|Microbial Nutrition and Metabolism|
Microorganisms—bacteria, fungi, algae, and protists—are found in every environment on the earth that supports life. Microorganisms have been found in hot springs where temperatures exceed 80 degrees Celsius as well as in rocks of Antarctic deserts.
To ensure survival in a variety of habitats, microorganisms have developed a fascinating variety of strategies for survival. The study of microbial ecology involves consideration of the mechanisms employed by microorganisms to obtain nutrients and energy from their environment.
To maintain life processes and grow, all cellular organisms require both a source of carbon (the principal element in all organic molecules) and a source of energy to perform the work necessary to transform carbon into all the molecular components of cytoplasm. Among plants and animals, two main nutritional modes have evolved to meet these requirements.
All plants are photoautotrophs, fixing carbon from inorganic carbon and obtaining energy from light. All animals are chemoheterotrophs, meeting their carbon needs by taking preformed organic molecules from the environment and extracting energy from chemical transformation of the same organic molecules.
Both of these nutritional modes, photoautotrophy and chemoheterotrophy, are found among microorganisms; for example, all algae are photoautotrophs,while all fungi are chemoheterotrophs. In addition, certain specialized bacteria exhibit a mode of nutrition, chemoautotrophy, found in no higher organisms.
Like photoautotrophs, chemoautotrophs are able to use carbon dioxide for all of their carbon requirements; however, they do not use light as an energy source. Instead, chemoautotrophic bacteria capture energy from inorganic chemical reactions, such as the oxidation of ammonia. Chemoautotrophic bacteria are highly specialized and can be found in unusual environments.
The most spectacular display of chemoautotrophic energy metabolism is exhibited at the hydrothermal vents found in certain locations on the ocean floor. There, where sunlight cannot penetrate, chemoautotrophic bacteria serve as the producers for a rich and diverse ecosystem.
An appreciation of the metabolic diversity displayed by microorganisms enhances understanding of the ways in which matter and energy are transformed in the biosphere. Consideration of microbial contributions to the flow of carbon, nitrogen, and other elements is critical to defining the balance of ecosystems and the effects of changes in environmental chemistry and species composition.
Microorganisms are, by definition, unseen, and many people become aware of them only in their negative manifestations as agents of disease and spoilage. In fact, however, the diverse metabolic activities of microorganisms make them a critical component of all the earth’s ecosystems and a source of many useful products for human industry.
Sixty percent of the dry mass of green plants is in the form of cellulose, although no animal that eats the plants is directly able to obtain carbon or energy from cellulose. Microorganisms that digest cellulose do so by secreting exoenzymes, proteins that cause cellulose to be broken into simpler molecular units that are absorbed by the microorganism.
Cellulose-digesting microorganisms are found in most terrestrial ecosystems and in the digestive tracts of animals, such as cattle and termites, that depend on cellulose-rich plant material as a nutrient source. By breaking down cellulose and other complex organic polymers, microorganisms make a significant contribution to the cycling of carbon in ecosystems.
Microorganisms also perform chemical transformations involving nitrogen, which is found in all cellular proteins and nucleic acids. Plants incorporate nitrogen from the soil in the form of nitrate or ammoniumions, and animals obtain nitrogen from the same organic compounds they use as carbon and energy sources.
When dead plant and animal tissue is decomposed by chemoheterotrophic microorganisms, the nitrogen is released as ammonia. A group of chemoautotrophic bacteria, the nitrifying bacteria, obtain their metabolic energy from the conversion of ammonia to nitrate; in this way, the nitrifiers convert the nitrogen released during decomposition to a form readily used by plants, thus contributing to soil fertility.
A second group of bacteria converts nitrate to atmospheric nitrogen gas, which cannot be used by plants; these bacteria are called denitrifiers because (in contrast to the nitrifiers) their metabolic activities cause a net loss of nitrogen from the soil.
Nitrogen lost from the environment by denitrification is replaced by ammonia released during decomposition and by the metabolic activity of nitrogen-fixing bacteria, so called because they “fix” nitrogen gas from the atmosphere in the form of ammonia.
Nitrogen fixation requires a great quantity of energy, and nitrogen-fixing bacteria are often found in symbiotic association with plants, especially legumes. The bacteria provide nitrogen in a usable form to the plant, while the plant provides carbon and energy in the form of organic compounds to the chemoheterotrophic nitrogen-fixing bacteria.
The presence of nitrogen-fixing bacteria is often indicated by the formation of characteristic nodules on the roots of plants involved in the associations. Free-living nitrogen fixers are also known, and these may play a significant role in the nitrogen balance of aquatic ecosystems.
Respiration and Fermentation
Chemoheterotrophic microorganisms are found both in aerobic environments,where oxygen is available, and in anaerobic environments, where oxygen is lacking. The availability of molecular oxygen may determine the type of energy metabolism employed by a microorganism.
Where oxygen is available, many microorganisms obtain energy by respiration. In respiration, electrons removed from organic nutrient sources are transferred through a complex sequence of reactions to molecular oxygen, forming water and carbon dioxide.
In the process, energy is made available to the organisms. In the absence of oxygen, some microorganisms are able to carry out a form of anaerobic respiration using nitrate or sulfate in place of oxygen. Denitrification is an example of anaerobic respiration.
Other anaerobic microorganisms employ fermentation. In fermentation, electrons removed from organic nutrient sources are transferred to organic molecules, forming fermentation products, such as alcohols and organic acids, which may be used as nutrient sources by other chemoheterotrophs.
A number of bacteria, the facultative anaerobes, are capable of performing either aerobic respiration or fermentation, depending on the availability of oxygen. These bacteria are able to achieve optimum growth in environments, such as soils, where the availability of oxygen may vary over time.
Effects and Uses
The contributions of microorganisms to the chemical transformations which characterize an ecosystem are many. Along with higher plants, photoautotrophic and chemoautotrophic microorganisms capture inorganic carbon dioxide and, using energy from sunlight or chemical reactions, synthesize organic molecules, which are used by animals and by chemoheterotrophic microorganisms as sources of carbon and energy. Through the processes of respiration and fermentation, chemoheterotrophs return inorganic carbon dioxide to the environment.
Much of this recycling of carbon from organic molecules to carbon dioxide depends on the activities of microbial decomposers, which are able to break down organic polymers, such as cellulose.
Nitrogen, released from organic molecules by chemoheterotrophs in the form of ammonia, may be made available to chemoheterotrophs in the form of nitrate by nitrifying bacteria. Nitrate which is lost from an ecosystem through the activities of denitrifiers may be returned by nitrogen-fixing microorganisms.
Although the nature of the microbial world has been known only since about the turn of the twentieth century, the metabolic activities of microorganisms have been exploited throughout human history. The manufacture of alcoholic beverages, cheeses, vinegars, and linen depends on the metabolic activities of microorganisms.
Farmers employed practices designed to optimize the availability of nitrogen to plants for centuries before the role of microorganisms in nitrogen cycling was understood. Composting and other decomposition processes, including sewage treatment, are consequences of the metabolic activities of mixed populations of microorganisms.
The number of organic compounds used as nutrients by one or another chemoheterotrophic microorganism is extraordinary. Some soil bacteria have been shown to use more than one hundred different organic molecules as their sources of carbon and energy.
By using selective enrichment techniques, it has been possible to isolate microorganisms capable of degrading pesticides, complex petroleum by-products, and other toxic chemicals previously assumed to be resistant to natural decomposition processes. Through application of appropriate engineering technologies, these microorganisms may play a part in solutions to toxicwaste disposal issues.