CHEMOSYNTHESIS: PHASES, ORGANISMS, DIFFERENCES WITH PHOTOSYNTHESIS - BIOLOGY - 2025

The chemosynthesis is a characteristic of certain biological process autotrophic organisms exploiting chemical energy to convert inorganic substances in organic matter. It differs from photosynthesis in that the latter uses energy from sunlight.

The organisms capable of chemosynthesis are generally prokaryotes such as bacteria and other microorganisms such as archaea, which extract energy from reactions that involve the oxidation of very small compounds.

Photograph of Riftia pachyptila, a chemosynthetic organism (Source: NOAA Okeanos Explorer Program, Galapagos Rift Expedition 2011 via Wikimedia Commons)

The most common examples of chemosynthetic bacteria are nitrifying bacteria, which oxidize ammonia to produce nitrogen dioxide, as well as sulfur bacteria, capable of oxidizing sulfuric acid, sulfur, and other sulfur compounds.

Origin of the concept

The microbiologist Sergei Winogradsky, in 1890, was the first scientist to speak about the possible existence of chemosynthetic processes, since he assumed that there must be a process similar to photosynthesis that uses an energy source other than sunlight.

However, the term "chemosynthesis" was coined in 1897 by Pfeffer. Winogradsky's theories were proven in 1977 during the expedition carried out by the submarine "Alvin" into deep ocean waters, around the Galapagos Islands.

During this expedition, the scientists aboard the submarine discovered some bacterial ecosystems that subsisted in the presence of inorganic matter and others in symbiosis with some invertebrate marine animals.

At present, various chemosynthetic ecosystems are known around the world, especially associated with marine and oceanic environments and, to a lesser extent, with terrestrial ecosystems. In these environments, chemosynthetic microorganisms represent important primary producers of organic matter.

Phases

Chemosynthesis almost always occurs at the interface of aerobic and anaerobic environments, where the end products of anaerobic decomposition and large amounts of oxygen are concentrated.

Like photosynthesis, chemosynthesis has well-defined phases: an oxidative and a biosynthetic. The first uses inorganic compounds and during the second organic matter is produced.

Oxidative phase

During this first phase and depending on the type of organism considered, different types of reduced inorganic compounds are oxidized, such as ammonia, sulfur and its derivatives, iron, some derivatives of nitrogen, hydrogen, etc.

In this phase, the oxidation of these compounds releases the energy that is used for the phosphorylation of ADP, forming ATP, one of the main energy currencies of living beings and, in addition, reducing power is generated in the form of NADH molecules.

A particularity of the chemosynthetic process has to do with which part of the ATP that is generated is used to propel the reverse transport of the electron chain, in order to obtain a greater quantity of reducing agents in the form of NADH.

In summary, this stage consists of the formation of ATP from the oxidation of the appropriate electron donors, whose biologically useful energy is used in the biosynthesis phase.

Biosynthesis phase

The biosynthesis of organic matter (carbon compounds) occurs thanks to the use of the energy contained in the high-energy bonds of ATP and the reducing power stored in the NADH molecules.

This second phase of chemosynthesis is "homologous" to that which occurs during photosynthesis, since the fixation of carbon atoms in organic molecules occurs.

In it, carbon dioxide (CO2) is fixed in the form of organic carbons, while ATP is converted into ADP and inorganic phosphate.

Chemosynthetic organisms

There are various types of chemosynthetic microorganisms, some being optional and others obligatory. This means that some depend exclusively on chemosynthesis to obtain energy and organic matter, and others do so if the environment conditions them.

Chemosynthetic microorganisms are not very different from other microorganisms, since they also obtain energy from electron transport processes where molecules such as flavins, quinones and cytochromes are involved.

From this energy, they are able to synthesize cellular components from sugars that are synthesized internally thanks to the reductive assimilation of carbon dioxide.

Some authors consider that chemosynthetic organisms can be divided into chemo-organoautotrophs and chemo-lithoautotrophs, according to the type of compound from which they extract energy, which can be organic or inorganic, respectively.

As far as prokaryotes are concerned, most of the chemosynthetic organisms are gram-negative bacteria, usually of the genus Pseudomonas and other related ones. Among these are the:

- Nitrifying bacteria.

- Bacteria capable of oxidizing sulfur and sulfur compounds (Sulfur Bacteria).

- Bacteria capable of oxidizing hydrogen (Hydrogen Bacteria).

- Bacteria capable of oxidizing iron (Iron Bacteria).

Chemosynthetic microorganisms use a type of energy that would be lost in the biosphere system. These constitute a large part of the biodiversity and population density of many ecosystems where the introduction of organic matter is very limited.

Their classification has to do with the compounds that they are capable of using as electron donors.

Nitrifying bacteria

They were discovered in 1890 by Winogradsky and some of the genera described so far form aggregates that are surrounded by the same membrane. They are commonly isolated from terrestrial environments.

Nitrification involves the oxidation of ammonium (NH4) to nitrites (NO2-) and of nitrites (NO2-) to nitrates (NO3-). The two groups of bacteria that participate in this process often coexist in the same habitat to take advantage of both types of compounds using CO2 as a carbon source.

Bacteria capable of oxidizing sulfur and sulfur compounds

These are bacteria capable of oxidizing inorganic sulfur compounds and depositing sulfur within the cell in specific compartments. Within this group are classified some filamentous and non-filamentous bacteria of different genera of facultative and obligate bacteria.

These organisms are capable of using sulfur compounds that are highly toxic to most organisms.

The compound most commonly used by this type of bacteria is H2S gas (sulfuric acid). However, they can also use elemental sulfur, thiosulfates, polythionates, metal sulfides, and other molecules as electron donors.

Some of these bacteria require acidic pH to grow, which is why they are known as acidophilic bacteria, while others can do so at neutral pH, closer to "normal".

Many of these bacteria can form "beds" or biofilms in different types of environments, but especially in mining industry drains, in sulphurous hot springs and in oceanic sediments.

They are usually called colorless bacteria, as they differ from other green and purple bacteria that are photoautotrophic in that they do not have pigments of any kind, and they do not need sunlight.

Bacteria capable of oxidizing hydrogen

In this group are found bacteria capable of growing in mineral media with atmospheres rich in hydrogen and oxygen and whose only carbon source is carbon dioxide.

Both gram negative and gram positive bacteria are found here, capable of growing in heterotrophic conditions and which can have different types of metabolisms.

Hydrogen accumulates from the anaerobic breakdown of organic molecules, which is achieved by different fermentative bacteria. This element is an important source of bacteria and chemosynthetic archaea.

Microorganisms capable of using it as an electron donor do so thanks to the presence of a hydrogenase enzyme associated with their membranes, as well as the presence of oxygen as an electronic acceptor.

Bacteria capable of oxidizing iron and manganese

This group of bacteria is capable of using the energy generated from the oxidation of manganese or iron in its ferrous state to its ferric state. It also includes bacteria capable of growing in the presence of thiosulfates as inorganic hydrogen donors.

From an ecological point of view, iron and magnesium oxidizing bacteria are important for detoxification of the environment, since they reduce the concentration of dissolved toxic metals.

Symbiotic organisms

In addition to free-living bacteria, there are some invertebrate animals that live in inhospitable environments and that associate with certain types of chemosynthetic bacteria to survive.

The discovery of the first symbionts occurred after the study of a giant tube worm, Riftia pachyptila, lacking a digestive tube and obtaining vital energy from the reactions carried out by the bacteria with which it is associated.

Differences with photosynthesis

The most distinctive characteristic of chemosynthetic organisms is that they combine the ability to use inorganic compounds to gain energy and reducing power, as well as to effectively bind carbon dioxide molecules. Something that can happen in the total absence of sunlight.

Photosynthesis is carried out by plants, algae and by some classes of bacteria and protozoa. It uses energy from sunlight to drive the transformation of carbon dioxide and water (photolysis) into oxygen and carbohydrates, through the production of ATP and NADH.

Chemosynthesis, on the contrary, exploits the chemical energy released from oxidation-reduction reactions to fix carbon dioxide molecules and produce sugars and water thanks to obtaining energy in the form of ATP and reducing power.

In chemosynthesis, unlike photosynthesis, no pigments are involved and oxygen is not produced as a by-product.

References

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