Oxygen | Nitric Acid | Sulfuric Acid | Carbon Dioxide
All animals, plants, fungi and many microbes breathe oxygen. Some microbes are particularly amazing because they can breathe a whole range of compounds other than oxygen such as iron, nitric acid, sulfuric acid, and carbon dioxide. Some of these different oxidizers are found in the Redox Mine.
Microbes that breathe oxygen are called "aerobes." Microbes that cannot breathe oxygen are collectively called "anaerobes."
Don't let the word oxidizer confuse you, since not all oxidizing agents contain oxygen. To eliminate confusion when talking about oxidizing agents (which often contain no oxygen) we call them "electron acceptors". Think of oxygen and other electron acceptors as a greedy elements that love electrons and will gladly accept them at every opportunity. When electrons are transferred from one molecule to another in a redox reaction, energy is released.
Different electron acceptors yield different amounts of energy. The electron acceptors deeper in this Redox Mine generally yield less energy. At the surface is oxygen, a powerful oxidizer which provides lots of energy. When oxygen runs out, organisms that can breathe another available oxidizer such as nitrate, survive.
In rocket ships fuel is burned. This burning is from the combination of fuel with oxygen, which releases chemical energy and blasts the rocket into space. Like rocket ships, microbes and other living things obtain energy from chemical energy. In humans, food (the fuel) like potatoes, hamburgers and ice cream, is combined with oxygen to provide energy. We breathe oxygen for energy to grow, move, talk and think.
Redox reactions involve the oxidation of one molecule and the reduction of another molecule. This is a difficult concept, but just remember that burning requires a fuel and an oxidizer. When a fuel such as sugar is oxidized, it loses electrons. Oxidation is the "loss of electrons" = Ole! When an oxidizer has accepted additional electrons, it is said to be reduced. (Since electrons are negatively charged, an increase in electrons means a reduction in electrical charge.) Thus, reduction is the addition of electrons.
When a redox reaction involves two different types of molecules and this energy is converted into metabolic energy, this is called respiration.
Fermentation is a special kind of redox reaction. In fermentation, the fuel and oxidizer are the same compound, such as sugar. The sugar is simultaneously oxidized and reduced.
Here are some of the compounds and the organisms that breathe them:
All large organisms must breathe oxygen. Only oxygen is powerful enough to give large organisms enough energy to function. Many bacteria also breathe oxygen.
Oxygen has not always been present in the atmosphere. Over 2 billion years ago, before the evolution of photosynthetic microbes that could produce oxygen, the atmosphere contained no oxygen.
This bacterium grows in the presence of oxygen and can form a protective endospore.
These fungi can breathe oxygen, or, when oxygen is lacking, they can obtain energy by fermentation.
This salt-loving bacterium requires oxygen to breathe as well as to degrade the herbicide 2,4-D.
Nitric acid breathers, are generally called nitrate reducers. These bacteria convert nitrate to nitrite. Some also convert nitrite to nitrous oxide and then to nitrogen gas. Many of these bacteria also have the ability to breathe oxygen.
Many different bacteria breathe sulfuric acid (or sulfate). These bacteria are inhibited by oxygen.
These organisms can breathe carbon dioxide. They combine carbon dioxide with hydrogen, which produces methane. Organisms that can breathe carbon dioxide are called methanogens.