Nutritional Types of Organisms

In terms of carbon utilization a cell may be heterotrophic or autotrophic. Heterotrophs obtain their carbon and energy for growth from organic compounds in nature. Autotrophs use C0₂ as a sole source of carbon for growth and obtain their energy from light (e.g. photoautotrophs) or from the oxidation of inorganic compounds (e.g. lithoautotrophs).

Most heterotrophic bacteria are saprophytes, meaning that they obtain their nourishment from dead organic matter. In the soil, saprophytic bacteria and fungi are responsible for biodegradation of organic material. Ultimately, organic molecules, no matter how complex, can be degraded to CO₂ (plus H₂ and H₂O). Probably no naturally-occurring organic substance cannot be degraded by the combined activities of the bacteria and fungi. Hence, most organic matter in nature is converted by heterotrophs to CO₂, only to be converted back into organic material by autotrophs that die and nourish heterotrophs to complete the carbon cycle.

Lithotrophic procaryotes have a type of energy-producing metabolism which is unique. Lithotrophs (also called lithoautotrophs or chemoautotrophs) use inorganic compounds as sources of energy, i.e., they oxidize compounds such as H₂ or H₂S or NH₃ to obtain electrons to feed in to an electron transport system and to produce ATP. Lithotrophs are found in soil and aquatic environments wherever their energy source is present. Most lithotrophs are autotrophs so they can grow in the absence of any organic material. Lithotrophic species are found among the Bacteria and the Archaea. Sulfur-oxidizing lithotrophs convert H₂S to  S^o and S^o to SO₄. Nitrifying bacteria convert NH₃ to NO₂ and NO₂ to NO₃; methanogenic archaea strip electrons off of H₂ as a source of energy and add them to CO₂ to form CH₄ (methane). Lithotrophs have an obvious impact on the sulfur, nitrogen and carbon cycles in the biosphere.

Photosynthetic bacteria convert light energy into chemical energy for growth. Most phototrophic bacteria are autotrophs so their role in the carbon cycle is analogous to that of plants. The planktonic cyanobacteria are the "grass of the sea" and their form of oxygenic photosynthesis generates a substantial amount of O₂ in the biosphere. However, among the photosynthetic bacteria are types of photosynthetic metabolism not seen in eucaryotes, including photoheterotrophy (using light as an energy source while assimilating organic compounds as a source of carbon), anoxygenic photosynthesis, and unique mechanisms of CO₂ fixation (autotrophy).

Photosynthesis has not been found to occur among the Archaea, but one archaeal species employs a light-driven non photosynthetic means of energy generation based on the use of a chromophore called bacteriorhodopsin. 

Adaptations to Environmental Conditions

Most procaryotes, whether they have been cultured and studied in the laboratory, or observed growing in their natural habitats, seem to be highly adapted to their specific environment by means of their macromolecular structure and/or their physiologic (metabolic) capabilities. The nutritional quality of the environment determines whether a particular organism will be present,  but so do various physical parameters such as the availability of light and  O₂, as well as the pH, temperature and salinity of the environment. As examples, the range of procaryotic responses to oxygen and temperature are discussed below.

Procaryotes vary widely in their response to O₂ (molecular oxygen). Organisms that require O₂ for growth are called obligate aerobes; those which are inhibited or killed by O₂, and which grow only in its absence, are called obligate anaerobes; organisms which grow either in the presence or absence of O₂ are called facultative anaerobes. Whether or not a particular organism can exist in the presence of O₂ depends upon the distribution of certain enzymes such as superoxide dismutase and catalase that are required to detoxify lethal oxygen radicals that are always generated by living systems in the presence of O₂

Procaryotes also vary widely in their response to temperature. Those that live at very cold temperatures (0 degrees or lower) are called psychrophiles; those which flourish at room temperature (25 degrees) or at the temperature of warm-blooded animals (37 degrees) are called mesophiles; those that live at high temperatures (greater than 45 degrees) are thermophiles. The only limit that seems to be placed on growth of certain procaryotes in nature relative to temperature is whether liquid water exists. Hence, growing procaryotic cells can be found in supercooled environments (ice does not form) as low as -20 degrees and superheated environments (steam does not form) as high as 120 degrees. Archaea have been detected around thermal vents on the ocean floor where the temperature is as high as 320 degrees!

Symbiosis

The biomass of procaryotic cells in the biosphere, their metabolic diversity, and their persistence in all habitats that support life, ensures that these microbe will play a crucial role in the cycles of elements and the functioning of the world ecosystem. However, the procaryotes affect the world ecology in another significant way through their inevitable interactions with insects, plants and animals. Some bacteria are required to associate with insects, animals or plants for the latter to survive. For example, the sex of offspring of certain insects is determined by endosymbiotic bacteria. Ruminant animals (cows, sheep, etc.), whose diet is mainly cellulose (plant material), must have cellulose-digesting bacteria in their intestine to convert the cellulose to a form of carbon that the animal can assimilate. Leguminous plants grow poorly in nitrogen-deprived soils unless they are colonized by nitrogen-fixing bacteria which can supply them with a biologically-useful form of nitrogen.

Bacterial Pathogenicity

Some bacteria are parasites of plants or animals, meaning that they grow at the expense of their eucaryotic host and may damage, harm, or even kill it in the process. Such bacteria that cause disease in plants or animals are pathogens. Human diseases caused by bacterial pathogens include tuberculosis, whooping cough, diphtheria, tetanus, gonorrhea, syphilis, pneumonia, cholera and typhoid fever, to name a few. The bacteria that cause these diseases have special structural or biochemical properties that determine their virulence or pathogenicity. These include: (1) ability to colonize and invade their host; (2) ability to resist or withstand the antibacterial defenses of the host; (3) ability to produce various toxic substances that damage the host. Plant diseases, likewise, may be caused by bacterial pathogens. More than 200 species of bacteria are associated with plant diseases, but a very small handful of genera are involved.

Figure 14. Borrelia burgdorferi. This spirochete is the bacterial parasite that causes Lyme disease. CDC.