Chemosynthesis

Chemosynthesis Definition

Chemosynthesis is the conversion of inorganic carbon-containing compounds into organic matter such as sugars and amino acids. Chemosynthesis uses energy from inorganic chemicals to perform this task.

The inorganic “energy source” is usually a molecule that has electrons to spare, such as hydrogen gas, hydrogen sulfide, ammonia, or ferrous iron. Like photosynthesis and cellular respiration, chemosynthesis uses an electron transport chain to synthesize ATP.

After having its electrons passed through the electron transport chain, the chemical fuel source emerges in a different form. Hydrogen sulfide gas, for example, is converted into solid elemental sulfur plus water.

The term “chemosynthesis” comes from the root words “chemo” for “chemical” and “synthesis” for “to make.” Its function is similar to that of photosynthesis, which also turns inorganic matter into organic matter – but uses the energy of sunlight, instead of chemical energy to do so.

Today chemosynthesis is used by microbes such as bacteria and archaea. Because chemosynthesis alone is less efficient than photosynthesis or cellular respiration, it cannot be used to power complex multicellular organisms.

A few multicellular organisms live in symbiotic relationships with chemosynthetic bacteria, making them a partial energy source. Giant tube worms, for example, host chemosynthetic bacteria which supply them with sugars and amino acids.

However, these tube worms are partially dependent on photosynthesis because they use oxygen (a product of photosynthetic organisms) to make their chemosynthesis more efficient.

Chemosynthesis Equation

There are many different ways to achieve chemosynthesis. The equation for chemosynthesis will look different depending on which chemical energy source is used. However, all equations for chemosynthesis typically include:

Reactants:

Products:

A commonly used example equation for chemosynthesis shows the transformation of carbon dioxide into sugar with the help of hydrogen sulfide gas:

12H₂S + 6CO₂ → C₆H₁₂O₆ (sugar molecule) + 6H₂O + 12S

This equation is sometimes reduced to its simplest possible ratio of ingredients. This shows the relative proportions of each ingredient necessary for the reaction, although it does not capture the full quantity of hydrogen sulfide and carbon dioxide necessary to create a single sugar molecule.

The reduced version looks like this:

2H₂S + CO₂ → CH₂O (sugar molecule) + H₂O + 2S

Function of Chemosynthesis

Chemosynthesis allows organisms to live without using the energy of sunlight or relying on other organisms for food.

Like chemosynthesis, it allows living things to make more of themselves. By turning inorganic molecules into organic molecules, the processes of chemosynthesis turn nonliving matter into living matter.

Today it is used by microbes living in the deep oceans, where no sunlight penetrates; but it is also used by some organisms living in sunny environments, such as iron bacteria and some soil bacteria.

Some scientists believe that chemosynthesis might be used by life forms in sunless extraterrestrial environments, such as in the oceans of Europa or underground environments on Mars.

It has been proposed that chemosynthesis might actually have been the first form of metabolism on Earth, with photosynthesis and cellular respiration evolving later as life forms became more complex. We may never know for sure if this is true, but some scientists believe it’s interesting to consider whether sunlight or chemical energy was the first fuel for life on Earth.

Types of Chemosynthetic Bacteria

Sulfur Bacteria

The example equation for chemosynthesis given above shows bacteria using a sulfur compound as an energy source.

The bacteria in that equation consumes hydrogen sulfide gas (12H₂S), and then produces solid, elemental sulfur as a waste product (12S).

Some bacteria that use chemosynthesis use elemental sulfur itself, or more complex sulfur compounds as fuel sources, instead of hydrogen sulfide.

Metal Ion Bacteria

The most well-known type of bacteria that use metal ions for chemosynthesis are iron bacteria.

Iron bacteria can actually pose a problem for water systems in iron-rich environments, because they consume dissolved metal ions in soil and water – and produce insoluble clumps of rust-like ferric iron, which can stain plumbing fixtures and even clog them up.

However, iron bacteria are not the only organisms that use metal ions as an energy source for chemosynthesis. Other types of bacteria use arsenic, manganese, or even uranium as sources of electrons for their electron transport chains!

Nitrogen Bacteria

Nitrogen bacteria are any bacteria that use nitrogen compounds in their metabolic process. While all of these bacteria use electrons from nitrogen compounds to create organic compounds, they can have very different effects on their ecosystem depending on what compounds they use.

Nitrogen bacteria can usually be divided into three classes:

1. Nitrifying bacteria:

Nitrifying bacteria grow in soils that contain ammonia. Ammonia is an inorganic nitrogen compound that is toxic to most plants and animals – but nitrifying bacteria can use it for food, and even turn it into a beneficial substance.

Nitrifying bacteria takes electrons from ammonia and converts the ammonia into nitrites, and ultimately nitrates. Nitrates are essential for many ecosystems because most plants need them to produce essential amino acids.

Nitrification is often a two-step process: one bacteria will convert ammonia into a nitrite, and then another bacteria species will convert that nitrite into a nitrate.

Nitrifying bacteria can turn otherwise hostile soils into fertile grounds for plants, and subsequently for animals.

2. Denitrifying bacteria:

Denitrifying bacteria use nitrate compounds as their source of energy. In the process, they break these compounds down into forms that plants and animals cannot use.

This means that denitrifying bacteria can be a very big problem for plants and animals – most plant species need nitrates in the soil in order to produce essential proteins for themselves, and for the animals that eat them.

Denitrifying bacteria compete for these compounds, and can deplete soil, resulting in limited ability for plants to grow.

3. Nitrogen fixing bacteria:

These bacteria are very beneficial to ecosystems, including human agriculture. They can turn nitrogen gas – which makes up most of our atmosphere – into nitrates that plants can use to make essential proteins.

Historically, fertility issues and even famine have happened when soil became depleted of nitrates due to natural processes or overuse of farmland.

Many cultures learned to keep soil fertile by rotating nitrogen-consuming crops with nitrogen-fixing crops.

The secret of nitrogen-fixing crops is that the plants themselves do not fix nitrogen: instead, they have symbiotic relationships with nitrogen-fixing bacteria. These bacteria often grow in colonies around the plants’ roots, releasing nitrates into the surrounding soil.

The image below shows the roots of a “nitrogen-fixing plant” – note the round nodules which are, in fact, colonies of nitrogen-fixing chemosynthetic bacteria:

Modern fertilizers are often made of artificial nitrates, like those compounds made by nitrogen fixing bacteria.

Methanobacteria

Methanobacteria are actually archaeabacteria – but scientists began studying them long before they fully understood the differences between archaeabacteria and “true bacteria.”

Both archaeabacteria and true bacteria are single-celled prokaryotes – which means they look pretty similar under the microscope. But modern methods of genetic and biochemical analysis have revealed that there are important chemical differences between the two, with archaeabacteria using many chemical compounds and possessing many genes not found in the bacteria kingdom.

One of the abilities found in archaeabacteria that is not found in “true bacteria” is the metabolic process that creates methane. Only archaeabacteria species can combine carbon dioxide and hydrogen to produce methane.

Methanobacteria live in a variety of environments – including inside your own body! Methanobacteria are found at the bottom of the ocean, in swamps and wetlands, in the stomachs of cows – and even inside human stomachs, where they break down some sugars we cannot digest in order to produce methane and energy.

Related Biology Terms

• Archaeabacteria – An ancient lineage of prokaryotes. Once thought to be a sub-type of bacteria, modern analysis has revealed that archaeabacteria are an entirely different lineage from modern bacteria.
• Bacteria – A modern kingdom of prokaryotes. Today, they are sometimes called “eubacteria” or “true bacteria” to differentiate them from archaeabacteria.
• Electron transport chain – A principle often used by cells to harvest energy from the environment. Electrons are passed through a series of proteins, which harvest their energy to produce life-giving molecules such as ATP.

Quiz

1. Which of the following is NOT true of chemosynthesis?
A. It is the process of using energy from chemicals to create organic compounds.
B. It cannot be completed without energy from sunlight.
C. It uses an electron transport chain to extract energy from electrons.
D. It requires both a starting carbon compound, and a source of chemical energy.

2. Which of the following is NOT true of the equation of chemosynthesis?
A. It requires a carbon-containing inorganic compound, such as carbon dioxide, on the reactant side.
B. It requires a source of chemical energy on the reactant side.
C. It ends with an organic molecule, such as a sugar, on the product side.
D. It ends with a transformed version of the chemical energy source on the product side.
E. None of the above.

3. Which of the following is NOT a type of chemosynthetic bacteria?
A. Iron bacteria
B. Methane-producing bacteria
C. Sulfur bacteria
D. Nitrogen-fixing bacteria
E. None of the above.