Bacteria are of such immense importance because of their extreme flexibility, capacity for rapid growth and reproduction, and great age. The oldest fossils known, nearly 3.5 billion years old, are fossils of bacteria-like organisms. In addition to shape, bacteria have traditionally been identified and classified on the basis of their biochemistry and the conditions under which they grow. The advent of molecular biology has made it possible to classify bacteria on the basis of similarities among DNA sequences. Bacteria may also be classified both by the mode by which they obtain their energy.
Autotrophs fix carbon dioxide to make their own food source.
While chemoautotrophs are uncommon, photoautotrophs are common and quite diverse. They include the cyanobacteria, green sulfur bacteria, purple sulfur bacteria, and purple nonsulfur bacteria. The sulfur bacteria are particularly interesting, since they use hydrogen sulfide as hydrogen donor, instead of water like most other photosynthetic organisms, including cyanobacteria. Respiration Bacteria have a wide range of envronmental and nutritive requirements. Most bacteria may be placed into one of three groups based on their response to gaseous oxygen.