Where does Oxygen Come from if it's Dark?

Amelia Ahnert
Jun 9
3 min read

We have plants to thank for the oxygen gas that we rely on to survive. About half comes from the plants we see around us while the other half is contributed by microscopic algae in the ocean called phytoplankton¹. All these organisms go through a process called photosynthesis, where carbon dioxide, water and sunlight are utilized to form oxygen and glucose. But what if there is a different source that we weren’t aware?

In 2013, scientists studying marine ecosystems in the Clarion-Clipperton zone (located between Hawaii and Mexico) collected data from the seafloor². One of the tests they ran sealed off a section of the seafloor to observe oxygen levels over several days. They expected that any organisms within the sealed section would consume any oxygen, leading to a decrease in oxygen levels over the test period. Instead, they observed a graph like the one below.

Oxygen Levels Based on Time Graph with a line increasing across time

What do you think this means? Well scientists blamed these results on faulty sensors and moved on with their research. It wasn’t until several years later when the scientists were running a different experiment with different equipment that they saw the same results again and started to question it.

What could the source of oxygen be so far down in the ocean? Light only can reach to 200-1000m below sea level². Phytoplankton, a microscopic organism, utilize the light near the surface of the ocean to perform photosynthesis. Some of the oxygen gas that forms is then carried to lower areas of the ocean by sinking cold water as part of convection currents. This has been thought to be the source of oxygen lower down, where photosynthesis can’t occur. The lack of oxygen and light contributes to the oddity of creatures we’ve discovered down there. We’ve only explored 5% of the ocean so far². With this new evidence of oxygen increasing even when sealed off from additional oxygen being added from above, what could be the source?

Let’s create a model of the ocean floor to look for a possible theory of dark oxygen. This model is a great K-12 lab!

Materials

Clear plastic cup
Round metal thumb tacks
Water
Epsom salt
Plastic spoon or stir stick
9V battery
Index card or paper
Tape
Graduated cylinder (recommended)
Digital scale (recommended)
Beaker (recommended)

First, poke metal thumb tacks through the bottom of the cup that are about 1.5 cm apart. The key here is that the 2 thumb tacks shouldn’t touch each other but need to be close enough that the 9V battery terminals can touch each of the tacks.

Second, make a 3.5% salt solution using water and epsom salt in their clear cup. Why 3.5% salinity? That’s the average salinity of the ocean! This is also an opportunity for students to measure using a graduated cylinder and digital scale. Mathematical calculations can be included to have students determine the amounts that they need.

Third, tape an index card to cover the top of the cup. This is a precaution to avoid touching the water when electricity is flowing. As a general reminder, this model should only be constructed with close adult supervision.

Fourth, hold a 9V battery to the bottom of the cup so that the two ends of the battery are touching the two thumb tacks. Observe what happens.

Bubbles! The energy from the battery is breaking the bonds in water to form hydrogen and OXYGEN gas. The current theory is that the metallic clusters (polymetallic nodules) scattering the sea floor in the Clarion-Clipperton zone act like a battery and produce oxygen gas. This has been named DARK OXYGEN since light isn't needed for its formation! If this is correct, then the unique animals that call the deep sea home may be relying on this oxygen for survival.

There’s push back on this theory from the deep sea mining industry who mine these metals for production of electronics and batteries ². Either way, more research needs to be done to determine the importance of these polymetallic nodules on the biodiversity of life on the seafloor.

What does this mean? Well, it shows us how little we understand the ocean and that there is so much more to discover! Plus if oxygen can be formed from rock, then this might impact how we use oxygen detection in our search for life on other celestial bodies.