Introduction
As discussed of microbial cells are structurally complex and carry out numerous functions. In order to construct new cellular components and do cellular work, organisms must have a supply of raw materials or nutrients and a source of energy.
Nutrients are substances used in biosynthesis and energy release and therefore are required for microbial growth. In this chapter we describe the nutritional requirements of microorganisms, how nutrients are acquired, and the cultivation of microorganisms.
THE COMMON NUTRIENT REQUIREMENTS
Analysis of microbial cell composition shows that over 95% of cell dry weight is made up of a few major elements: carbon, oxygen, hydrogen, nitrogen, sulfur, phosphorus, potassium, calcium, magnesium, and iron.
These are called macroelements or macronutrients because they are required by microorganisms in relatively large amounts. The first six (C, O, H, N, S, and P) are components of carbohydrates, lipids, proteins, and nucleic acids.
The remaining four macroelements exist in the cell as cations and play a variety of roles. For example, potassium (K) is required for activity by a number of enzymes, including some of those involved in protein synthesis.
Calcium (Ca2), among other functions, contributes to the heat resistance of bacterial endospores. Magnesium (Mg2) serves as a cofactor for many enzymes, complexes with ATP, and stabilizes ribosomes and cell membranes. Iron (Fe2 and Fe3) is a part of cytochromes and a cofactor for enzymes and electron-carrying proteins.
In addition to macroelements, all microorganisms require several nutrients in small amounts. These are called micronutrients or trace elements. The micronutrients—manganese, zinc, cobalt, molybdenum, nickel, and copper—are needed by most cells.
However, cells require such small amounts that contaminants from water, glassware, and regular media components often are adequate for growth. In nature, micronutrients are ubiquitous and probably do not usually limit growth.
Micronutrients are normally a part of enzymes and cofactors, and they aid in the catalysis of reactions and maintenance of protein structure. For example, zinc (Zn2) is present at the active site of some enzymes but can also be involved in the association of regulatory and catalytic subunits
(e.g., E. coli aspartate carbamoyltransferase). Manganese (Mn2) aids many enzymes that catalyze the transfer of phosphate groups. Molybdenum (Mo2) is required for nitrogen fixation, and cobalt (Co2) is a component of vitamin B12. Enzymes, Control of protein activity.
Besides the common macroelements and trace elements, microorganisms may have particular requirements that reflect their specific morphology or environment. Diatoms need silicic acid (H4SiO4) to construct their beautiful cell walls of silica [(SiO2)n].
Although most procaryotes do not require large amounts of sodium, many archaea growing in saline lakes and oceans depend on the presence of high concentrations of sodium ion (Na).
Protist classification: Stramenopiles Phylum Euryarchaeota: The Halobacteria (section 20.3) Finally, it must be emphasized that microorganisms require a balanced mixture of nutrients. If an essential nutrient is in short supply, microbial growth will be limited regardless of the concentrations of other nutrients.
REQUIREMENTS OF CARBON, HYDROGEN, OXYGEN, AND ELECTRONS
All organisms need carbon, hydrogen, oxygen, and a source of electrons. Carbon is needed for the skeletons or backbones of all the organic molecules from which organisms are built. Hydrogen and oxygen are also important elements found in organic molecules.
Electrons are needed for two reasons. As will be described more completely in chapter 9, the movement of electrons through electron transport chains and during other oxidation-reduction reactions can provide energy for use in cellular work.
Electrons also are needed to reduce molecules during biosynthesis (e.g., the reduction of CO2 to form organic molecules).
The requirements for carbon, hydrogen, and oxygen often are satisfied together because molecules serving as carbon sources often contribute hydrogen and oxygen as well.
For instance, many heterotrophs—organisms that use reduced, preformed organic molecules as their carbon source—can also obtain hydrogen, oxygen, and electrons from the same molecules.
Because the electrons provided by these organic carbon sources can be used in electron transport as well as in other oxidation-reduction reactions, many heterotrophs also use their carbon source as an energy source.
Indeed, the more reduced the organic carbon source (i.e., the more electrons it carries), the higher its energy content. Thus lipids have a higher energy content than carbohydrates. However, one carbon source, carbon dioxide (CO2), supplies only carbon and oxygen, so it cannot be used as a source of hydrogen, electrons, or energy.
This is because CO2 is the most oxidized form of carbon, lacks hydrogen, and is unable to donate electrons during oxidation-reduction reactions. Organisms that use CO2 as their sole or principal source of carbon are called autotrophs.
Because CO2 cannot supply their energy needs, they must obtain energy from other sources, such as light or reduced inorganic molecules. A most remarkable nutritional characteristic of heterotrophic microorganisms is their extraordinary flexibility with respect to
carbon sources. Laboratory experiments indicate that there is no naturally occurring organic molecule that cannot be used by some microorganism. Actinomycetes, common soil bacteria, will degrade amyl alcohol, paraffin, and even rubber.
Some bacteria seem able to employ almost anything as a carbon source; for example, Burkholderia cepacia can use over 100 different carbon compounds. Microbes can degrade even relatively indigestible human-made substances such as pesticides.
This is usually accomplished in complex microbial communities. These molecules sometimes are degraded in the presence of a growth-promoting nutrient that is metabolized at the same time—a process called cometabolism.
Other microorganisms can use the products of this breakdown process as nutrients. In contrast to these bacterial omnivores, some microbes are exceedingly fastidious and catabolize only a few carbon compounds.
Cultures of methylotrophic bacteria metabolize methane, methanol, carbon monoxide, formic acid, and related one-carbon molecules.
Parasitic members of the genus Leptospira use only long-chain fatty acids as their major source of carbon and energy