Chemoautotroph

Biochemistry, Cell Biology, Ecology

Chemoautotroph Definition

Chemoautotrophs are cells that create their own energy and biological materials from inorganic chemicals.

In nature, “autotrophs” are organisms that don’t need to eat because they make their own biological materials and energy. This term comes from the Greek “auto” for “self” and “troph” for “to eat” or “to feed.”

Autotrophs form the basis for all food chains: they are the organisms which create sugars, proteins, lipids, and other materials for life. All other organisms survive by eating autotrophs, or other organisms that are fed by the autotroph food chain.

The two major types of autotrophs are chemoautotrophs and photoautotrophs. Photoautotrophs use energy from sunlight to make their biological materials. These include green plants and photosynthesizing algae.

This flow chart may be useful in determining whether an organism is an autotroph, and if so, which kind it is:

Troph flow chart

Chemoautotrophs, on the other hand, derive energy for their life functions from inorganic chemicals. They feed on chemicals that are good electron donors, such as hydrogen sulfide, sulfur, or iron.

Like all autotrophs, chemoautotrophs are able to “fix” carbon. They take atoms of carbon from inorganic compounds, such as carbon dioxide, and using it to make organic compounds such as sugars, proteins, and lipids.

Chemoautotrophs are commonly found in environments where plants cannot survive, such as at the bottom of the ocean, or in acidic hot springs.

Some types of chemoautotrophs also play critical roles in plant-based ecosystems. Although plants perform carbon fixation in these ecosystems, many plants rely on chemoautotroph bacteria to fix nitrogen, which is necessary to make amino acids and proteins.

Function of Chemoautotroph

Basis of Ecosystems Without Sunlight

Chemoautotrophs form the basis of the energy pyramid for ecosystems where photosynthesizers can’t survive. Without chemoautotrophs, life would only be able to exist where energy could be derived from sunlight.

They are the basis of some deep sea ecosystems, such as those existing around deep sea hydrothermal vents.

Scientists have speculated that chemoautotrophs could form the basis of life on planets which receive less sunlight than earth.

Nitrogen Fixation

One type of chemoautotroph, Nitrosomonas, plays the crucially important role of fixing nitrogen in the soil of some ecosystems. Like most chemoautotrophs, Nitrosomonas can take toxic chemicals – in this case ammonia – and turn it into materials for life.

Nitrosomonas harvests nitrogen from ammonia, and fixes it into organic compounds which can then be used to produce amino acids, proteins, and other crucial materials for life.

Possible Origin of Life

We have no fossils of the first cells on Earth, so we are not able to say at this time what they were like. We know that they had to be autotrophs, since they would have had to produce all of their own organic materials.

Some scientists think that the first cells were likely photoautotrophs, deriving energy form sunlight – but other scientists think the first cells may have been chemoautotrophs, and that photosynthesis may have evolved later.

If true, this would make all of us descendants of chemoautotrophs!

Examples of Chemoautotrophs

Example #1: Nitrosomonas

Nitrosomonas is a genus of nitrogen-fixing bacteria.

As you may have guessed, “nitrogen fixation” means taking nitrogen from inorganic compounds, such as ammonia, and assembling it into organic compounds, such as amino acids.

Nitrogen fixation is crucial for many ecosystems, even some which rely primarily on plants. Many plants cannot fix their own nitrogen – meaning that they require nitrogen-fixing bacteria in the soil, or they cannot obtain the nitrogen compounds they need to live.

Nitrogen fixation is an extremely important concept in agriculture, where many crops cannot perform nitrogen fixation themselves. To ensure that the soil contains enough organic nitrogen compounds to grow, farmers much either make sure that there are enough nitrogen-fixing bacteria present to sustain their crops, or add artificial nitrogen compounds in the form of fertilizers.

Of note, some nitrogen-fixing bacteria have developed permanent symbiotic relationships with certain plant species. These include the symbiotic bacteria found in nodules on the roots of “nitrogen-fixing” plants such as legumes. However, these nitrogen-fixing bacteria are not chemoautotrophs, as they have evolved to rely upon their plant hosts for food. They are no longer autotrophs, as they are no longer able to feed themselves.

Other types of nitrogen-fixing bacteria remain independent, and are still chemoautotrophs.

Example #2: Iron Bacteria

Iron bacteria are a type of bacteria that obtain energy by oxidizing ferrous iron which is dissolved in water.

Because they obtain their energy from iron, they can live in water with iron concentrations that would kill most organisms. Iron bacteria can be found in iron-rich wells, rivers, and hot springs.

They are sometimes thought of as a pest because the oxidized iron they produce can stain sinks, toilets, clothes, and other materials if it gets into a water supply. This is particularly common with well water, which does not pass through the same filtration process that municipal tap water goes through.

However, iron bacteria have also been an ally to some industries. In the industry of iron mining, research is being done into how to use these bacteria to capture and purify iron that might not otherwise be accessible to humans because it is dissolved in water or mixed with other minerals.

Example #3: Methanogens

Methanogens are bacteria that produce methane. They are chemoautotrophs, which energy from the electrons found in hydrogen gas to produce methane and other organic compounds.

Methanogens can be found at the bottom of the ocean, where they can create huge methane bubbles beneath the ocean floor. They can also be found in swamps and marshes, where they are responsible for producing methane “swamp gas.”

Some methanogens live in the guts of ruminants such as cows, and to a lesser extent in the guts of humans. That means there are chemoautotrophs living right in your own body!

Methane is an extremely powerful greenhouse gas, with the power to trap much more heat from the sun then carbon dioxide. Because small amounts of methane can be such a powerful greenhouse gas, many environmentalists are concerned about the beef industry, which raises millions of cattle whose guts play host to many trillions of methane-producing bacteria.

It is thought that reducing cattle raising and beef consumption would be one powerful way to fight man-made climate change caused by greenhouse gases.

Related Biology Terms

  • Autotroph – Any organism which makes derives energy from inorganic sources, and uses it to create organic molecules.
  • Carbon fixation – A process by which carbon from inorganic molecules, such as carbon dioxide, is assembled into organic molecules such as sugars, proteins, and lipids.
  • Photoautotroph – An organism that uses energy from sunlight to fuel its cellular activities and create organic molecules.

Quiz

1. Which of the following is NOT true of a chemoautotroph?
A. They obtain their own energy from inorganic chemicals.
B. They make their own biological materials out of inorganic chemicals.
C. They form the basis of ecosystems without sunlight.
D. None of the above.

Answer to Question #1

2. Which of the following is not a type of chemoautotroph?
A. Iron bacteria
B. Nitrosomonas
C. Green algae
D. Methanogens

Answer to Question #2

3. Which of the following is NOT a concern humans have about chemoautotrophs?
A. Methanogens produce methane, which is a powerful contributor to global warming.
B. Chemoautotrophs can pose harmful competition to human crops and livestock.
C. Iron bacteria produce iron deposits that can stain sinks, clothing, and hair.
D. Nitrogen-fixing bacteria are needed to allow some crops to grow.

Answer to Question #3
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