6-Carbon Sugar (1)
Figure 13.4 Calvin cycle carbon fixation process. *(No.) refers to number of molecules.
Photosynthetic organisms utilize part of the carbon they fix as an energy source, particularly in the dark (Section 13.1.2). However, the rest is used to produce the organic compounds they need for growth. It is this net fixation of carbon that sustains the ecosystems dependent on these primary producers, directly or indirectly, for their carbon and energy supply.
Of course, there are considerable variations in the rates of organic carbon assimilation around the world. Tropical rain forests, for example, have some of the highest rates of carbon uptake on Earth, with annual net fixation of up to 1.5 kg C/m2 of land surface area. Freshwater wetlands are almost as productive. Forests, grasslands, and cultivated plants in the middle latitudes typically assimilate 50% as much carbon, although intensive agriculture can more than double the production of the rain forest. The open ocean, deserts, and the Arctic tundra fix much less carbon (typically 0.05, 0.1, and 0.2kg/m2, respectively, per year). Sunlight intensity, temperature, nutrient supply, and water availability are all factors (see Chapters 14 and 15).
The use of sunlight for photosynthesis is not very efficient. On average, only about 0.1% of the energy available is utilized. Even with intensive agriculture, this value is unlikely to exceed 1%. Each gram of carbon fixed represents about 10 kJ of stored energy.
It is estimated that photosynthesis provides our planet with a cumulative carbon assimilation rate of approximately 1011 metric tons per year. Based on the stoichiometry in equation (13.1), this means that at the same time, almost 3 x 1011 metric tons per year of oxygen is produced. The long-term maintenance of this essential activity, however, is sensitive to disruption. Whether measured in acreage of rain forest lost to deforestation, wetlands drainage, or accidental tanker spills and resultant oil slicks, humans certainly have the potential to cause severe environmental stress.
Chemolithoautotrophic Carbon Fixation Although they utilize chemical energy stored in inorganic molecules rather than light, chemolithoautotrophs also utilize the Calvin cycle to fix inorganic carbon. Worldwide, a substantial amount of carbon is fixed through their action. In fact, in some environments in which sunlight is absent, they may be the major or only primary producers. Surrounding thermal vents in the deep ocean, for example, the release of hydrogen sulfide serves as an energy source for sul-fide-oxidizing bacteria and archaea, which then form the base of an extensive ecosystem, including giant worms and clams. It also appears that the abiotic release of hydrogen gas in deep deposits of basalt (a volcanic rock) in the Pacific northwestern region of the United States may support specialized autotrophic bacteria that serve as the base of a microbial ecosystem.
Nitrifying (see more in Section 13.2.2) and sulfur-oxidizing (Section 13.3.2) bacteria may be important in waste treatment and aquatic and soil environments, and sulfur-and iron-oxidizing (Section 13.4.2) bacteria play a major role in acid mine drainage. However, in these cases it is not the carbon fixation that is of greatest environmental significance. It recently has been suggested, however, that in drinking water systems using chloramination (an ammonium/chlorine combination) for disinfection, the nitrification that sometimes occurs may actually contribute materially to the otherwise low organic carbon content of the water.
Methanogenesis Methanogenesis, the production (genesis) of methane, is a form of carbon reduction carried out solely by a group of strictly anaerobic archaea referred to collectively as methanogens (Section 10.6.3). One reaction commonly utilized is actually an anaerobic respiration in which CO2 [equation (13.2)], or occasionally, CO [equation (13.3)], is reduced to methane during oxidation of H2. Many methanogens can ferment formic acid [equation (13.4)] or methanol [equation (13.5)], with some of the molecules being oxidized to CO2 and others reduced to CH4. A few methanogens can similarly ferment methyl amines or, less commonly, methyl sulfides. Many can also ferment acetic acid, oxidizing part of the molecule to CO2 while reducing the other part of same molecule to CH4 [equation (13.6)]:
Often under methanogenic conditions, such as in landfills and anaerobic digesters, the net gas production is almost two-thirds methane and one-third carbon dioxide, with small amounts of other gases. Additional details about methanogenesis are given in Sections 13.1.1 and 16.2.1.
4CHOOH ! CH4 + 3CO2 + 2H2O 4CH3OH ! 3CH4 + CO2 + 2H2O CH3COOH ! CH4 + CO2
396 MICROBIAL TRANSFORMATIONS 13.1.2 Carbon Oxidation
The oxidation of organic carbon compounds provides energy to a cell. Even in photosyn-thetic organisms, it is the oxidation of the fixed carbon produced (with the energy from sunlight) that provides the cells with most of the energy they use.
The two overall types of energy-yielding metabolism are respiration (Section 5.4.3) and fermentation (Section 5.4.2). In respiration, an inorganic molecule acts as the terminal electron acceptor, whereas in fermentation, an organic molecule serves this function. Fermentation is an anaerobic process, whereas there are both aerobic and anaerobic forms of respiration.
Respiration Respiration provides more energy to the cell then fermentation, so is favored whenever it is possible. Aerobic respiration, in which molecular oxygen (O2) is the terminal electron acceptor, provides the most energy; thus, aerobic organisms are likely to dominate wherever oxygen is available.
Since our atmosphere is 21% oxygen, it might be expected that oxygen is readily available except in sealed systems. However, oxygen is poorly soluble in water, with only 9.1 mg/L of dissolved oxygen (DO) present at saturation (equilibrium with the atmosphere) at 20°C. Also, oxygen diffusion through pores in solid matrices (such as soil and composting material) occurs by a factor of 4 orders of magnitude more slowly in water than in air. Thus, in aqueous systems (such as wastewater, liquid sludges, surface waters, river and lake sediments, groundwater, and flooded soils) the oxidation of even small amounts of organic material can quickly deplete DO, leading to anaerobic conditions.
In the absence of oxygen, many bacteria can carry out respiration using nitrate as the terminal electron acceptor (Section 5.4.3). This denitrification (Section 13.2.1) provides about 85% of the energy available aerobically from oxidation of the same organic compound with oxygen. The nitrate is reduced first to nitrite, then eventually to nitrogen gas. Thus, nitrite can also be utilized, but yields still less energy. Denitrifying bacteria include a wide diversity of aerobic bacteria that can utilize nitrate and nitrite as alternative electron acceptors when oxygen is absent. The process itself is discussed in more detail as part of the nitrogen cycle (Section 13.2).
Ferric iron can also be utilized for respiration by some aerobic bacteria, providing a similar amount of energy to nitrite. Some other metals may also be utilized by facultatively or strictly anaerobic bacteria.
In the absence of oxygen, nitrate, nitrite, and ferric iron, some anaerobic bacteria and archaea can utilize sulfate as an electron acceptor, producing hydrogen sulfide. This is particularly important in salt marshes and other systems where sulfate is abundant (Section 13.3). Once the sulfate is also depleted, methanogens can utilize CO2 as an electron acceptor, producing methane [equation (13.2)].
Under anaerobic conditions, particular bacteria, especially some sulfate reducers, may be able to utilize other "unusual" electron acceptors. Examples include the use of a number of chlorinated organic compounds, including important environmental contaminants such as polychlorinated biphenyls (PCBs), trichloroethylene (TCE), tetrachloroethylene [commonly called perchloroethylene (PCE)], and vinyl chloride, during which a hydrogen is substituted for a chlorine atom. This represents both a chemical reduction and a dechlorination of the organic molecule and results in the release of a chloride ion (CP). A similar reaction may occur with brominated and perhaps other halogenated compounds. Thus, the more general term reductive dehalogenation is often used to describe this process.
Fermentation Although it is usually bacteria, archaea, some fungi, and a few protozoa that are thought of as being anaerobic, higher organisms also carry out fermentations. During vigorous exercise, for example, not enough oxygen can be delivered to the muscles, leading to buildup of the fermentation product lactic acid. This is what makes your arms and legs feel heavy and tired. Gradually, as you recover, the lactic acid is transported to the liver and converted there for other uses.
Because an organic compound serves as the electron acceptor in fermentation, some of the organic material is reduced while oxidizing other organic molecules. In the examples above for methanogenesis, one reaction involves four molecules of formic acid [equation (13.4)], one of which is reduced to CH4 while the other three are oxidized to CO2. For the fermentation of the more reduced compound methanol [equation (13.5)], the ratio of these two products is reversed. With acetic acid [equation (13.6)], on the other hand, of the two carbons of a single molecule, one is oxidized while the other is reduced.
Was this article helpful?