The boundaries of the biosphere are determined by the presence of living organisms or “traces” of their vital activity.
The biosphere includes the upper part of the lithosphere, the entire hydrosphere, and the lower part of the atmosphere.
Living matter has formed an insignificantly thin layer in the total mass of the Earth’s geospheres. Its mass is 2420 billion tons, which is more than 2000 times less than the mass of the lightest shell of the Earth – the atmosphere. But this insignificant mass of living matter is found almost everywhere: at present, living things are absent only in areas of extensive glaciation and in the craters of active volcanoes.
Living organisms are unevenly distributed in the geological envelopes of the Earth.
The lithosphere is the upper solid shell of the Earth.
Its thickness ranges from 50-200 km. The spread of life in it is limited and sharply decreases with depth. The overwhelming majority of species are concentrated in the upper layer, which is several tens of centimeters thick. Some species penetrate into a depth of several meters or tens of meters (burrowing animals – moles, worms; bacteria; plant roots).
The greatest depth at which some species of bacteria have been found is 3-4 km (in groundwater and oil-bearing horizons). The spread of life deep into the lithosphere is hindered by various factors. Plant penetration is impossible due to the lack of light. For all forms of life, significant obstacles are the constantly increasing density of the environment and the temperature of rocks with depth.
Hydrosphere – the watery shell of the Earth, is a collection of oceans, seas, lakes and rivers.
Unlike the lithosphere and the atmosphere, it is completely assimilated by living organisms. Even at the bottom of the World Ocean, at depths of more than 11 km (the deepest depression of the World Ocean – Mariana – 11022 m), various types of living beings (animals, bacteria) were found. However, most of the species live in the hydrosphere within 150–200 m from the surface. This is due to the fact that light penetrates to such a depth. Consequently, in the lower horizons, the existence of plants and many species that depend on plants for food is impossible. The spread of organisms at great depths is ensured by the constant “rain” of excrement, the remains of dead organisms falling from the upper layers, as well as predation.
The atmosphere is the gaseous shell of the Earth, which has a certain chemical composition: about 78% nitrogen, 21% oxygen, 1% argon and 0.03% carbon dioxide.
Only the lowest layers of the atmosphere enter the biosphere. Life in them cannot exist without a direct connection with the lithosphere and hydrosphere. Large woody plants reach several tens of meters in height, placing their crowns upward. Flying animals – insects, birds, bats – rise hundreds of meters. Some species of birds of prey rise 3-5 km above the Earth’s surface, looking out for their prey. Finally, ascending air currents passively carry bacteria, spores of plants, fungi, and seeds upwards for tens of kilometers. However, all of the listed flying organisms or introduced bacteria are only temporarily in the atmosphere. There are no organisms constantly living in the air.
The upper boundary of the biosphere is considered to be the ozone layer located at an altitude of 8–10 km at the poles, 17–18 km at the equator, and 20–25 km above the Earth’s surface over other territories. It protects all life on our planet from powerful ultraviolet solar radiation, largely absorbing these rays. Life cannot exist above the ozone layer.
Thus, the main part of the species of living organisms is concentrated on the boundaries of the atmosphere and lithosphere, atmosphere and hydrosphere, forming a relatively “thin film of life” on the surface of our planet.