Autotrophic Bacteria

Autotrophic bacteria are those bacteria that can synthesize their own food. They perform several reactions involving light energy (photons) and chemicals in order to derive energy for their biological sustainability. In order to do so, they utilize inorganic compounds like carbon dioxide, water, hydrogen sulfide, etc. for converting them into organic compounds like carbohydrates, proteins, etc. which are known to be energy-providing molecules in biological systems. Thus, autotrophic bacteria are self-sustaining organisms much similar to plants in terms of nutritional dependency.

The Role of Bacteria in our Ecosystem

The natural world consists of both living and nonliving things. Living things require energy to maintain their physical bodies and to perform the work every day. This energy is obtained from the food they eat. This food is produced by the primary producers of nature. Green plants are known as the primary producers that provide food for all the other living organisms including human beings. So Plants are known as autotrophs that produce their own food and all other animals that depend on these plants directly or indirectly to obtain food are known as heterotrophs. There are also some bacteria present in nature that produce their own food. These microorganisms are known as autotrophic bacteria.

All the organisms like green plants and bacteria produce their food in organic compound forms such as carbohydrate-protein etc. These are essentially the organic compounds transformed by them from inorganic substances with the help of sunlight as the source of energy. The green plants have chlorophyll and chloroplast that trap the energy from sunlight. This process is known as photosynthesis. Autotrophic bacteria also use light to convert inorganic compounds into organic compounds. Some types of bacteria also generate food using the energy from the chemical compounds. So these two categories of bacteria are differentiated into photoautotrophic bacteria and chemoautotrophic bacteria respectively. Like green plants, these two types of bacteria also need a source of carbon ( C ). Based on the source of carbon these bacteria have also been differentiated by their unique properties that are taught in the higher classes.  These Bacterias can be free-living and can sustain their life in any extreme environment or can remain inside the body of multicellular organisms. The autotrophic bacteria are mostly free-living and are found in soil, water bodies or oceans etc. In the process of photosynthesis, oxygen is also produced which is also an essential substance for all other living creatures.

Types of Autotrophic Bacteria

As stated above, autotrophic bacteria are capable of producing their own food by using light (i.e. by photosynthesis) or chemical compounds. Hence, depending on the type of substrate used for producing food and energy there are two types of autotrophic bacteria. They are given below:

1. Photoautotrophic Bacteria: These bacteria produce food and energy by photosynthesis. They utilize sunlight in order to produce compounds that provide them with energy. 

2. Chemoautotrophic Bacteria: These are the bacteria that produce their food and energy by using chemical compounds. 

One common thing in both types of bacteria is the requirement of a carbon source essential for the synthesis of their own food. The different types of autotrophic bacteria examples are given below and are explained by their unique properties.

Photoautotrophic Bacteria

By definition, photoautotrophic bacteria make use of light energy for photosynthesis by which they convert it into chemical energy. This is also an example of photosynthetic bacteria as they are one and the same. They are very much similar to plants in this matter as they involve photosynthesis. These bacteria have the capability to carry out the photosynthetic process either in presence of oxygen or the absence of oxygen. 

The types and examples of different types of photoautotrophic bacteria are given below:

• Aerobic Photosynthetic Bacteria: This bacteria is an example of photosynthetic bacteria utilising oxygen to carry out photosynthesis. Cyanobacteria or blue-green algae is an example of photosynthetic bacteria that make use of oxygenic photosynthesis. In this type of photosynthesis, water molecules are used as electron donors and oxygen is produced in the reaction. In distinction to the plants, they may have photosynthetic pigments like Chlorophyll-a but are devoid of chloroplasts. The reaction that occurs is: 

6CO2 + 12H2O + light → C6H12O6 + 6CO2 + H2O

These bacteria are filamentous and colonial. They are capable of nitrogen fixation. The autotrophic bacteria examples included in this category are Nostoc, Anabaena, etc.

• Anaerobic Photosynthetic Bacteria: The most common example of photosynthetic bacteria is anaerobic photosynthetic bacteria. Instead of water they use hydrogen sulphide or thiosulphate as a reducing agent. Instead of chlorophylls, they have bacteriochlorophyll for carrying out photosynthesis. Examples include green sulphur bacteria, purple non-sulphur bacteria, purple sulphur bacteria, etc.  The reaction that undergoes is as follows:

CO2 + 2H2S + light → [CH2O] + 2S + H2O.

• Purple Sulphur Bacteria: Found in the hot sulphur springs and stagnant water these bacteria survive in anaerobic conditions. They use hydrogen sulphide or thiosulphates in photosynthesis. They contain bacteriochlorophyll ‘a’ and ‘b’ which is present in the plasma membrane. They are similar to PSII types of plants. Examples of purple sulphur bacteria include Chromatiaceae, Spiribacter curvatus, etc.

• Purple Non-sulphur Bacteria: Unlike the sulphur bacteria these bacteria use hydrogen as a reducing agent instead of hydrogen sulphide giving them their name. They are very important since they produce beneficial substances such as polyphosphates, vitamins, pigments, hydrogen, extracellular nucleic acids and growth-promoting substances. They also are capable of increasing plant yields by these substances and help in providing resistance to environmental stress and improving biomass quality. 

• Green Sulphur Bacteria: Found in the deep oceans and in extremely low light conditions, they thrive in anoxic environments and also near the thermal vents. In this case, the electron donor is sulphide, hydrogen or ferrous ion. They also have bacteriochlorophyll ‘c’, ‘d’ and ‘e’ along with bacteriochlorophyll ‘a’ in the plasma membrane and chlorosomes. They fix carbon by reverse tricarboxylic acid. They are similar to the PSI system of higher plants. 

Chemoautotrophic Bacteria

As per the definition given above, chemoautotrophic bacteria synthesize their food and energy by making use of chemical substances. This makes them capable of surviving in extreme environments.

Chemoautotrophic bacteria example is similar to the examples of chemosynthetic autotrophs since they both mean the same. They use chemosynthesis instead of photosynthesis for producing energy and they lack photosynthetic pigments. The chemical energy is produced from the oxidation of inorganic compounds for example - hydrogen sulfide, hydrogen, carbon monoxide, ammonia, methane, iron salts, nitrite, etc. which serve as the carbon sources in the reaction. The energy released from oxidation is saved in the form of ATP for the synthesis of organic compounds.

Examples of chemosynthetic bacteria include aerobic and anaerobic bacteria. Based upon the source of carbon they can be classified as sulphur bacteria, hydrogen bacteria, iron bacteria, nitrogen bacteria, methanotrophs, etc. Due to the utilization of chemical substances, they are important in the nutrient cycling of nitrogen, phosphorus, sulphur, iron, etc. The chemosynthetic bacteria examples are explained below:

• Sulphur Bacteria: These kinds of bacteria oxidise hydrogen sulphide or thiosulphates to molecular sulphur or sulphates. The chemosynthetic bacteria examples mentioned here and included are Beggiatoa. Thiothrix, Sulfolobus, Thiobacillus, etc. 

2H2S + O2 → 2H2O + 2S + energy

• Nitrogenous Bacteria: These bacteria convert ammonia to nitrite. This is followed by nitrite and then nitrate. The oxidation reaction leads to the release of energy. The converted nitrate is in turn utilized by the plants. Examples include Nitrbacter, Nitrobacter. The reaction that goes in this bacteria is:

NH₃ + O₂ → NO₂ + H₂O + energy (Nitrosomonas)

2NO₂ + O₂ → NO₃ + energy (Nitrobacter)

• Hydrogen Bacteria: These bacteria use hydrogen for carrying out their chemical reaction. The aerobic bacteria which are hydrogen-oxidizing bacteria use oxygen as an electron acceptor while the anaerobic bacteria utilize nitrogen dioxide or sulphate as the electron acceptor. The chemical equation is given below:

2H₂ + O₂ → 2H₂O + energy.

• Methanotrophs: For these bacteria, methane is the carbon source for producing food and driving energy. They can be either aerobic or anaerobic. The aerobic bacteria oxidize methane into formaldehyde which moving forward is used in diverse pathways to form organic molecules. Examples are Methylomonas, Methylococcus capsulatus, etc. They collect formaldehyde by the RuMP pathway and it is used to produce animal food. 

• Iron Bacteria: These are the bacteria that convert ferrous ions to ferric ions by oxidation. Living in iron-rich environments like hot lava beds and hydrothermal vents they are extremely chemosynthetic. The reaction that they undertake for energy production is:

4FeCO3 + O2 + 6H2O → 4Fe(OH)3 + 4CO2 + energy

Conclusion

This article gives brief information about autotrophic bacteria. The examples and the reactions underline their unique properties and specialities that help them survive extreme environments on earth.