The Diversity of Microbial Metabolism
The diverse environments on Earth today present energy, and carbon and other nutrients in varying forms. They also vary with respect to temperature, acidity, and the availability of byproducts from other organisms. Microbes thrive in a vast array of these environments.
Microorganisms vary with regard to the sources of energy they use for assembling macromolecules and other cellular components from smaller molecules. Phototrophs obtain their energy from light; chemotrophs use chemicals as energy sources. (Troph is derived from a Greek word meaning "to feed.") Many organisms use organic compounds as sources of energy; these are the chemoorganotrophs. In contrast, the chemolithoautotrophs use inorganic chemicals as energy sources.
Microorganisms also vary with respect to the source of carbon they use. Autotrophs are able to build organic molecules from carbon dioxide. Heterotrophs, the "other feeders," obtain their carbon from organic compounds - amino acids, fatty acids, sugars, and so on - of autotrophs.
These terms are often combined. So, a "photoautotroph" is an organism that, like plants, gets its energy from light and its carbon from CO2. Decomposers are often chemoheterotrophs; they may obtain energy and carbon from the same source.
So what metabolic classes might microbes found in a deep-sea hydrothermic vent fall within? The lack of sunlight makes them dependent on chemical energy; thus, they are chemotrophs. Carbon dioxide dissolved in the ocean is their source of carbon; they are autotrophs. Organic material from decomposing phototrophs is not abundant, so these organisms rely on inorganic sources for energy. They may use H2 (present in magmatic gases), reduced sulfur compounds or methane as a source of energy. They are also thermophiles, growing optimally at temperatures above 45° C. Thermophilic chemolithoautotrophs serve as primary producers, the first organisms in food chains that include animals such as tube worms and giant clams.
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