15.8: Essential chemicals cycle through ecosystems.

What is necessary for life? Energy and some essential chemicals top the list. New energy continually comes to earth from the sun, fulfilling the first need. And everything else is already here. The chemicals cycle around and around, using the same pathway taken by energy—the food chain. Plants and other producers generally take up the molecules from the atmosphere or the soil. Then as animals consume the plants or other animals, the chemicals move up the food chain. As the plants and animals die, detritivores and decomposers return the chemicals to the abiotic environment. From a chemical perspective, life is just a continuous recycling of molecules.

Chemicals cycle through the living and non-living components of an ecosystem. Each chemical is stored in a non-living part of the environment called a “reservoir.” Organisms acquire the chemical from the reservoir, the chemical cycles through the food chain, and eventually it is returned to the reservoir.

We can get a deeper appreciation of the functioning of ecosystems and the ecological problems that can occur when they are disturbed—particularly by humans—by investigating a few of these cycles in more detail. Here we explore three of the most important chemical cycles: carbon, nitrogen, and phosphorus.

Carbon Carbon is found largely in four compartments on earth: the oceans, the atmosphere, terrestrial organisms, and fossil deposits. Plants and other photosynthetic organisms obtain most of their carbon from the atmosphere, where carbon is in the form of carbon dioxide (CO₂). As we saw in Chapter 4, photosynthetic organisms use carbon dioxide in photosynthesis, separating the carbon molecules from CO₂ and using them to build sugars and other macromolecules. Carbon then moves through the food chain as organisms eat plants and are themselves eaten (FIGURE 15-16). The oceans contain most of the earth’s carbon. Here, many organisms use dissolved carbon to build shells (which later dissolve back into the water, after the organism dies).

Most carbon returns to its reservoir as a consequence of organisms’ metabolic processes. Organisms extract energy from food by breaking carbon-carbon bonds, releasing the energy stored in the bonds, and combining the released carbon atoms with oxygen. They then exhale the end product as CO₂.

The burning of fossil fuels is adding significantly to the atmospheric carbon reservoir. Fossil fuels are created when large numbers of organisms die and are buried in sediment lacking oxygen. In the absence of oxygen, at high pressures, and after very long periods of time, the organic remains are transformed into coal, oil, and natural gas. Trapped underground or in rock, these sources of carbon played little role in the global carbon cycle until humans in industrialized countries began using fossil fuels to power their technologies. Burning coal, oil, and natural gas releases large amounts of carbon dioxide, thus increasing the average CO₂ concentration in the atmosphere—the current level of CO₂ in the atmosphere is the highest it has been in almost half a million years. This has potentially disastrous implications, as we’ll see in Chapter 16.

Although, on average, the level of CO₂ in the environment is increasing, there is a yearly cycle of ups and downs in the CO₂ levels in the northern hemisphere. This is due to fluctuations in the ability of plants to absorb CO₂. Many trees lose their leaves each fall and, during the winter months, relatively low rates of photosynthesis lead to low rates of CO₂ consumption, causing an annual peak in atmospheric CO₂ levels. During the summer, trees have their leaves, sunlight is strong, and photosynthesis (consuming CO₂) occurs at much higher levels, causing a drop in the atmospheric CO₂ level.

Nitrogen Nitrogen is necessary to build a variety of molecules critical to life, including all amino acids, the components of every protein molecule. The chief reservoir of nitrogen is the atmosphere, but even though more than 78% of the atmosphere is nitrogen gas (N₂), for most organisms this nitrogen is completely unusable. The problem is that atmospheric nitrogen consists of two nitrogen atoms bonded tightly together, and these bonds need to be broken to make the nitrogen usable for living organisms. Only through the metabolic magic (chemistry, actually) of some soil-dwelling bacteria, the nitrogen-fixers, can most nitrogen enter the food chain (FIGURE 15-17).

These bacteria chemically convert, or “fix,” nitrogen by attaching it to other atoms, including hydrogen, to produce ammonia and related compounds. These compounds are then further modified by other bacteria into a form that can be taken up by plants and used to build proteins. And once nitrogen is in plant tissues, animals acquire it in the same way they acquire carbon: by eating the plants. Nitrogen returns to the atmosphere when animal wastes and dead animals are broken down by soil bacteria (decomposers) that convert the nitrogen compounds in tissues back to nitrogen gas.

The availability of nitrogen is like a bottleneck limiting plant growth. Because it is necessary for the production of every plant protein, and because all nitrogen must first be made usable by bacteria, plant growth is often limited by nitrogen levels in the soil. For this reason, most fertilizers contain nitrogen in a form usable by plants.

Phosphorus Every molecule of ATP and DNA requires phosphorus. But no (or barely any) phosphorus is available in the atmosphere. Instead, soil serves as the chief reservoir. Like nitrogen, phosphorus is chemically converted—“fixed”—into a form usable by plants (phosphate) and then absorbed by their roots. It cycles through the food chain as herbivores consume plants and carnivores consume herbivores. As organisms die, their remains are broken down by bacteria and other organisms, returning the phosphorus to the soil. The pool of phosphorus in the soil is also influenced by the much slower process of formation of rock on the seafloor, its uplifting into mountains, and its eventual weathering, releasing its phosphorus (FIGURE 15-18).

Like nitrogen, phosphorus is often a limiting resource in soils, constraining plant growth. Consequently, fertilizers usually contain large amounts of phosphorus. This is beneficial in the short run, but it can have some disastrous unintended consequences. As more and more phosphorus (and nitrogen) is added to soil, some of it runs off with water and ends up in lakes, ponds, and rivers. It also acts as a fertilizer in these habitats, making spectacular growth possible for algae. But eventually the algae die and sink, creating a huge source of food for bacteria. The bacterial population increases and may use up too much of the dissolved oxygen, causing fish, insects, and many other organisms to suffocate and die.

The increase in nutrients, particularly nitrogen and phosphorus, in an ecosystem, is called eutrophication. The consequent rapid growth of algae and bacteria, followed by large-scale die-offs of other organisms, is increasingly a problem in both small and large bodies of water, including more than half of the lakes of Asia, Europe, and North America (FIGURE 15-19). Lake Erie, on the U.S.-Canadian border, for example, has experienced eutrophication as a result of all the phosphorus- and nitrogen-containing wastewater that drains into the lake from the extensive surrounding farmlands. A eutrophication-caused algae bloom there in August 2014 was responsible for the growth of toxic bacteria in municipal water supplies. The governor of Ohio was forced to declare a state of emergency, banning half a million people from using water for drinking, cooking, or even bathing. Given the lower use of fertilizers in South America and Africa, eutrophication is less common there.