11.1 The Prokaryotic Groups

• The prokaryotes are grouped into two domains, the Archaea and Bacteria.

  • Each domain is divided into phyla, each phylum into classes, each class into order into families, each family into genera, and finally, each genus into species.

11.2 Gram-Negative Bacteria

• The proteobacteria, which include most of the gram-negative, chemoheterotrophic bacteria, are presumed to have arisen from a common photosynthetic ancestor.

• As a group, the alphaproteobacteria includes most of the proteobacteria that are capable of growth with very low levels of nutrients.

  • Some have unusual morphology, including protrusions such as stalks or buds known as prosthecae.

• Members of the photosynthetic phylum Chlorobi (representative genus: Chlorobium) are called green sulfur bacteria.

  • Members of the phylum Chloroflexi (representative genus: Chloroflexus) are called green nonsulfur bacteria.

• However, there are photosynthetic gram-negative bacteria that are genetically included in the proteobacteria.

  • These are the purple sulfur bacteria (gammaproteobacteria) and the purple nonsulfur bacteria (alphaproteobacteria and betaproteobacteria).

11.5 Diversity within the Archaea

• Extremophiles, as they are known, include halophiles, thermophiles, and acidophiles.