Autotrophs are organisms that are capable of producing their own food by using various inorganic components like water, sunlight, air, and other chemical substances.
- Autotrophs are the source of all the organic compounds found on the planet that are utilized by organisms that cannot prepare their own food.
- The term autotroph is composed of two words; ‘auto’ meaning self and ‘troph’ meaning food, indicating that these organisms can prepare their own food.
- These are also called producers in ecology as these produce organic compounds from inorganic compounds, which then pass through different trophic levels in the food chain.
- Autotrophs are one of the most important living organisms on the planet, as all other living organisms depend on them for food and energy.
- Depending on the type of autotrophs, these either utilize solar energy as the primary source of energy or the energy obtained from chemical reactions.
- All autotrophs utilize a single inorganic source of carbon which is then converted into organic compounds in the presence of energy.
- Green plants are the most well-known group of autotrophs as these are practically responsible for all the organic components of the environment.
- Besides, there are other autotrophs like green algae, phytoplankton, and bacteria that also produce organic compounds in land and aquatic environments.
- Autotrophs are located in the first trophic level of most food chains which are then consumed by heterotrophs in order to maintain the flow of energy in the chain.
- The number of autotrophs in an ecological niche is important as it affects the population of all other groups of living organisms.
- An increase in the number of autotrophs in an area causes an increase in the number of heterotrophs, whereas the decrease in number results in starvation and a reduction in the number of other organisms as well.
- The term autotrophy is often used with autotrophs which indicates the feeding habit of these organisms.
Types of Autotrophs
Autotrophs can be grouped into two categories depending on the source of energy they utilize to produce their food.
- Photoautotrophs are autotrophs that utilize solar energy and carbon dioxide to prepare their food by the process of photosynthesis.
- The photoautotrophs have a photosynthetic reaction center consisting of chlorophyll. The chlorophyll pigment is responsible for the transduction of radiation in cells.
- Photoautotrophs can be further divided into two groups based on the number of reaction centers in the photosynthetic apparatus and the ability to utilize water; oxygenic photoautotrophs and anoxygenic photoautotrophs.
- Oxygenic photoautotrophs utilize water as a source of reducing power in order to generate oxygen. Anoxygenic photoautotrophs depend on environmental reducing power in order to assimilate carbon dioxide.
- Photoautotrophs had been considered to be solely responsible for life on Earth; however, the idea has been challenged by the discovery of chemolithoautotrophs.
- Photoautotrophs include green plants and algae that contain chlorophyll. The chlorophyll is involved in capturing energy from sunlight which is utilized to assemble carbon dioxide into glucose.
- Green plants are producers of the terrestrial and aquatic ecosystems, which are consumed by heterotrophs known as consumers.
- The energy assembled by green plants is transferred to other animals through the food chain, which forms the basis of the ecosystem.
- Besides green plants, there are numerous photosynthetic bacteria that contain other photosynthetic pigments like rhodopsin, carotenoids, etc.
- Photoautotrophs are also important in the carbon cycle as these utilize carbon dioxide released by heterotrophs during respiration.
- Chemoautotrophs are autotrophs that utilize energy obtained from a chemical reaction involving oxidation in order to prepare their food.
- These differ from photoautotrophs in that they do not depend on sunlight for energy.
- The source of carbon, in the case of chemoautotrophs, is an oxidized form of carbon like carbon dioxide.
- The most well-known group of chemoautotrophs includes the chemolithoautotrophic that are found in rocks and utilize inorganic sources like ferrous ion, hydrogen, and hydrogen sulfide.
- These autotrophs are found in extreme habitats like deep-sea vents, acidic environments, and deep tranches.
- All chemoautotrophs are microorganisms belonging to the Archea and Bacteria domains. These have been studied intensely over the years to discover their role in the evolution of living beings on the planet.
- Chemoautotrophs are also studied for their role in astrobiology because of their ability to survive in extreme conditions.
- The energy used by these microorganisms is obtained from chemical reactions occurring in the environment. The energy is then transformed into cellular energy by the microorganisms.
Examples of Autotrophs
The following are some of the examples of autotrophs;
a. Green plants
- Green plants are the most important example of autotrophs as they are responsible for almost all the biomass on the planet.
- Green plants are photoautotrophs that capture solar energy in order to reduce carbon dioxide into glucose.
- Green plants contain chlorophyll in their cells which are the photosynthetic center of these organisms. The chlorophyll captures rays of different wavelengths and uses the energy to move electrons to the water molecule.
- These plants are called producers, and these provide food to other plants as well as heterotrophs. These from the primary trophic level in the food chain.
- The food is prepared in the green parts of the plant, which is then transferred to other parts via plant tissues.
- Green plants are found in most ecosystems where these are the primary source of food and energy for all other living organisms.
b. Green Sulfur Bacteria
- Green sulfur bacteria are microorganisms that are strictly anaerobic, photoautotrophs that assemble carbon compounds by utilizing sulfur compounds as electron donors.
- The photosynthetic center of these bacteria is similar in structure and function to the photosystem of plants and cyanobacteria.
- The light-induced electron transfer in green sulfur bacteria is non-cyclic which results in the generation of NADPH.
- The green sulfur bacteria live in sulfur-rich environments with low-light intensities. In order to capture enough light, these have an extensive antenna system so as to capture all available photons.
- Unlike photosynthesis in green plants, photosynthesis in green sulfur bacteria doesn’t produce oxygen.
- Methanogens are a group of bacteria that produce methane and other organic compounds by utilizing the electrons found in hydrogen gas.
- These bacteria occur in underground areas like the bottom of the sea, deep-sea trenches, and vents where they produce large bubbles of methane gas.
- Most methanogens utilize acetate as the primary carbon source, resulting in the production of acetyl-CoA during autotrophic CO2 fixation.
- The process of production of methane by these bacteria is known as methanogenesis, and the process is driven by energy obtained via different chemical reactions.
- The biological production of methane is not as effective as the industrial processes; however, the methane produced by these organisms has a higher tolerance against impurities.
- Some of the examples of methanogenic bacteria include Methanococcales, Methanobacteriales, Methanosarcina, etc.
d. Nitrogen-fixing bacteria
- Nitrogen-fixing bacteria are a group of bacteria that are involved in the conversion of molecular nitrogen into the organic form. The organic form of nitrogen can then be taken up by plants as a nutrient.
- These bacteria are chemoautotrophic bacteria that utilize the energy obtained from the chemical reaction for the transfer of electrons from the donor to the receiver.
- The molecular nitrogen in the environment is converted to nitrate by these bacteria, which is then taken by plants for the production of amino acids.
- Nitrogen fixation is an important process in the nitrogen cycle as well as in plants as plants depend on the process for their need for nitrogen.
- Some of these bacteria occur in a symbiotic relationship with different plants, which enables the transformation of unusable inorganic nitrogen into the usable organic form.
- Some examples of nitrogen-fixing bacteria include Azotobacterium, Azospirillum, Rhizobium, etc.
Autotrophs in the Food Chain
- Autotrophs form the first trophic level of the ecological food chains. Autotrophs are termed producers as these produce the food and energy, which is then transferred to the organisms present in the upper trophic levels.
- Autotrophs are the primary source of energy in all food chains that provides energy to consumers and decomposers.
- About 10% of the energy produced by autotrophs is transferred to the next trophic level, whereas the rest is stored in the ecosystem.
- Autotrophs form the largest trophic level in the ecological pyramid in terms of biomass as well as energy.
- Autotrophs are directly consumed by primary consumers, resulting in the transfer of energy. The energy then slowly moves through the chain to finally reach the apex predators.
- B. Jagannathan, J.H. Golbeck. Photosynthesis: Microbial. Encyclopedia of Microbiology (Third Edition). Academic Press. 2009. Pages 325-341. https://doi.org/10.1016/B978-012373944-5.00352-7.
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