The biosphere is made up of every living organism on Earth, plus the air, water, rock, and soil that support them. It spans about 20 kilometers from top to bottom, stretching from deep ocean trenches to high in the atmosphere, though almost all life is concentrated in a much narrower band: from about 500 meters below the ocean surface to roughly 6 kilometers above sea level.
What makes the biosphere unique among Earth’s layers is that it isn’t a single physical zone. It’s the overlap of three other spheres: the atmosphere (air), the hydrosphere (water), and the lithosphere (rock and soil). Anywhere those zones can support life, that’s the biosphere.
The Living Components
Every organism in the biosphere fills one of three roles. Producers, primarily plants and photosynthetic microorganisms, capture energy from sunlight and convert carbon dioxide and water into sugars and oxygen. Consumers eat producers or other consumers. Decomposers break down dead organisms and waste, returning nutrients to the soil and water so the cycle can start again.
A landmark census published in the Proceedings of the National Academy of Sciences estimated the total living mass on Earth at roughly 550 gigatons of carbon. The breakdown is striking:
- Plants: about 450 gigatons, or 80% of all life on the planet, almost entirely on land
- Bacteria: about 70 gigatons (15%), much of it in deep underground environments
- Fungi, archaea, protists, animals, and viruses: together less than 10% of total biomass
- Animals: roughly 2 gigatons, with most animal biomass living in the ocean rather than on land
That last point surprises most people. All the world’s mammals, birds, insects, and fish combined represent a tiny sliver of the biosphere’s living material. Plants dominate so thoroughly because trees and other land vegetation are massive carbon stores, while animals are comparatively small and short-lived.
How Many Species the Biosphere Contains
Scientists have formally described about 1.2 million species, roughly 950,000 of which are animals. But the actual number of species alive today is far larger. A widely cited 2011 estimate projected about 8.75 million living species, including 7.8 million animals, 298,000 plants, 611,000 fungi, and 63,900 protists. Even that figure may be conservative. Newer molecular techniques are pushing estimates dramatically higher for groups like bacteria (potentially millions of species rather than thousands), fungi (possibly 6.3 million), and insects (over 20 million). Some projections for all life on Earth run into the trillions when microbial diversity is fully accounted for.
The Non-Living Parts That Keep It Running
Living things can’t exist without the non-living, or abiotic, components of the biosphere. These include sunlight, water, temperature, nutrients, and the gases in the atmosphere. Each one shapes where and how life can survive.
Sunlight is the primary energy source. Plants, algae, and photosynthetic bacteria capture solar energy and convert it into chemical energy that feeds nearly every food web on the planet. In places sunlight can’t reach, like hydrothermal vents on the deep ocean floor, certain bacteria extract energy from inorganic chemicals instead.
Water is essential for every living cell. Temperature dictates which organisms can survive in a given place, since very few species tolerate conditions below freezing. Inorganic nutrients like nitrogen and phosphorus control how abundant life can become in any environment. In soils, the particle size, pH, and nutrient content together determine which plants can grow. In aquatic systems, dissolved oxygen plays the same gatekeeper role.
Where the Three Spheres Overlap
The biosphere isn’t neatly layered. It exists wherever the atmosphere, hydrosphere, and lithosphere meet in conditions that allow life. A forest sits on the lithosphere, draws water from the hydrosphere, and exchanges gases with the atmosphere. An ocean coral reef depends on all three simultaneously. Even the atmosphere itself carries life: bacteria, fungal spores, and single-celled organisms drift through the air in substantial numbers, with studies over tropical oceans detecting tens of thousands of bacterial cells and fungal spores per cubic meter. These microorganisms can travel thousands of kilometers on air currents before settling back to the surface.
Deep underground, bacteria and archaea thrive in rock and sediment far from sunlight, making up a significant share of the planet’s total biomass. In the ocean, life extends from sunlit surface waters down into the pitch-dark trenches. The biosphere, in other words, reaches into every physical zone on Earth where liquid water and an energy source exist.
How Matter Cycles Through the Biosphere
The biosphere doesn’t just contain life. It actively recycles the materials life needs. Atoms move continuously between living organisms and non-living reservoirs in loops called biogeochemical cycles. The two most important are the carbon cycle and the nitrogen cycle.
In the carbon cycle, plants and phytoplankton (microscopic ocean organisms) pull carbon dioxide from the atmosphere and convert it into biomass like leaves, stems, and roots through photosynthesis. Animals eat that biomass and release carbon dioxide back through respiration. When organisms die, decomposers break them down, returning carbon to the soil or water. Over longer timescales, carbon gets locked in rocks, fossil fuels, and ocean sediments before eventually cycling back.
Nitrogen follows a similar path: bacteria in soil and water convert atmospheric nitrogen into forms plants can absorb, animals eat those plants, and decomposition returns nitrogen to the soil where bacteria process it again. These cycles connect every living thing in the biosphere to the non-living world around it, ensuring that the same atoms get reused over and over across millions of years.