Chemosynthetic bacteria are one type of autotrophic organism, a life form that derives its nutrition from nonfood sources. The other type of autotroph is the photosynthetic organism, which includes most plants and some kinds of bacteria. Photosynthetic organisms create nutrients using light. Chemosynthetic organisms use chemical reactions to convert inorganic substances into nutrients. They are described by the type of inorganic molecule that they use as an input for their reactions.
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Bacteria that Use Metal Ions
Chemosynthesis generally works by oxidising an inorganic substance. Oxidising means that the reaction takes electrons away from the oxidised molecule. The electrons go to another molecule to facilitate its transformation into another compound---in the case of chemosynthetic bacteria, into organic molecules. Strains of bacteria use iron, arsenic, manganese and uranium as their sources of electrons, and they are identified by the metal that they use.
These bacteria inhabit wetlands, areas high in sewage and intestinal tracts. They combine carbon dioxide and hydrogen, which frees the oxygen that they need to live and produces methane as a byproduct.
Sulphur bacteria are found deep in the sea. The first colonies were discovered near the Galapagos Islands around deep-sea hydrothermal vents. Hydrogen sulphide pours from the volcanic vents, creating an environment that scientists thought was inhospitable to life. The sulphur bacteria that live there turn the hydrogen sulphide into elemental sulphur, while at the same time producing organic molecules that they use as nutrients. Sulphur bacteria are also found in cold seeps in the Gulf of Mexico, where hydrogen sulphide seeps out of the sediment into the sea with no accompanying heat. Around cold seeps, sulphur bacteria are found inside tubeworms. The worms absorb hydrogen sulphide and funnel it to the bacteria, which in turn produce organic molecules that the tubeworm shares.
These chemosynthetic bacteria carry out reactions involving nitrogen compounds. There are three categories of nitrogen bacteria: nitrifying, denitrifying and nitrogen-fixing. Nitrifying bacteria live in soil where ammonia compounds are found. Ammonia contains nitrogen and hydrogen. Some bacteria use these compounds to create nitrites, a compound of nitrogen and oxygen. Other bacteria continue the process, converting the nitrites into nitrates, compounds of nitrogen and oxygen that contain more oxygen than do nitrites, and which plants can absorb through their root systems and use as nutrients. All of the bacteria that take part in this process are called nitrifying bacteria.
Denitrifying bacteria impoverish soil because they destroy the nitrates that provide nutrients for plants. These bacteria derive nourishment from the transformation of nitrates into other compounds, including nitrogen and nitrites, which plants are unable to convert into the molecules that they need.
Bacteria that fix nitrogen live in the roots of certain plants, including legumes. They turn atmospheric nitrogen into nitrates. Atmospheric nitrogen is a stable compound, so the reactions that these bacteria carry out are difficult to replicate. Fritz Haber, a German chemist, won the Nobel Prize for Chemistry in 1918 for his process of fixing nitrogen artificially. The Haber process is used to create commercial fertilisers in a mechanised imitation of what nitrogen-fixing bacteria do in legume roots.
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