We breathe because our body needs oxygen, a gas which makes up 21 % of the Earth’s atmosphere. But this oxygen comes from photosynthetic organisms like plants. But it is not well known that most of the oxygen we breathe comes from organisms in the ocean.
This a true fact as more than half of the oxygen in our atmosphere comes from marine photosynthesizers such as phytoplankton and seaweed. Both take in carbon dioxide, water, and energy from the sun to produce food for themselves, releasing oxygen in the process. This process is photosynthesis which is done in the ocean.
Photosynthesizers have existed in the ocean for a long time. Land plants made an appearance in the fossil record just 470 million years ago before dinosaurs wandered the earth. But the oceans were producing oxygen for billions of years before that. The oldest known fossil belongs to a marine cyanobacterium, a tiny-blue green photosynthesizer that was probably releasing oxygen 3.5 billion years ago. So, in reality, all the plants in the ocean should get credit for all of the oxygen that comes from land plants as well, as land plants evolved from green marine algae. In a race to put oxygen in the atmosphere, the ocean has a major head start.
But the ocean’s long history of photosynthesis would not be too relevant to our daily life if not for the photosynthesizers that inhabit it today. The most impressive out of these is a cyanobacterium called Prochlorococcus which is estimated to be the most abundant photosynthesizer on the planet and considered responsible for producing 20% of the oxygen in the atmosphere. The impressive prochlorococcus is possibly responsible for one in every five breaths you take. Even more amazing is the fact that scientists discovered this super-abundant photosynthesizer only in 1988.
Sunlight is essential for photosynthesis so all marine photosynthesizers have to live in what scientists’ term as the “photic zone” i.e. the layer at the top of the ocean which gets illuminated by sunlight. The photic zone extends down to nearly 656 feet (200 meters) below the surface of the ocean. It’s somewhat difficult to put a depth limit on it, because photosynthesizers continue photosynthesis even further down than estimated. There is barely any sunlight visible at eight hundred and eighty-six feet below the ocean’s surface but corallinales, a type of red algae, defy all odds and photosynthesizes at that depth. The red color of Corallinales is due to a pigment which enables it to absorb blue and green light, which is very nearly the only kind of light that manages to filter down to such improbable depths populated by Corallinales. This photosynthesizer manages to produce oxygen despite having access to only the tiniest fraction of sunlight.
So now we know that marine photosynthesizers are indispensable to our survival because not only did they evolve the earliest, they photosynthesize the most in spite of dwelling the deepest.