The process of chemosynthesis states the synthesis of organic compounds by bacteria or other living organism utilizing the energy derived from chemical reactions involving inorganic compounds, typically in the absence of light.
Earth's majority life form is based in a good chain which typically involves sunlight, as plant synthesis food through photosynthesis. In deep ocean, where no light is present leads to no plant life. So, instead they derive energy from chemical compounds rather than light energy because of its absence. The phenomenon of utilization of chemical energy instead of light energy by organisms are called chemosynthesis. The main regions where chemosynthetic organisms found are hydrothermal vents in the deep sea ocean.
Origin of chemosynthesis
The process chemosynthesis was first identified by group of brilliant scientists in 1977 while doing a research expedition near the Galapagos Islands across the coast of Ecuader found hot vents on the surface of ocean expel chemical substance of hot fluid. There is founding that surrounding these hydrothermal vents exist a community of several animal species living in complete absence of sunlight. These amazing populations have been found at varied dispersal centers and subduction zones across the globe.
What are Chemosynthetic bacteria?
Typically, chemosynthetic bacteria involves a community of autotrophic bacteria that utilize chemical energy to synthesis their own food. Like photosynthetic bacteria, chemosynthetic bacteria require a source of carbon such as carbon dioxide as well as energy in order to produce their food.
Most of the part, these chemosynthetic bacteria are aerobic and thus depend on oxygen to complete the chemosynthesis process. Although, some species such as Sulfuricurvum kujiense have seen to been in association with anaerobic chemosynthesis.
As they have the ability to synthesis food using chemical energy, make these bacteria survive in different habitats and environment with harsh and extreme conditions. They survive in these harsh environment by associating with other organism through symbiosis with them.
In contrast, photosynthesis which is common in eukaryotic organisms and cyanobacteria, the chemosynthesis process are mostly found in prokaryotic organisms typically bacteria and archae.
Examples of chemosynthetic bacteria include the following:
Types of Chemosynthetic Bacteria
As above said, chemosynthesis involve survival of several types of bacteria (chemosynthetic bacteria) without the use of light energy rather they use chemical energy to synthesis their food.
Here, the energy used to synthesis food products is derived from different organic chemical compounds involving variety of chemical reactions. For the same reason, there are various types of chemosynthetic bacteria present that use chemical compounds ad their carbon source of energy.
Some of the chemosynthetic bacteria also exist in hot sunny environment and thus exposed to extreme sunlight. However, they don't depend on sunlight for their source of energy.
Bacteria such as Paracoccus utilize sulphur compounds like hydrogen sulphide, hydrogen thiosulfate, and inorganic sulphur components. The oxidation process occurs in several stages depending on the organism and the type of sulphur compound being oxidized.
However, some organisms use inorganic sulphur and keep it stored until they are required to use it again.
Nitrogen bacteria are those bacteria whose source of energy is nitrogen or nitrogen like compounds. These bacteria are divided into three categories including nitrifying bacteria, denitrifying bacteria and nitrogen fixing bacteria.
In the case of nitrifying bacteria ammonia is the first compound to get oxidized to hydroxylamine in the cytoplasm by the enzyme ammonium monooxygenase.
Later, the hydroxylamine gets oxidized to form nitrite by the enzyme hydro hydroxylamine oxidoreductase in the periplasm.
The above process generates a proton (one proton for each Ammonia molecule). On the other hand, the denitrifying bacteria oxidize nitrate compounds as a source of energy to produce Ammonium ion. Then this Ammonia gets converted in to atmospheric nitrogen which can be used for plants, animals and bacteria.
Methane bacteria/ Methanobacterium
this is typically common among chemosynthetic archae bacteria. Some studies have suggested that some of the chemosynthetic bacteria in the archae bacteria domain use methane as their source of energy to perform chemosynthesis.
Bacteria are like Hydrogenovibrio marinus and Helicobacter pylori oxidize hydrogen as the source of energy under microaerophilic Condition.
Mostly these bacteria have shown to be anaerobic and therefore existing areas with little or no oxygen condition. This is largely due to The fact that enzyme hydrogenase used for the oxidation purpose functions effectively in anaerobic environment.
Those bacteria such as acidithiobacillus ferrooxidans and leptospirillum ferroxidans use iron as the source of energy to perform chemosynthesis effectively. This occurs effectively under various conditions depending on the type of organism. For example low pH and oxic- Anoxic condition.
During chemosynthesis, chemosynthetic bacteria with no photosynthetic capability, have to rely completely on the energy produced by the oxidation of inorganic compounds to produce food only sugar while the nitrogen fixing bacteria tend to convert atmospheric nitrogen gas into nitrate. These processes are used to produce a Proton which is utilized for the carbon dioxide fixation.
Typically, all these biochemical reactions take place in cytoplasm in the presence of membrane bound respiratory enzymes. For example, in the case of hydrogen oxidation, group 1 Nife hydrogen present in the cytoplasm catalyze the reaction to produce two electrons molecules and one Proton from a hydrogen molecule. These electrons are then transported to the quinone pool in the electron transport chain.
Whereas in the case of hydrogen sulphide, Hydrogen sulfide gets oxidized to produce electrons and hydrogen ions also known as protons. These are separated from the compound as electrons and protons. Later, these electrons and protons are transferred to two electron transport chains (ETC) in the membrane. as the electron enter the ETC, proton get pumped out of the cell on the other hand electrons are accepted by the oxygen and attracts the hydrogen ion protons does forming the water molecule through ATP synthase enzyme, protons that are formed out of the cell are channeled back to the cell with their kinetic energy being stored as ATP and for the used for synthesis of sugar (food).
Carbon fixation in chemosynthetic bacteria
There are metabolic pathways used for Carbon fixation depending on the type of bacteria, the source of carbon and their habitat.
Below enlisted some most common Pathways to fix carbon in chemosynthetic bacteria.
Calvin Benson cycle
In which cycle, the photosynthetic enzyme rubisco ribulose 1, 5 Bisphosphate carboxylase/ oxygenase. This leads to generation of a six carbon molecule which in fact is converted into two molecules of 3 phosphoglycerate 3-PGA. It is referred to as the carbon fixation process involving the conversion of carbon dioxide into organic molecules.
The carbon product 3-PGA is again converted to produce G3P Glyceraldehyde 3-Phosphate through the energy stored in ATP and NADPH which was generated via oxidation process.
One molecule undergoes in the process to generate RuBP and the other molecule leaves the Calvin cycle to form sugar (food).
Kreb Reverse Cycle
Unlike Calvin Cycle, This particular cycle produces pyruvate molecules as their end product. This cycle is also referred to as the Reductive Tricarboxylic Acid Cycle. The process initiates with the fixation of carbon dioxide (2 molecules). It results in the formation of acetyl coenzyme A that gets reduced to produce pyruvate. The product pyruvate is later used for the synthesis of organic molecules.
Some other process used for carbon fixation
the process also known as 3-Hydroxypropionate cycle which fixes carbon dioxide to produce Malyl-CoA in the presence of acetyl CoA and propionyl CoA carboxylase. Then, this Malyl CoA splits to form glyoxylate and acetyl CoA. typically, this pathway forms the pyruvate which is ultimately used for the synthesis of organic molecules as food material for the cell.
Reductive Acetyl CoA pathway
In this, acetyl CoA produced by the fixation of two molecules of carbon dioxide. Here the hydrogen molecule act as electron donor in the reaction where CO2 act as an electron acceptor.
Dicarboxylate /4-hydroxybutyrate cycle
It is found common in bacteria present in micro aerobic and anaerobic conditions. For example in Desulfurococcales.
This pathway converts acetylcoA and two molecules of carbon into succinyl CoA. Some enzymes like pyruvate synthase and phosphoenol pyruvate (PEP) carboxylase.
Significance of chemosynthetic bacteria
Chemosynthesis is a process where chemosynthetic bacteria synthesis food using chemical energy. Thus, unlike photosynthesis, these organism are independent of light energy for the
Production of organic molecules. This features makes them important primary producer of energy in various habitats like micro aerobic and anaerobic environments and also these habitats contain oxidants in the form of sulfates and nitrates.
In deep-sea vent ecosystems, for instance, the absence of sunlight means that photosynthesis cannot take place. Because of the ability of some bacteria to manufacture food through chemosynthesis, they play an important role as producers in this ecosystem.
This behavior has also been shown to benefit other organisms through a symbiotic relationship. For instance, in various environments, nitrogen-fixing bacteria have been shown to form symbiotic relationships that benefit a variety of organisms (diatoms, algae, legumes, sponges, etc.). Here, they are able to convert nitrogen (abundant in nature) into useable forms.
Here, these bacteria can catalyze atmospheric nitrogen to produce ammonia (using an enzyme known as nitrogenase) which is then used by plants for the synthesis of nitrogenous biomolecules.
One of the other symbiotic relationships that have received significant attention is between tubeworms (Riftia pachyptila) and chemosynthetic bacteria in hydrothermal vents. In this environment, water temperatures are extremely high due to geothermal heat. Moreover, these worms live at the seafloor (environment lacking light energy).
Despite the unfavorable conditions in this environment (extremely high temperatures and lack of light), the availability of hydrogen sulfide allows bacteria to carry out chemosynthesis.