Chemosynthesis involves the conversion of carbon compounds as well as others molecules to organic substances. In this biochemical process, methane, or an inorganic compound like hydrogen sulfide or gas, is converted into a form that can serve as an energy source. However, the photosynthesis process’s energy source (the series of reactions in which water and carbon dioxide are transformed to oxygen and glucose) uses sunlight energy to drive the process.

The concept that microorganisms could exist on inorganic compounds was suggested by Sergei Nikolaevich Vinogradnsii (Winogradsky) in 1890, in response to studies conducted on bacteria that were found to be living on iron, nitrogen or sulfur. The theory was proved in 1977, when the deep submersible sea Alvin discovered tube worms as well as other living things around hydrothermal vents on the Galapagos Rift. Harvard Student Colleen Cavanaugh suggested and later confirmed that the tube worms survived due to their association with the chemosynthetic bacteria. Chemosynthesis’s discovery is believed to be the work of Cavanaugh.

Organisms that generate energy through the an oxidation process of electron donors are referred to as chemotrophs. When the molecules have organic components, they are referred to as Chemoorganotrophs. When the molecules are inorganic they are called chemolithotrophs. Conversely organisms that make use of solar energy are known as phototrophs.

Chemosynthetic Bacteria Definition

Chemosynthetic bacteria comprise an autotrophic group of bacteria that utilize chemical energy to create food. Similar to photographsynthetic bacteria, the chemosynthetic bacterium require carbon sources (e.g. carbon dioxide) and also an energy source to produce themselves food.

Most of the time the majority of these bacteria are aerobic, and therefore depend on oxygen to carry out this process effectively. However, certain species (e.g. Sulfuricurvum Kujiense) have been linked to anaerobic chemical synthesis.

Because of their ability to manufacture their own food using chemical energy, these organisms are able to survive in a variety of habitats/environments including harsh environments with extreme conditions as free-living organisms or in association with other organisms through symbiosis with other organisms.

In contrast to photosynthesis, which is found in the cyanobacteria and eukaryotic organisms Chemosynthetic reactions are mainly performed in prokaryotic microorganisms (particularly archaea and bacteria)

Chemosynthetic bacteria can be found in:

• Venenivibrio stagnispumantis
• Beggiatoa
• T. neapolitanus
• T. novellus
• ferrooxidans

Example of Chemosynthesis

In addition to bacterial as well as archaea, a few larger organisms depend on chemical synthesis. One example is the massive tube worm, which is located in huge numbers in the vicinity of deep vents of hydrothermal energy. Every worm is home to chemosynthetic bacteria inside an organ known as the trophosome. The bacteria convert sulfur from the worm’s surroundings to create the nutrition that animals need. By using hydrogen sulfide for the fuel source, the process for chemosynthesis involves:

12 H2S + 6 CO2 → C6H12O6 + 6 H2O + 12 S

This is similar to the process of producing carbohydrate through photosynthesis, but the photosynthesis process releases oxygen gases and chemosynthesis results in solid sulfur. The sulfur granules that are yellow are visible within the cytoplasm of bacteria that are involved in the reaction.

A different example of chemosynthesis was identified in 2013 when a bacterium was found in basalt, which is below the sediments of the ocean floor. These organisms were not associated with a hydrothermal vent. It has been suggested that the bacteria utilize hydrogen from the reduction of minerals present in seawater that bathe the rock. The bacteria can react with carbon dioxide and hydrogen to make methane.

Where Does Chemosynthesis Occur?

Chemosynthesis is observed in hydrothermal ventsand caves in isolation methane clathrates in whale falls as well as cold seeps. The process could allow life beneath on Mars as well as the moon of Jupiter Europa. in addition to other locations within the solar system. Chemosynthesis is possible in the presence of oxygen however it isn’t needed.

Chemoautotrophs and Chemoheterotrophs

Chemoautotrophs get its energy through chemical reactions, and create organic compounds using carbon dioxide. Chemosynthesis’s energy source could be elemental sulfur, hydrogen sulfur, molecular hydrogen manganese, ammonia, or iron. Examples of chemoautotrophs comprise methanogenic archaea and bacteria that live in deep vents in the sea. The word “chemosynthesis” was originally coined by Wilhelm Pfeffer in 1897 to describe energy production by oxidation of inorganic molecules by autotrophs (chemolithoautotrophy). Under the modern definition, chemosynthesis also describes energy production via chemoorganoautotrophy.

Chemoheterotrophs are unable to fix carbon and create organic compounds. Instead, they can use inorganic energy sources, such as sulfur (chemolithoheterotrophs) or organic energy sources, such as proteins, carbohydrates, and lipids (chemoorganoheterotrophs).

Types of Chemosynthetic Bacteria

Chemosynthesis, as mentioned earlier, allows diverse kinds of bacteria (chemosynthetic bacteria) to thrive without the need to rely on sunlight or other organisms to feed.

The energy utilized for the production of food products comes from a range of organic chemicals, and consequently various chemical reactions. Because of this, there are various kinds of chemosynthetic bacteria that differ based on the chemical compounds they make use of as an energy source.

Chemosynthetic bacteria are found in sunny locations and thus are exposed to the sun. However, they don’t depend on sunlight for an energy source.

Sulfur bacteria 

They (e.g. Paracoccus) are able to oxidize sulfur compounds such like hydrogen sulfur (sulfides) Thiosulfates, inorganic sulfur. Based on the species of organism, or the sulfur compound that is being utilized the process of oxidation is carried out in a variety of stages. In certain organisms, like inorganic sulfur can be stored until it is needed to be used. 

Nitrogen bacteria

Separated into three groups, which comprise nitrifying bacteria, denitrifying bacteria, and nitrogen fixing bacteria. For the nitrifying bacteria, ammonia gets first converted into hydroxylamine within the cells (by the enzyme ammonium monooxygenase).

The hydroxylamine is then converted to generate nitrite in the periplasm via hydroxylamine’s the oxidoreductase. This process creates one proton (one proton for every ammonium molecule). When compared with the nitrifying bacteria, denitrifying organisms are able to oxidize nitrate compounds and serve as an energy source.

Methanobacteria/methane bacteria

Some researchers have suggested that certain bacteria make use of methane as an source of energy for the process of chemosynthesis, it is frequent in archaebacteria that produce chemosynthetic toxins.

Hydrogen bacteria

The bacteria such like Hydrogenovibrio marineus and Helicobacter Pylori are known to oxidize hydrogen and use it as an energy source when under conditions of microaerophilia.

In the majority of cases the bacteria are anaerobic and thrive in areas that have low or no oxygen. This is due in large part to the fact that the enzyme utilized to oxidize substances (Hydrogenase) is effective under anaerobic conditions.

Iron bacteria

Acidithiobacillus ferrooxidans as well as Leptospirillum ferrooxidans are two of the bacteria that can oxidize iron. The process has been proven to take place under different conditions dependent on the species (e.g. low pH, the oxic-anoxic).

Chemosynthetic bacteria, which are not photosynthetic, must rely on the energy generated by the an oxidation process of these substances (inorganic) to produce foods (sugars) while nitrogen fixing bacteria turn nitrogen gas to Nitrate. These processes all create a proton that is used in the fixation of carbon dioxide.

The majority of these reactions take place within the cytoplasm, in there is membrane-bound respiration enzymes. In the instance of hydrogen oxidation NiFe hydrogenases in group 1 located in the cytoplasm activate the reaction and produce two proton and electron (hydrogen with an electric charge) from the hydrogen molecules (H2 2H+ 2H+, and 2e-). These electrons then get transported into the quinone pool of the transport chain for electrons.

In the case of hydrogen sulfide the compound undergoes an oxidation process to release hydrogen ions and electrons (referred by their protons, given that they are separated from compound and electrons, and acquire the charge of a positive). The result of this reaction are sulfur, electrons and as protons. Protons and electrons then enter in the chain that transports electrons (at the membrane).

When electrons enter the chain and protons are released out from the cell. Electrons, on the contrary on the other hand, are taken in by oxygen and draw the protons (hydrogen ions) and form water molecules. Through an enzyme called ATP synthase, protons which were previously expelled out of cells return to the cell. Their power (kinetic energy) stored as ATP and used to aid in sugar synthesizing.

Carbon Assimilation in Chemosynthetic Bacteria (fixation)

The type of bacteria they are, the habitat in which they live and carbon source There are many of metabolic pathways that are used to fixate. The most popular pathways are:

Calvin-Benson Cycle

In this cycle, the enzyme RuBisCo (ribulose 1, 5-bisphosphate carboxylase/oxygenase) facilitates the addition of molecular carbon dioxide to ribulose 1, 5-bisphosphate. This process generates a six-carbon compound that is, in turn, converted into two molecules of 3-PGA (3-phosphoglycerate). This process is also known as carbon fixation because it involves the conversion of carbon dioxide to organic molecules.

By storing energy by ATP as well as NADPH (generated by the process of oxidation) The carbon-based substance (3-PGA) is converted into a different carbon compound to create G3P (Glyceraldehyde 3-phosphate) during this reduction stage.

When one of these molecules departs from its Calvin chain (to make the sugar molecule or carbohydrate) and the other one is involved in the formation of RuBP.

Krebs Reverse Cycle

In contrast to the Calvin cycle carbon fixation in Krebs Reverse Cycle results in the creation of Pyruvate. Also called it is the Reductive Tricarboxylic Acid Cycle, this cycle begins with fixation of two carbon dioxide. This leads to the production of acetyl coenzyme (acetyl-CoA) which is later reductively carboxylated in order to create the pyruvate. The pyruvate created by the process is utilized to create organic cell material.

Some of the other processes used by these bacteria include:

• 3-Hydroxypropionate Bicycle – Also known as the 3-hydroxypropionate cycling, this pathway bonds carbon dioxide and forms Malyl-CoA when it is in combination with Propionyl CoA carboxylases as well as acetyl. Then, it is divided to produce acetyl CoA and Glyoxylate. In the end, the pathway is responsible for the production of pyruvate, which is utilized for the production of various organic substances that are required by cells.
• Reductive Acetyl-CoA Pathway – In this pathway two carbon dioxide are joined to create acetyl-CoA. Usually, hydrogen is the electron source in this process and Carbon Dioxide as the electron acceptor.
• Dicarboxylate/4-Hydroxybutyrate Cycle – This cycle is common among bacteria found in anaerobic and microaerobic habitats (e.g. Desulfurococcales). Like the 3-hydroxypropionate/4-hydroxybutyrate cycle, this cycle converts cetyl-CoA and two molecules of carbon into succinyl-coenzyme (CoA). The enzymes involved in this process include Pyruvate synthase, as well as phosphoenolpyr (PEP) carboxylase.

Importance of Chemosynthetic Bacteria

In essence, chemosynthesis refers the process by the chemosynthetic bacteria digest food by using chemical energy. So, in contrast to photosynthesis they aren’t dependent upon light for their production. This makes them the primary producers in a variety of environments that have oxidants such like sulfates and nitrates.

In deep-sea vent ecosystems for example lack of sunshine in these ecosystems means that photosynthesis can’t take place. Due to the capacity of certain bacteria to produce food by chemosynthesis, they play a crucial function as food producers in this eco-system.

The same behavior has been demonstrated that it can benefit different organisms by way of an symbiotic connection. In various situations, nitrogen-fixing bacteria have been found to establish symbiotic relationships which can benefit many different organisms (algae diatoms, algae, legumes as well as sponges). They are capable of converting nitrogen (abundant in nature) into usable forms.

These bacteria catalyze atmospheric carbon dioxide to create ammonia (using an enzyme called nitrogenase) which is later used by plants for the production of biochemicals containing nitrogen.

Another of the interactions that has attracted considerable attention is that among tubeworms (Riftia Pachyptila) and chemosynthetic bacteria found in hydrothermal vents. In this particular environment the temperature of the water is extremely high because of geothermal heat. Additionally, the worms reside in the ocean (environment that is devoid of sunlight).

Despite the harsh conditions of this particular environment (extremely high temperatures and a lack of sunlight) the presence of hydrogen sulfide permits bacteria to perform chemical synthesis.

With a highly vascularized plume of gills that the worm can to absorb dissolved carbon dioxide and oxygen and hydrogen sulfur (the hemoglobins of these organisms is capable of binding oxygen and sulfur compounds). They then move to special cells called the bacteriocytes, where the chemosynthetic bacteria live.

Utilizing oxygen and sulfide in the process, bacteria create energy (ATP) which is later used to transform carbon dioxide in sugars. These sugars are then utilized by the mollusks to provide food.

The symbiotic relationship has been identified as well with Bivalves of Solemyid and Lucinid, Achinoids, Ciliate protists, Marine sponges, Mussels.

Some of the traits that have been linked to the symbiont (chemosynthetic bacteria) include:

• Contain a Gram-negative envelope
• Shapes vary from small coccoid-like endosymbionts that measure around 0.25um in diameter up to massive (about 10um length) rod-shaped Chemotrophic bacteria
• Based on the species, they might be endosymbionts or simply attach to body surface of hosts