Autotrophic bacteria synthesize their own food. They derive energy from light or chemical reactions. They utilize simple inorganic compounds like carbon dioxide, water, hydrogen sulfide, etc. and convert them into organic compounds like carbohydrates, proteins, etc. to supplement their energy requirements.
Also read: Shapes of Bacteria
What are the two types of Autotrophic Bacteria?
How autotrophic bacteria make food? Depending on the types of sources utilized, autotrophic bacteria are categorized into two types. The two different types of autotrophic bacteria are:
- Photoautotrophs – or photosynthetic. They derive energy from sunlight.
- Chemoautotrophs – or chemosynthetic. They use chemical energy to prepare their food.
Apart from energy requirements, both types of bacteria need a carbon source to synthesize their food, e.g. carbon dioxide and other compounds.
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1. Photoautotrophic Bacteria Types and Examples
Photoautotrophic bacteria trap light energy and convert it into chemical energy. They make their own food like plants. Photoautotrophic bacteria may perform oxygenic photosynthesis or anoxygenic photosynthesis.
Photosynthetic bacteria are used as biofertilizers, for bioremediation, waste water treatment and purification of polluted water.
Oxygenic Photosynthetic Bacteria
Cyanobacteria (blue-green algae) perform oxygenic photosynthesis. They use H2O as an electron donor and oxygen is produced in the reaction. They do not possess chloroplasts but photosynthetic pigments like chlorophyll-a are present in the cytosol.
6CO2 + 12H2O + light energy → C6H12O6 + 6O2 + 6H2O
Cyanobacteria are filamentous or colonial, they may also perform nitrogen fixation and have specialized cells for that known as heterocyst. E.g. Nostoc, Anabaena, etc.
Anoxygenic Photosynthetic Bacteria
Most of the photosynthetic bacteria are anoxygenic, i.e. they do not utilize water as an electron donor, instead, they use H2S, H2 or thiosulphate as reducing agent and hydrogen sources. They contain a photosynthetic pigment known as bacteriochlorophyll (BChl), which is like chlorophylls in plants.
Examples include green sulphur bacteria, purple sulphur bacteria, purple non-sulphur bacteria, phototrophic acidobacteria and heliobacteria, FAPs (filamentous anoxygenic phototrophs).
CO2 + 2H2A + light energy → [CH2O] + 2A + H2O ; where H2A can be any electron donor, e.g. H2S, H2 etc.
Purple sulphur bacteria (PSB)
They are found in hot sulphur springs and stagnant water. These bacteria thrive in anaerobic or oxygen poor environments. They belong to the order Chromatiales of proteobacteria. They utilize hydrogen sulfide or thiosulphates as a reducing agent and release sulphur. The main pigments are bacteriochlorophyll ‘a’ and ‘b’ located in the plasma membrane. Their photosystem is similar to PSII of higher plants.They are of two types:
- Chromatiaceae – contains intercellular sulphur granules
- Ectothiorhodospiraceae – contains extracellular sulphur granules
Purple non-sulphur bacteria (PNSB)
They mainly use hydrogen as a reducing agent. They belong to the order Rhodospirillales.
Purple photosynthetic bacteria are very important as they produce various beneficial substances such as polyphosphates, vitamins, pigments, hydrogen, extracellular nucleic acids and growth promoting substances for plants. They can increase the plants yield, resistance to environmental stress and improve biomass quality.
They are also helpful in bioremediation of heavy metals and reducing emission of greenhouse gases.
Green sulphur bacteria (GSB)
They are obligate anaerobe and generally non-motile. They belong to the Chlorobiaceae family. They are found deep in the ocean in extremely low light and anoxic environment and near thermal vents. The electron donor is sulphide, hydrogen or ferrous ion. Carbon fixation is done by reverse tricarboxylic acid or RTCA cycle. They contain bacteriochlorophyll ‘c’, ‘d’ and ‘e’ along with bacteriochlorophyll ‘a’. Pigments are present in the plasma membrane and chlorosomes. Their photosystem is similar to PSI of higher plants.
Chlorobium tepidum is a mixotroph, which derives carbon from both organic and inorganic compounds.
2. Chemoautotrophic Bacteria Types and Examples
Have you ever wondered how some bacteria survive in such extreme environments? They do so by relying on chemical compounds to get energy.
Chemoautotrophic bacteria perform chemosynthesis, which utilizes chemical energy. They lack photosynthetic pigments. Here carbon sources can be carbon dioxide, hydrogen sulphide, methane, etc. The chemical energy is produced from oxidation of inorganic compounds such as hydrogen, H2S, carbon monoxide, ammonia, methane, iron salts, nitrite, etc. The energy liberated from oxidation is trapped in ATP for the synthesis of organic compounds.
They can be aerobic or anaerobic. Depending on the source, where they derive energy from, they are categorised into various types such as sulphur bacteria, hydrogen bacteria, iron bacteria, nitrogen bacteria, methanotrophs, etc. They play an important role in nutrient recycling such as nitrogen, phosphorus, sulphur, iron, etc.
They oxidise, hydrogen sulphide or thiosulphates to molecular sulphur or sulphates. E.g. Beggiatoa, Thiobacillus, Thiothrix, Sulfolobus, etc.
2H2S + O2 → 2H2O + 2S + Energy
Nitrifying bacteria convert ammonia to nitrite and then to nitrate. In this oxidation process, energy is released. Nitrate is utilized by plants. E.g. Nitrosomonas, Nitrobacter
NH3 + O2 → NO2 + H2O + Energy (Nitrosomonas)
2NO2 + O2 → NO3 + Energy (Nitrobacter)
As the name implies, they oxidise molecular hydrogen. Aerobic hydrogen-oxidizing bacteria use O2 as an electron acceptor, whereas anaerobic hydrogen bacteria use nitrogen dioxide or sulphate as an electron acceptor. E.g. Helicobacter pylori, Hydrogenobacter thermophilus, Hydrogenovibrio marinus, etc.
2H2 + O2 → 2H2O + Energy
They use methane as a carbon source and to derive energy. They can be aerobic or anaerobic. Aerobic methanotrophs oxidize methane to formaldehyde, which is then utilized in various pathways to form organic compounds. Anaerobic methanotrophs utilize other compounds as electron acceptors. E.g. Methylomonas, Methylococcus capsulatus, etc. It assimilates, formaldehyde by the RuMP pathway. It is used to produce animal feed. Some methanotrophs assimilate formaldehyde by serine pathway.
They oxidise ferrous ions to ferric ions. They are present in iron-rich environments like hot lava bed, hydrothermal vents. E.g. Thiobacillus ferrooxidans, Geobacter metallireducens, Zetaproteobacteria, Gallionella, Ferrobacillus, etc.
4FeCO3 + O2 + 6H2O → 4Fe(OH)3 + 4CO2 + Energy