Biogeochemistry

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biogeochemistry

[‚bī·ō‚jē·ō′kem·ə·strē]
(geochemistry)
A branch of geochemistry that is concerned with biologic materials and their relation to earth chemicals in an area.

Biogeochemistry

 

a part of geochemistry that studies the geochemical processes occurring in the biosphere with the participation of organisms. The migration of chemical elements on the earth cannot be understood without considering the influence of organisms. Biogeochemical processes are reflected on geological charts. The tasks of biogeochemistry were formulated for the first time in the USSR by the academician V. I. Vernadskii and were developed in a specially created biogeochemical laboratory (now the V. I. Vernadskii Institute for Geochemistry and Analytical Chemistry of the Academy of Sciences of the USSR). The problems of biogeochemistry are widely studied in the USSR and abroad.

Biogeochemistry is concerned not with individual specimens or types of organisms but rather with their aggregate—so-called living matter, expressed in the mass, chemical composition, and energy it brings to the biogeochemical processes. Living matter is distributed unevenly over the earth’s surface. Areas of accumulation or concentration of the mass are known—for example, plankton in the oceans and seas, forests on dry land, and humus and peat in the soils. The density of the population is also uneven and depends to a significant degree upon the soil and climatic zones. Plant organisms constitute the basic mass of living matter. (About 1 percent of the solar energy that reaches the earth—which is the equivalent of 3 x 1014 kg of carbon—is absorbed by plants; this corresponds approximately to the mass of living matter on the earth.) The mass of living matter alone does not give a correct notion of the intensity of its participation in the biogeochemical processes. The multiplication rate of the organisms—that is, the total product of organic matter formed over a certain period of time—is of enormous significance. This applies particularly to the lower organisms, such as bacteria, fungi, and algae, which have a high rate of reproduction. All the known chemical elements and their isotopes are part of living matter, but the basic mass of any organism is made up of a limited number of known chemical elements (see Table 1), which under the conditions of the biosphere form readily soluble and mobile compounds—for example, the gases CO2 or NH3 H2O, the ions H+, OH-, NO3-, SO42-, PO43-, Na+, K+, Ca2+, Mg2+, and also the heavy metals, which form highly oxidized compound ions.

Table 1. Average content of some chemical elements in the earth’s crust, soils, and organisms (percent by mass, 1968 data)
ElementEarth’s crust (sedimentary rock)Soil coverOrganisms (plants)
B1 X 10-21 X 10-31 X 10-4
C1.02.018.0
N6 X 10-21 X 10-13 X 10-1
052.849.070
F5 X 10-22 X 10-21 X 10-3
Na0.660.632 X 10-2
Mg1.340.637 X 10-2
Al10.457.12 X 10-2
Si23.833.01.5 X 10-1
P7 X 10-28 X 10-27 X 10-2
S3 X 10-18 X 10-25 X 10-2
Ci1.6 X 10-21 X 10-210-2
K2.281.363 X 10-1
Ca2.531.373 X 10-1
Ti0.454.6 X 1011 X 104
Mn6.7 X 10-28 X 10-21 X 10-3
Fe3.33.82 X 10-2
Cu5.7 X 10-32 X 10-32 X 10-4
Sr4.5 X 10-23 X 10-210-4
Zr2 X 10-23 X 10-210-4
l1 X 10-45 X 10-41 X 10-3
Ba8 X 10-25 X 10-210-4
U3 X 10-45 X 10-55 X 10-7

The chemical elements that do not—unlike Ti, Zr, and Th—form soluble and mobile compounds in the biosphere, in spite of their marked quantity in the rocks of the earth’s crust, are found only in very small quantities in organisms. Organisms do not fully repeat the chemical composition of the environment but rather actively select various compounds. Often one or another type of organism accumulates a certain chemical element—that is, the chemical composition of the organisms is a characteristic feature for a certain species. Hence, organisms fulfill a geochemical function in participating in the biogenic migration of one or another chemical element. For example, calcium has long been used by organisms for forming the skeleton in the form of CaCO3. This very ancient geochemical function was characteristic for many lower organisms. Later on, along with a skeleton made up of CaCO3, organisms appeared with a skeleton consisting of calcium phosphate (above all among the brachiopods), and this skeleton became established in all the higher organisms. In many ancient lower organisms (up to the marine sponges inclusively) a skeleton of silicic acid is also encountered. This indicates the direction of evolution in organisms.

The participation of living matter in biogeochemical processes is manifested directly or indirectly. Thus, after the death of the organisms, living matter participates directly in the formation of diatomaceous earth, limestones, coals, petroleum, and so forth. Green plants create the entire mass of oxygen in the present-day atmosphere of the earth as a result of photosynthetic activity. Marine algae concentrate significant quantities of iodine; after their death, they become buried in the marine silts, and the process of converting organic detritus into the substance of petroleum occurs. As a result of the pressing of the liquid oil out of the buried silts into porous rock (sand and other collectors), muddy waters containing a large quantity of iodine are forced out.

The indirect effect of organisms and the products of their vital activities on the geochemical processes is even more varied. For example, microorganisms participate in the oxidation of compounds containing iron, manganese, and other elements. This leads to their precipitation out of natural solutions and deposition in sediments. The microorganisms reduce sulfates, forming biogenic sulfur deposits and so on. Geochemical processes change over time under the influence of living matter. Thus, when there was still no biosphere on the earth, uranium, germanium, and vanadium were concentrated in sedimentary iron ores, but with the appearance of the biosphere, these three elements also accumulated in certain fossil coals and bitumens.

Along with H2O and CO2, living matter plays an exceptional role in the processes of weathering and the formation of sedimentary rock (biogenic sediments in the seas and oceans). Also of interest is the participation of organisms in the processes of separating pairs of chemical elements which are close in terms of their properties—for example, Si/Ge, Fe/Mn, K/Na, and Ca/Sr. In turn, the environment is also reflected in the composition of the organisms. Within the limits of the so-called biogeochemical provinces, forms of organisms arise which accumulate sometimes significant quantities of a chemical element—that is, an intensive biogenic migration occurs. It is also known that organisms participate in disrupting the isotope composition of a number of light chemical elements (carbon, oxygen, and sulfur). As a rule, the organisms absorb predominantly the lighter isotopes in the biogenic processes.

Man also plays an enormous biogeochemical role as a result of his geological activities. Each year as much as several dozen tons of rock are extracted from the earth for each living person. Man influences the chemical and isotope composition of the atmosphere, the biosphere, and the earth’s crust, and this influence has continuously grown with each century.

REFERENCES

Vernadskii, V. I. Khimicheskoe stroenie biosfery Zemli i ee okruzheniia. Moscow, 1965.
Vinogradov, A. P. “Khimicheskii elementarnyi sostav organizmov moria.” Trudy Biogeokhimicheskoi laboratorii AN SSSR, 1935–44, vols. 3, 4, and 6.

A. P. VINOGRADOV

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