Life on Earth is dependent on a continuous supply of oxygen, primarily generated through photosynthesis. A common assumption is that the world’s forests and jungles are the main suppliers of this atmospheric gas. However, the true source is distributed between the land and the sea, with the vast majority of production occurring in an often-overlooked environment. Understanding the origin of the oxygen we breathe requires looking past the terrestrial landscape and into the expansive, sunlit waters of the global ocean.
The Definitive Answer: Marine vs. Terrestrial Contributions
The answer to whether the ocean produces more oxygen than trees is a definitive yes. The ocean is responsible for generating roughly half of the oxygen through photosynthesis. Scientific consensus places the ocean’s contribution to gross oxygen production at about 50 to 80 percent. This massive output means that for every ten breaths a person takes, approximately five to eight can be attributed to the ocean. Terrestrial environments, including all forests and land plants, generate the remaining portion.
The Microscopic Powerhouse: How the Ocean Generates Oxygen
The enormous scale of oceanic oxygen production is achieved not by large, visible sea plants, but by a collection of microscopic organisms known as phytoplankton. These tiny, single-celled organisms, which include diatoms, coccolithophores, and various types of cyanobacteria, drift in the sunlit upper layer of the water column. Like their terrestrial counterparts, phytoplankton use the energy from the sun to convert carbon dioxide and water into sugars and, as a byproduct, release oxygen.
The sheer numbers and widespread distribution of these organisms compensate for their individual size. One particular species of cyanobacteria, Prochlorococcus, is the smallest photosynthetic organism on Earth, yet it alone is estimated to produce up to 20 percent of the planet’s oxygen. Phytoplankton are the base of the marine food web and are found across the approximately 70 percent of the Earth’s surface covered by ocean. This vast, continuous biological activity in the surface waters establishes the ocean as the primary source of global oxygen.
The Scale of Terrestrial Oxygen Production
Land-based ecosystems, primarily forests and grasslands, contribute the remaining 20 to 50 percent of the world’s gross oxygen production. Trees and other plants perform photosynthesis in the same manner as phytoplankton, utilizing carbon dioxide, water, and sunlight to release oxygen into the atmosphere. The visibility of massive forests, such as the Amazon, often leads to the perception that they are the Earth’s main source of oxygen.
The terrestrial environment has a distinct structural difference from the ocean that affects its overall contribution. Much of a tree’s biomass, such as the wood in its trunk and branches, is non-photosynthesizing tissue that requires oxygen for cellular respiration. While a single large tree produces significant oxygen, the collective global impact of microscopic phytoplankton across the entire ocean surface remains greater. Land plants are therefore a significant, but secondary, contributor to the planet’s total oxygen output.
Understanding the Net Global Oxygen Budget
The concept of “gross production” must be distinguished from the long-term “net budget” of oxygen in the atmosphere. The vast majority of oxygen generated through photosynthesis is quickly consumed again by living organisms. This consumption occurs through cellular respiration, where organisms breathe oxygen to release energy from organic matter, and through the decomposition of dead material by microbes.
The atmospheric oxygen that has accumulated over geologic time and remains stable in the air is a residual amount. This long-term stability is maintained by the burial of organic carbon, which effectively removes it from the active cycle of production and consumption. When organic matter sinks or is buried without fully decomposing, the corresponding oxygen produced during its creation is left behind in the atmosphere. This small, continuous process of carbon burial ultimately sustains the long-term oxygen reservoir.