CHEMOSYNTHESIS
Chemosynthesis is the synthesis of organic compounds by bacteria or other living organisms using energy derived from reactions involving inorganic chemicals, typically in the absence of sunlight.
Chemosynthesis
can be defined as the biological production of organic compounds from C-1
compounds and nutrients, using the energy generated by the oxidation of
inorganic (e.g., hydrogen gas, hydrogen sulfide, ammonium) or C-1 organic
(e.g., methane, methanol) molecules.
PROCESS
OF CHEMOSYNTHESIS
Chemosynthesis is the process by which food (glucose) is made by bacteria using chemicals as the energy source, rather than sunlight. Chemosynthesis occurs around hydrothermal vents and methane seeps in the deep sea where sunlight is absent. During chemosynthesis, bacteria living on the sea floor or within animals use energy stored in the chemical bonds of hydrogen sulfide and methane to make glucose from water and carbon dioxide (dissolved in sea water). Pure sulfur and sulfur compounds are produced as by-products.
Chemosynthesis is a biosynthesis performed by living organisms. It is through this process that a more complex chemical compound is produced. It often does so by combining simpler chemical entities or precursors. Examples of chemical synthesis, in particular, include organic synthesis and dehydration synthesis. Chemoautotrophs, for instance, are organisms that perform chemosynthesis. They include certain groups of bacteria such as sulfur-oxidizing gamma proteobacteria, epsilon proteobacteria, and neutrophilic iron-oxidizing bacteria, and certain archaea such as methanogenic archaea. Certain eukaryotes form symbiosis with bacteria that can fix carbon dioxide for them. For instance, the giant tube worms have bacteria in their trophosome that can produce sugars and amino acids from carbon dioxide with hydrogen sulfide as the energy source. This form of chemosynthesis results in the formation of carbohydrate as well as solid globules of sulfur.
Ecosystems depend upon the ability of some
organisms to convert inorganic compounds into food that other organisms can
then exploit (or eat!). The majority of life on the planet is based on a food
chain which revolves around sunlight, as plants make food via photosynthesis.
However, in environments where there is no sunlight and thus no plants,
organisms instead rely on primary production through a process called chemosynthesis,
which runs on chemical energy. Together, photosynthesis and chemosynthesis fuel
all life on Earth.
Photosynthesis occurs in plants and some bacteria, wherever there is
sufficient sunlight – on land, in shallow water, even inside and below clear
ice. All photosynthetic organisms use solar energy to turn carbon dioxide and
water into sugar (food) and oxygen: 6CO2 + 6H2O -> C6H12O6 + 6O2.
Chemosynthesis occurs in bacteria and other organisms and involves the use of energy released by inorganic chemical reactions to produce food. All chemosynthetic organisms use energy released by chemical reactions to make a sugar, but different species use different pathways. For example, at hydrothermal vents, vent bacteria oxidize hydrogen sulfide, add carbon dioxide and oxygen, and produce sugar, sulfur, and water: CO2 + 4H2S + O2 -> CH20 + 4S + 3H2O. Other bacteria make organic matter by reducing sulfide or oxidizing methane.
EXAMPLES OF CHEMOSYNTHETIC BACTERIA
Chemosynthetic
bacteria include a group of autotrophic bacteria that use chemical energy to
produce their own food. Like photosynthetic bacteria, chemosynthetic bacteria
need a carbon source (e.g. carbon dioxide) as well as an energy source in order
to manufacture their own food.
For
the most part, these bacteria are aerobic and therefore rely on oxygen to
complete this process successfully. However, some species (e.g. Sulfuricurvum
kujiense) have been associated with anaerobic chemosynthesis.
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.
- Venenivibrio stagnispumantis
- Beggiatoa
- T. neapolitanus
- T. novellus
- ferrooxidans
TYPES OF CHEMOSYNTHETIC BACTERIA
Chemosynthesis
allows different types of bacteria (chemosynthetic bacteria) to survive without
relying on light energy or other organisms for food.
Here,
the energy used to manufacture food materials is derived from a variety of inorganic
chemicals and thus different chemical reactions. For this reason, there are
different types of chemosynthetic bacteria based on the type of compounds they
use as an energy source.
Some
chemosynthetic bacteria live in sunny environments and are therefore exposed to
sunlight. However, they do not rely on sunlight as a source of energy
Sulfur bacteria - These bacteria
(e.g. Paracoccus) oxidize such sulfur compounds as hydrogen sulfide (sulfides)
thiosulfates and inorganic sulfur etc. Depending on the organism, or the type
of sulfur compound being used, the oxidation process takes place in several
stages.
In some of the organisms, for instance, inorganic sulfur will be stored until they are required for use.
Nitrogen
bacteria -
Divided into three groups that include nitrifying bacteria, denitrifying
bacteria, and nitrogen-fixing bacteria. In the case of
nitrifying bacteria, ammonia is first oxidized to hydroxylamine in the cytoplasm (by ammonium
monooxygenase).
The hydroxylamine is then oxidized to
produce nitrite in the periplasm by hydroxylamine oxidoreductase. This process
produces a proton (one proton for each molecule of ammonium). As compared to
nitrifying bacteria, denitrifying bacteria oxidize nitrate compounds as a
source of energy.
Methanobacteria/methane bacteria - Although some
scientists have suggested that some bacteria use methane as a source of energy
for chemosynthesis, this is particularly common among chemosynthetic
archaebacteria.
Hydrogen
bacteria -
Such bacteria as Hydrogenovibrio marinus and Helicobacter pylori oxidize
hydrogen as a source of energy under microaerophilic conditions.
For
the most part, these bacteria have been shown to be anaerobic and therefore
thrive in areas with very little to no oxygen. This is largely due to the fact
that the enzyme used for oxidation purposes (Hydrogenase) functions effectively
in anaerobic conditions.
Iron bacteria - Acidithiobacillus ferrooxidans and
Leptospirillum ferrooxidans are some of the bacteria that oxidize iron. This
process has been shown to occur under different conditioned depending on the
organism (e.g. low pH and oxic-anoxic).
During chemosynthesis, chemosynthetic
bacteria, being non-photosynthetic, have to rely on energy produced by
oxidation of these compounds (inorganic) in order to manufacture food (sugars)
while nitrogen-fixing bacteria convert nitrogen gas into nitrate. All these
processes serve to produce a proton used in carbon dioxide fixation.
IMPORTANCE OF CHEMOSYNTHESIS
Essentially,
chemosynthesis refers to the process through which chemosynthetic bacteria
process food using chemical energy. Therefore, compared to photosynthesis,
these organisms are not dependent on light energy for production. This makes
them important primary producers in various habitats that contain such oxidants
as nitrates and sulfates.
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 (algae, diatoms, 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.
Using
a highly vascularized gill-like plume, the worm is able to take in dissolved
carbon dioxide, oxygen, and hydrogen sulfide (the hemoglobin of these organisms
are capable of binding oxygen and sulfides). They are then transported to
specialized cells known as bacteriocytes where chemosynthetic bacteria
reside.
Using the sulfide and oxygen, the bacteria produce energy (ATP) that is then used to convert carbon dioxide into sugars. These sugars are then used by the mollusk as a source of food.
Such symbiotic relationships have also been
identified with:
- olemyid and lucinid bivalves
- Achinoids
- Ciliate protists
- Marine sponges
- Mussels