Ganymede, and Callisto. Does that mean it’s available for life that might exist in their hypothetical oceans?

“The production of oxidants on the surface and their delivery to the ocean can effectively input O₂ to the latter even sans photosynthesis,” the authors explain. Europa’s icy shell isn’t all solid ice. Scientists think that briny liquid can percolate through the ice, and that could potentially deliver surface dark oxygen to the ocean.

There’s a third pathway for dark oxygen called microbial dismutation. Though it’s biotic, it doesn’t rely on photosynthesis. It could be an overlooked source of oxygen.

The evidence we have so far says that worlds like Earth are extremely rare, while environments like Europa could be widespread. “To round off our preliminary venture into this eclectic subject, we reiterate our
prefatory statement that marine habitable settings implausible for photosynthesis, especially on icy worlds with subsurface oceans, are likely widespread in the Universe,” the authors write in their conclusion.

“Therefore, if dark oxygen production is feasible and commonplace on this class of worlds – whether via seawater electrolysis or the prior two routes – then our analysis may broadly encapsulate the profound consequences of dark oxygen for the prevalence of abiogenesis, complex multicellularity, and perhaps even technological intelligence in the Cosmos,” the authors explain.

The fact that we’ve only now discovered dark oxygen on the ocean floor should make us all pause. We’re discovering things about nature that could be critical in the search for life and habitable worlds. If we can confirm that the so-called ocean moons really do have oceans and that dark oxygen is either produced in or transported to those oceans, then we have to adapt our thinking about habitability. Proximity to a star may not be critical, which would simultaneously broaden our understanding while deepening the mystery of life in the cosmos.

That’s the intriguing part of science. It’s equal part mysteries and answers.