Biogeochemistry

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biogeochemistry Biogeochemistry refers to the coupling of biological, chemical, and geological processes that together drive the major chemical cycles on the Earth's surface. Life plays a major role in this cycling, and there are few chemical reactions on the surface of the Earth that are not affected. The chief biological catalysts of these cycles tend to be micro-organisms, and in particular, bacteria. Micro-organisms do not have the same visual impact as large animals and plants, but the power of these small microscopic organisms is in their great metabolic diversity. For 70 per cent of the history of life on Earth (before 3.8 billion years (Ga) ago) bacteria were the only type of organism present and it is their activity that has shaped the surface chemistry of our planet, including the atmosphere and thus the climate. Indeed, their development of oxygen-producing photosynthesis both dramatically changed the atmosphere and made possible the subsequent development of metazoans. Today the atmosphere still provides a clear signature of active biogeochemical cycles in the form of a mixture of oxidized gases (e.g. oxygen, O₂) and reduced gases (e.g. methane, CH₄) that is far from chemical equilibrium. This chemical disequilibrium is produced by life and would provide clear evidence for life and active biogeochemical cycles on other planets.

Biological activity accelerates the speed of natural chemical reactions, such as rock weathering. For example, the weathering of pyrite is increased up to 1 million times. Biological activity also makes possible a range of unique reactions that could not otherwise take place. The importance of oxygen from photosynthesis has already been mentioned, but oxygen is only a waste product. The production of new organic compounds from atmospheric carbon dioxide (CO₂), and thus the formation of new cells and an energy source, are the crucial products for photosynthetic organisms. This photosynthetic ‘fixation’ of CO₂ traps energy from sunlight and is considered to be the base of all ecosystems on Earth, since herbivores, and indirectly carnivores, rely on it for their energy supply. However, after the death of these organisms their carbon and nutrients must be recycled or photosynthetic production would eventually cease because of lack of nutrients. The decomposition of organic matter is therefore just as important as its production through photosynthesis in maintaining life on Earth. These two processes drive the biological carbon cycle and the cycles of associated elements (for example, nitrogen, sulphur, and phosphorus—key elements in biomolecules). In addition, if carbon dioxide were not returned to the atmosphere through decomposition, the greenhouse effect of carbon dioxide would decrease and the planet would start to freeze (in about 4–10 years).

Other reactions unique to life, except at extremely high temperatures and pressures, include nitrogen fixation, nitrification and denitrification, sulphate and sulphur reduction, and methane formation. These processes have a direct effect on element cycles and can have major global impacts. For example, most commercial sulphur deposits are of biological origin, and the majority of atmospheric methane (which is 60 times more powerful as a greenhouse gas than CO₂) is of bacterial, not volcanic, origin.

A small amount of biologically produced organic matter leaks from the biosphere and becomes buried and preserved in the geosphere. Over geological time, this accumulates in vast quantities, making the geosphere the largest store for most elements on Earth. For example, vast amounts of organic matter are stored in sedimentary rocks, some of it in the form of fossil fuels. Naturally, this material would eventually be returned to the surface by the geological processes of mountain building and volcanic eruptions, so completing the element cycle. These processes, however, are very slow, taking approximately 300million years. In contrast, the biological cycle is extremely fast (taking years to tens of years), and it is the interplay between the slowly turned over but vast geological reservoirs and the rapidly turned over but small biological reservoirs that controls the global biogeochemical cycles. There is evidence that biological interaction with the geological reservoirs has kept the climate stable for life despite changes in heat output, volcanic and tectonic activity, and solar radiation during the development of the Earth.

R. John Parkes

Bibliography

Schlesinger, W. H. (1997) Biogeochemistry: an analysis of global change. Academic Press, London.

biogeochemistry The scientific study of the effects of living things on subsurface geology; or with the distribution and fixation of chemical elements in the biosphere. Its principles are applied to the systematic collection and analysis of plants in the exploration for mineral deposits. It is also the study of the chemistry of organic sediments and of the chemical composition of fossils and fossil fuels. See also geobotanical exploration.

biogeochemistry Science concerned with the effects of living things on subsurface geology; or with the distribution and fixation of chemical elements in the biosphere. Its principles are applied to the systematic collection and analysis of plants in the exploration for mineral deposits. It is also the study of the chemistry of organic sediments and of the chemical composition of fossils and fossil fuels. See also GEOBOTANICAL EXPLORATION.