Nutrient/energy cycling refers to the way in which the limited materials, atoms, com pounds, and ions, are reused at the earth's surface. The main systems of life—cells, organisms made of cells, and ecosystems made of populations of species—join the biosphere as a whole as functioning energy-driven systems. Thermodynamically, these systems are not closed but open—that is, they admit of material and energetic flow across their boundaries. All complex systems in the cosmos, including those of life, are energy-driven systems. (Whereas organisms take in some materials and produce others, ecosystems can be fully recycling, using energy to turn wastes into food.) What has been nominated the fourth law of thermodynamics, Morowitz's cycling law (after U.S. biophysicist Harold Morowitz), states that the flow of energy from a source to a sink through a steady-state system will lead to at least one cycle in the system. Just as the materials of life are not special, nor is its tendency to become cyclical and complex. The second law of thermodynamics, which says that systems become less organized when isolated (that is, when they are left alone, not fed with new energy or materials), was based on a study of energy in artificially closed conditions. A more complete, extended version, which applies to life and other open systems, says that gradients, differences across a distance, are reduced. Cycle formation occurs and even gives rise to physical and nonliving chemical systems with nascent identity. In the presence of gradients—differences of temperature, pressure, and chemical concentration (electron potential)— cycles are generated. Thermodynamically, the gradient represents a previous improbability. The cycling system functions to dissipate a pre-existing improbable state in accord with the second law. Just as life's carbon, nitrogen, sulfur, hydrogen, oxygen, and phosphorus are elements that make up nonliving matter, so living matter—in which cyclical chemistry and growing complexity appear in regions of energy flow—is not unique.
It is in the context of thermodynamics and ecology that we can best appreciate the cycling of nutrients and wastes, for each does sometimes transform into the other on the earth's surface. The main elements of life are found in the environment because life is an open ther-modynamic system that has, over evolutionary time, progressively integrated more of the earth's surface, more chemical elements and compounds into its cyclical processes. Bio-mineralization, in which minerals can be laid down under precise genetic control, includes crystals of magnetite and calcium phosphate (apatite), made in bacteria and in our inner ear's balance organs, respectively. Life orchestrates, beyond the body of its flesh proper, an expanding mineral house including bony infrastructure and shelly walls. Trace metals, inorganic phosphorus, nitrate, carbonate, and silica in seawater are scavenged for functional uses in and around cells. Technology—for example, the extraction and processing of silicates to make silicon computer chips for the information industry—is part of an ancient process of recycling materials. Indeed, long before man, diatoms (a kind of alga) that need silica to make their frustules, and sponges (a kind of animal) making spicules, depleted the ocean of silica for their own biotechno-logical purposes.
Seen from space, the earth is a system far from equilibrium that would never be predicted under the standard mixing rules for chemicals in an isolated system. That is because life has found a very elaborate way to capture and degrade, in ever more complex chemical material cycles, the energy of the sun. Unlike nonliving energy-driven systems that cycle and become more complex (for example, chemical patterns or a tornado until it dissipates a barometric pressure gradient), cyclical chemistry on earth has spawned reproduc-tion—a process for making, more or less faithfully, finely tuned "vehicles" of energy degradation. But earth's locally complex organisms and cells are not only excellent energy degraders, producing wastes and even using energy to turn wastes back into food; in addition, the lack of perfect fidelity leads naturally to new systems able to recognize and use new gradients, and thus in turn producing new wastes.
The nutrients that organisms need are ultimately the atoms that cyclically compose their bodies. Although many complex models exist showing chemical changes in and out of cells as atoms circumnavigate the planet, all are provisional in detail. The interactions of cycles such as the carbon, nitrogen, and sulfur cycles are even more complex; investigation into their details is underway by biogeochemists. Seemingly minor sulfur compounds formed by microbes over the ocean, for example, can react to form the nuclei of raindrops that fall, feeding algae eaten by other organisms that carry the compounds of their bodies to new places, producing still other changes. But something is known about the main cycles and their major players. Phosphorus, for example, which is needed as the "backbone" of DNA and RNA, is unlike the other elements of organisms because it is not available in a gaseous form. Thus, from a global cycling perspective, the phosphorus-rich feces of sea birds (such guano is known to create entire islands) is a means of distributing this biologically limiting element. Beginning more than 3 billion years ago, carbon dioxide has been taken from the atmosphere and used to construct cells and their cyclical aggregations.
The oldest and most prodigious cyclers of materials on the planet are the bacteria, whose cells provided the site for the evolution of all the major modes of metabolism by which energy is used to produce cell chemistry and its waste products. These waste products, in an ancient and more efficient version of the recycling efforts instituted by humanity, are incorporated again as nutrients. Bacteria, for example, produce carbon dioxide in methanogenesis, fermentation, and respiration, and incorporate it into bodies by using light or chemical reactions. Organisms that use light for energy are known as phototrophs; those that use chemical gradients are chemotrophs. Cells that eat other cells tend to be chemo-organ-otrophs, using the energy stored in the organic compounds of other bodies. Some organisms, such as the newly discovered abundant denizens deeper in the rocks of the biosphere, are chemolithotrophs; they tap into inorganic chemical reactions to metabolically maintain and reproduce themselves.
Bacteria are about 50 percent dry weight carbon, and 12 percent nitrogen; apart from the other elements mentioned, potassium, magnesium, sodium, calcium, and iron are required by cells for functions such as building enzymes and cell walls. Bacteria get these elements as salt ions in solution, or from solid rocks or minerals in rocks. On a cell level gradients, such as the oxidation/reduction gradient between the hydrogen-rich organic compounds of eukaryotic cells (cells with nuclei) and the oxygen from the atmosphere, run complex metabolic cycles. Whether eaten alive or exposed to decay after death by fungi and bacteria, organisms take from the environment and each other as they propagate increasingly energy-seeking systems in a swirl of thermodynamic cyclical activity beyond any single life form. Fermentation, photosynthesis, sugar metabolism (including glycolysis and the citric acid cycle), and production of nucleic acids (genes) are all part of the overarching sun-driven energy cycle by which the limited atoms of earth's surface are co-opted, used up, and reused, in the evolving systems of life.
—Lynn Margulis and Dorion Sagan
See also: Agricultural Ecology; Agriculture and Biodiversity Loss: Genetic Engineering and the Second Agricultural Revolution; Atmosphere; Atmospheric Cycles; Carbon Cycle; Evolution; Five Kingdoms of Nature; Food Webs and Food Pyramids; Lichens; Microbiology; Nitrogen Cycle; Pollution; Protoc-tists; Soil; Topsoil Formation
Margulis, Lynn, Clifford Matthews, and Aaron Haselton, eds. 2000. "Five-kingdom Classification Scheme: Superkingdom Prokaryota." In Environmental Evolution. Appendix E. 2d ed. Cambridge: MIT Press; Rambler, Mitchell B., Lynn Margulis, and Rene Fester, eds. 1989. Global Ecology: Towards a Science of the Biosphere. Boston: Academic; Sagan, Dorion, and Lynn Margulis. 1993. Garden of Microbial Delights: A Practical Guide to the Subvisible World. Dubuque, IA: Kendall/Hunt.
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