Humans and almost all other animals depend on oxygen in the atmosphere or water to respire—that is, to produce energy at the cellular level necessary for survival. Most sea animals extract oxygen directly from ocean water, while land animals breathe air from Earth’s atmosphere, which consists of about 21 percent pure oxygen.
Oxygen has not always been a given element in the air; in fact, its presence is a relatively recent development in Earth’s history. Until around 600 million years ago, our atmosphere was composed of less than five percent oxygen, instead being mainly a nitrogen and carbon dioxide mixture dating back to Earth’s formative volcanic activity over four billion years ago.
Fortunately for us, organisms evolved that could use carbon dioxide, along with solar radiation, to produce metabolic energy and oxygen—a process called photosynthesis. While we may think of photosynthesis as the life process of land plants, algae and a variety of other microscopic organisms called phytoplankton had been using photosynthesis long before terrestrial plants appeared. These organisms that reduce carbon dioxide and produce oxygen are generally known as primary producers, a term indicative of their role in creating the necessary environment for more complex life to flourish.
As the ocean’s primary producers diversified and spread, atmospheric oxygen increased to roughly the level of today, setting the stage for aquatic animals and plants to make the transition onto land. Seasonal phytoplankton blooms still account for over half the photosynthesis and subsequent atmospheric oxygen production on Earth. It’s hard to believe, but we owe every breath we take to a biological product of marine animals mostly invisible to the naked eye.