5.10A: The Energetics of Chemolithotrophy
- Outline the characteristics associated with chemolithotrophs
A lithotroph is an organism that uses an inorganic substrate (usually of mineral origin) to obtain reducing equivalents for use in biosynthesis (e.g., carbon dioxide fixation) or energy conservation via aerobic or anaerobic respiration. Known chemolithotrophs are exclusively microbes; no known macrofauna possesses the ability to utilize inorganic compounds as energy sources. Macrofauna and lithotrophs can form symbiotic relationships, in which case the lithotrophs are called “prokaryotic symbionts”. An example of this is chemolithotrophic bacteria in deep sea worms or plastids, which are organelles within plant cells that may have evolved from photolithotrophic cyanobacteria-like organisms.
Chemotrophs are organisms that obtain energy through the oxidation of electron donors in their environments. These molecules can be organic (chemoorganotrophs) or inorganic (chemolithotrophs). The chemotroph designation is in contrast to phototrophs, which utilize solar energy. Chemotrophs can be either autotrophic or heterotrophic. Chemoautotrophs generally fall into several groups: methanogens, halophiles, sulfur oxidizers and reducers, nitrifiers, anammox bacteria, and thermoacidophiles. Chemolithotrophic growth could be dramatically fast, such as Thiomicrospira crunogena with a doubling time around one hour.
In chemolithotrophs, the compounds – the electron donors – are oxidized in the cell, and the electrons are channeled into respiratory chains, ultimately producing ATP. The electron acceptor can be oxygen (in aerobic bacteria), but a variety of other electron acceptors, organic and inorganic, are also used by various species. Unlike water, the hydrogen compounds used in chemosynthesis are high in energy. Other lithotrophs are able to directly utilize inorganic substances, e.g., iron, hydrogen sulfide, elemental sulfur, or thiosulfate, for some or all of their energy needs.
Key Points
- Chemotrophs are organisms that obtain energy by the oxidation of electron donors in their environments. These molecules can be organic (chemoorganotrophs) or inorganic ( chemolithotrophs ).
- In chemolithotrophs, the compounds – the electron donors – are oxidized in the cell, and the electrons are channeled into respiratory chains, ultimately producing ATP.
- The electron acceptor can be oxygen (in aerobic bacteria ), but a variety of other electron acceptors, organic and inorganic, are also used by various species.
Key Terms
- chemolithotroph : chemoautotroph
- symbiont : An organism that lives in a symbiotic relationship; a symbiote.
- chemotroph : an organism that obtains energy by the oxidation of electron-donating molecules in the environment
- lithotroph : An organism that obtains its energy from inorganic compounds (such as ammonia) via electron transfer.