Biogeochemical Cycling and Energy Flow

Biogeochemical cycles are the cyclical paths that elements take as they flow through living (biotic) and non-living (abiotic) components of ecosystems. These cycles are important, because a fixed and limited amount of the elements that make up living cells exists on the earth and in the atmosphere. Thus, in order for an ecosystem to sustain its characteristic life forms, elements must continuously be recycled. For example, if the organic carbon that animals use as an energy source and exhale as carbon dioxide (CO2) were not eventually converted back to an organic form, we would run out of organic carbon to build cells. The carbon and nitrogen cycles are particularly important, because they involve stable gaseous forms, carbon dioxide and nitrogen gas, which enter the atmosphere and thus have global impacts.

While elements continually cycle in an ecosystem, energy does not. Instead, energy must be continually added to an ecosystem, fueling the activities required for life.

Understanding the cycling of nutrients and the flow of energy is becoming increasingly important as the burgeoning human population impacts the environment in a major way. For example, industrial processes that convert nitrogen gas (N2) into ammonia-containing fertilizers have increased food production substantially, but they have also altered the nitrogen cycle by increasing the amount of fixed nitrogen, such as ammonium and nitrate, in the environment. Pollution of lakes and coastal areas with these nutrients has far-reaching effects, including depletion of dissolved O2, which leads to the death of aquatic animals. It also decreases the biodiversity in terrestrial ecosystems. Excavation and burning of coal, oil, and other carbon-rich fossil fuels provides energy for our daily activities, but releases additional CO2 and other carbon-containing gases into the atmosphere. Fossil fuels, the ancient remains of partially decomposed plants and animals, are nutrient reservoirs that are unavailable without human intervention, and therefore would not normally participate in biogeochemical cycles. The increase of carbon-containing gases in the atmosphere raises global temperatures because the gases absorb infrared radiation and reflect it back to earth.

When studying biogeochemical cycles, it is helpful to bear in mind the role of a given element in a particular organism's metabolism. Elements are used for three general purposes:

■ Biomass production. The element is incorporated into cell material. All organisms, for example, require nitrogen to produce amino acids. Plants and many prokaryotes assimilate nitrogen by incorporating ammonia (NH3) to synthesize the amino acid glutamate (see figure 6.30a). Some prepare for this step using the process of assimilatory nitrate reduction, which converts nitrate (NO3:) to ammonia. Once glutamate has been synthesized, the amino group can then be transferred to other carbon compounds in order to produce the necessary amino acids. Animals cannot

774 Chapter 30 Microbial Ecology incorporate ammonia and instead require amino acids in their diet. Some prokaryotes can reduce atmospheric nitrogen to form ammonia, which can then be incorporated into cellular material. ■ amino acid synthesis, p. 161

■ Energy source. A reduced form of the element is used to generate energy in the form of ATP; the energy-yielding reactions oxidize the energy source. Reduced carbon compounds such as sugars, lipids, and amino acids are used as energy sources by chemoorganotrophs. Chemolithotrophs can use reduced inorganic molecules such as hydrogen sulfide (H2S), ammonia (NH3), and hydrogen gas (H2) (see table 4.4). ■ energy source, p. 134

■ Terminal electron acceptor. Electrons from the energy source are transferred to an oxidized form of the element during respiration; this reduces the terminal electron acceptor. In aerobic conditions, O2 is used as a terminal electron acceptor. In anaerobic conditions, some prokaryotes can use nitrate (NO3), nitrite (NO2), sulfate (SO4), and carbon dioxide (CO2) as terminal electron acceptors (see figure 6.20). ■ terminal electron acceptor, p. 134

Carbon Cycle

All organisms are composed of organic molecules including proteins, lipids, and carbohydrates. Consumers eat plants as well as other consumers to acquire organic carbon to build biomass and to oxidize to gain energy. Decomposers use the remains of primary producers and consumers for the same purposes. As the organic carbon is degraded, respiration and some fermentations release CO2, which must then be converted back to an organic form to complete the cycle (figure 30.9).

A fundamental aspect of the carbon cycle is carbon fixation, the defining characteristic of primary producers. These organisms all convert CO2 into an organic form. The mechanisms used are described in chapter 6. Without the activities of the primary producers, no other organisms, including humans, could exist as we depend on them to generate the organic carbon we require. ■ carbon fixation, p. 159

The organic carbon travels through the food chain as primary producers are eaten by primary consumers, which are then eaten by secondary consumers. Through these events, one form of biomass is transformed into another. Not all of the organic material

Respiration

Methane-oxidizing bacteria

Methane CH4 Reduced carbon

(CH2O)n Organic compounds

Combustion

CO2 Oxidized carbon s1

Photosynthesis

• Cyanobacteria

Aerobic

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