Forests feel alive in a way that’s easy to love. They’re visible, towering, and green — obvious symbols of oxygen and life. Photosynthesis, the process through which plants turn light into their food, makes them seem like Earth’s natural life-support system. But the story of where our oxygen comes from doesn’t end on land. In fact, it doesn’t even begin there.
First, we need to talk more about photosynthesis, but before you yawn, consider that it is one of the most powerful processes on Earth. Using sunlight as energy, plants pull carbon dioxide from the air and water from the soil, turning them into sugar for growth and releasing oxygen as a byproduct — what a miracle! Over millions of years, this quiet chemical process reshaped the planet itself during The Great Oxidation Event, where oxygen built up in the atmosphere, making complex life possible and allowing animals to breathe. Early algae mats and plants helped regulate the Earth’s carbon cycle, and some time after, forests cooled the climate and stabilized soils. It’s no exaggeration to say that photosynthesis turned a hostile planet into a livable one.
But here’s the thing: oxygen doesn’t primarily come from forests, actually, not even from plants. While trees may dominate our imaginations, they aren’t working alone. What if the largest contributors to the air we breathe aren’t towering at all? What if they’re so small that we rarely notice them? To find them, we have to leave the forest behind and look toward… the coast?!
Yes, out in the open ocean, far from forests and shorelines, live the planet’s most important oxygen producers: phytoplankton. Defining them is a complex ordeal, since some can be bacteria or protists but, in essence, most are single cell plants. These microscopic organisms drift near the ocean’s surface and use sunlight to photosynthesize just like plants do, except they exist in staggering numbers, to the point that their seasonal blooms can be seen from space!
Together, phytoplankton and other photosynthetic microorganisms produce over half of the oxygen present on Earth, quietly releasing massive amounts of oxygen into the atmosphere with every sunlit day. They are small, fragile, and largely unseen — but their collective impact is ginormous. Every breath you take is likely owed not to a tree, but to the sea.
What makes phytoplankton easy to overlook is exactly what makes them vulnerable. Because they’re invisible to the naked eye, we tend to ignore them and the oceans that sustain them. Treating seas as if they were endless and secondary to land has resulted in warming waters, ocean acidification, and pollution that are already disrupting plankton communities, threatening the systems that quietly keep our atmosphere breathable. So, the next time you picture oxygen in your mind, imagine not just leaves reaching for sunlight, but microscopic life drifting through blue water. As the Lorax once said, “Unless someone like you cares a whole awful lot, nothing is going to get better.” So, let’s care.
