The Ocean’s Invisible Gift

Air bubbles at Blue Hole Dive Site, Egypt. Photo by Mostafa Ashraf Mostafa

I want you to take a deep breath in. Feel the fresh, cool air filling your lungs, bringing your body the oxygen that is essential for you and almost all life on Earth.

It makes me want to hug a tree.

Since kindergarten, we have been taught that the oxygen in our atmosphere comes from rainforests and terrestrial plants. But what if I told you that was only partly true? Astonishingly, less than half the oxygen on Earth is provided by biomass on land.

So Where Does Our Air Come From?

The ocean!

Well, not the water itself, but the photosynthesizing marine plants and plantlike organisms living in the ocean. Although estimates can vary, science tells us that seaweed and phytoplankton provide around 70% of the oxygen in our biosphere.

*TEM (Transmission electron microscopy) image of Prochlorococcus marinus*

In fact, just one particular species of plankton produces more oxygen than all the tropical rainforests on land combined.

3,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000.

Wow. I needed 48 zeros to type out just how many of these Prochlorococcus organisms live in the ocean. There are trillions upon trillions of these microbes globally, but in just one liter of seawater, we might find up to 200 million of them.

The History of Air

Until about 2.33 billion years ago, there was very little oxygen on Earth. Then, in a period scientists have named the Great Oxygenation Event, oxygen first began to accumulate in the atmosphere due to the proliferation of microbes like Prochlorococcus in the ocean.

These plantlike organisms use the process of photosynthesis to capture light energy which converts carbon dioxide and water into sugar and oxygen. The plankton only need to consume the sugar, so the oxygen is released into the environment.

First, the excess oxygen was absorbed by iron and other minerals in the water, which led to the mineral-rich banded iron formations in rocks, now visible in places like the Lake Superior region. Once the minerals became oversaturated with oxygen, the air molecules rose from the ocean and entered our atmosphere billions of years ago. In comparison, trees and other land plants started growing and producing oxygen only a mere 460 million years ago.

*Banded iron formation caused by oxygen in the ocean being absorbed by iron and other minerals over millions of years, Karijini National Park, Western Australia*

But wait, photosynthesis needs sunlight, so do all these plants and organisms live in shallow waters or near the surface? For the most part, yes, but sunlight can reach much greater depths than we would think. There are many marine plants that capture the blue-green wavelength of sunlight we cannot see. One type of red algae, Rhodoliths, has a color pigment that can trap sunlight at depths up to 268 meters, which means it is growing in waters that would appear pitch black to the human eye.

*Image Provided by Massachusetts Department of Elementary and Secondary Education*

Seaweed’s Role in Air Production

Macroalgae, or seaweed, also produce significant amounts of oxygen for our planet. Like grasses and ferns on land, thousands of species of seaweed are the foundation of biodiverse habitats forming vast underwater forests for marine creatures.

Kelp forests, composed predominantly of large brown algae, are among the most productive and dynamic ecosystems in the world. Similar to tropical rainforests, kelp forests provide habitat, food, and nursery grounds for an array of organisms such as fish, invertebrates, and marine mammals like whales and dolphins.

*Kelp forest in the Pacific Ocean highlighting the Ecklonia maxima species*

Kelp is also one of the fastest growing species on the planet. Giant kelp (Macrocystis pyrifera) can grow 18 inches in a single day and one plant can reach heights of up to 100 feet. This means kelp could play a key role in carbon sequestration by absorbing carbon from the water and atmosphere before being processed for long term carbon storage.

One company, Running Tide, is pioneering this concept with biodegradable floats that will break down as the seaweed grows, allowing the entire system to sink and potentially sequester carbon in the ocean as it accumulates under the deep-sea sediment. The startup is currently conducting field tests in the North Atlantic.

Threats of Climate Change: Dead Zones, Acidification, and Sea Level Rise

Although oceans cover 75% of the Earth's surface, it’s easy to underestimate just how crucial the oceans truly are to life on Earth.

Of the many growing threats due to climate change, rising ocean temperature is what is truly most concerning to me. Today, the ocean is absorbing 90% of the excess heat caused by greenhouse gas emissions. Since water has a higher heat capacity than air, the ocean can store large amounts of heat without rapidly increasing in temperature.

Despite a seemingly small temperature increase of just 1.5 degrees Fahrenheit over the past century, the ocean is already facing significant and harmful effects.

Thermal expansion and melting ice is causing sea level rise, which threatens our global coastal communities. Fish migrating to warmer waters are leading to uninhabitable underwater deserts scientists are calling “dead zones.” The excess absorption of carbon dioxide lowers the pH levels of the ocean, making it more acidic. This acidification dissolves calcium carbonate shells and skeletons, which leads to coral bleaching and the destruction of biodiverse coral reef habitats.

*A fire coral that experienced severe bleaching in the 2016 mass bleaching event. Photo Credit to The Ocean Agency / XL Catlin Seaview Survey / Richard Vevers*

The warmer waters are even affecting our microbial friends who have been filling our lungs with air for millennia.

A recent study discovered that Prochlorococcus and other oxygen-producing plankton are actually creatures called mixotrophs, meaning that they can switch between photosynthesizing like plants and eating like animals. While plants emit oxygen as a byproduct, animals emit carbon dioxide through digestion and decomposition. In 2023, researchers found that under warming conditions, mixotrophic microbes shift from plant-like carbon sinks to animal-like carbon emitters.

The study’s simulations suggest that this “flip of a switch” would occur when global temperatures rose above 4 degrees Fahrenheit. If we do not address the human-induced factors causing global climate change, we are on pace to breach 2 to 4 degrees before the end of this century. This phenomenon would drastically reduce the world’s oxygen supply!

So for every second breath you take, thank the microalgae and seaweed growing in our underwater ecosystems. And when you find yourself advocating for climate solutions, don’t forget to defend our oceans like your breath depends on it—because it does.