D) chemoautotrophs
Explanation:
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
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.
