All organisms require an energy source to drive energy-consuming life processes. Energy can be tapped from light or chemicals (organic chemicals and inorganic chemicals). Organisms that can utilize radiant energy (sunlight) are called phototrophs. Chemotrophs are organisms that can harvest energy from chemicals.
Organisms that conserve energy from organic chemicals are called chemoorganotrophs. Thousands of different organic chemicals can be used by one or another microorganism. Indeed, all-natural and even most synthetic organic compounds can be metabolized. Energy is conserved from the oxidation of the compound and is stored in the cell in the energy-rich bonds of the compound adenosine triphosphate (ATP).
Aerobes obtain energy from an organic compound in the presence of oxygen, anaerobes obtain energy in the absence of oxygen and facultative anaerobes can break down organic compounds in both aerobic and anaerobic conditions.
The oxidation of inorganic compounds to yield energy is known as chemolithotrophy. Many prokaryotes can tap the energy available from the oxidation of inorganic compounds. This phenomenon was discovered by the Russian microbiologist Winogradsky. Organisms that carry out chemolithotrophic reactions are called chemolithotrophs. Like phototrophic organisms, chemolithotrophic bacteria are also autotrophs.
Chemolithotrophy occurs only in prokaryotes and is widely distributed among species of Bacteria and Archaea. Several inorganic compounds can be oxidized; for example, H2, H2S (hydrogen sulfide), NH3 (ammonia), and Fe21 (ferrous iron). Typically, a related group of chemolithotrophs specializes in the oxidation of a related group of inorganic compounds, and thus we have the “sulfur” bacteria, the “iron” bacteria, and so on.
The capacity to conserve energy from the oxidation of inorganic chemicals is a good metabolic strategy because competition from chemoorganotrophs, organisms that require organic energy sources, is not an issue. In addition, many of the inorganic compounds oxidized by chemolithotrophs, for example, H2 and H2S, are actually the waste products of chemoorganotrophs. Thus, chemolithotrophs have evolved strategies for exploiting resources that chemoorganotrophs are unable to use, so it is common for species of these two physiological groups to live in close association with one another.
Sunlight is available in many microbial habitats on Earth, phototrophic microorganisms living in those areas harvest energy from sunlight. They contain pigments that allow them to convert light energy into chemical energy, and thus their cells appear colored. Unlike chemotrophic organisms, phototrophs do not require chemicals as a source of energy.
Purple bacteria appeared on Earth long before oxygenic phototrophs evolved. Green sulfur bacteria were some of the first phototrophs to evolve on Earth.
Two major forms of phototrophy are known in prokaryotes.
- Oxygenic photosynthesis: oxygen (O2) is produced. Among microorganisms, oxygenic photosynthesis is characteristic of cyanobacteria and algae (oxygenic phototrops).
- Anoxygenic photosynthesis: does not yield O2. Purple sulfur bacteria, green bacteria, and heliobacteria are anoxygenic phototrophs.
Among phototrophic bacteria are species that use inorganic compounds as their source of electrons and are called photolithotrophs. For example, Chromatium okenii.
Some phototrophic microorganisms use organic compounds such as fatty acids and alcohols as electron donors and are therefore photoorganotrophs. For example, Rhodospirillum rubrum.
Heterotrophs and Autotrophs
All organisms require carbon in some form either in small or large amounts to synthesize cell components. Organisms that can use carbon dioxide (CO2) as their major or even sole source of carbon are termed autotrophs. Other organisms require organic compounds as their carbon source and are known as heterotrophs.
Chemoorganotrophs are by definition heterotrophs. By contrast, most chemolithotrophs and phototrophs are autotrophs. For example chemolithotrophic bacteria of the genus Nitrosomonas are able to oxidize ammonia into nitrite, thereby obtaining sufficient energy to assimilate the carbon of CO2 into cell component (CO2 fixation).
Autotrophs are sometimes called primary producers because they synthesize new organic matter from CO2 for both their own benefit and that of chemoorganotrophs. Autotrophs can transform inorganic compounds into carbohydrates, proteins, nucleic acids, lipids, vitamins, and other complex organic substances required for the cells.
Heterotrophs either feed directly on the cells of primary producers or live off products they excrete. Virtually all organic matter on Earth has been synthesized by primary producers, in particular, the phototrophs. Autotrophs are responsible for the cycling of elements in nature through biological processes.
Heterotrophs rely on autotrophs for their foods and are also called consumers of the food chains. All the organisms that cause diseases of humans, animals, and plants are heterotrophs. They constitute the greater part of the microbial population in our immediate environment. Heterotrophs vary considerably in their nutritional requirements, particularly with respect to their organic carbon source, nitrogen sources, and vitamins. For example, E. coli has simple nutritional requirements than lactobacilli.
The nutritional and physical requirements of this group of bacteria are not known yet so we can not cultivate them in an artificial medium. Such bacteria are propagated only through animal inoculation. For example, Mycobacterium leprae can be cultured by infection suckling mice or nine-banded armadillos. Other obligate intracellular bacteria are rickettsia, chlamydias, and spirochetes.
References and further readings
- Madigan Michael T, Bender, Kelly S, Buckley, Daniel H, Sattley, W. Matthew, & Stahl, David A. (2018). Brock Biology of Microorganisms (15th Edition). Pearson.
- Pelczar, M. J., Chan, E. C. S., & Krieg, N. R. (2001). Microbiology: Concepts and applications. New York: McGraw-Hill.