Acetogenic prokaryotes

Abstract

This chapter circumscribes the acetogens, a physiologically defined group of the domain Bacteria that are anaerobes, using the acetyl-CoA pathway as a mechanism for the reductive synthesis of acetyl-CoA from CO 2, for a terminal-electron-accepting, energy-conserving process, and for mechanism for the fixation (assimilation) of CO 2 in the synthesis of cell carbon. Three main metabolic features of these organisms were defined, such as the use of chemolithoautotrophic substrates (H 2 -CO 2 or CO-CO 2) as sole sources of carbon and energy under anoxic conditions, the capacity to convert certain sugars stoichiometrically to acetate, and the ability to O-demethylate methoxylated aromatic compounds and metabolize the O-methyl group via the 420 acetyl-CoA pathway. Acetogens have been assigned to more than 20 different genera and they differ in their morphology, cytology, and physiology. The most frequently isolated acetogenic species to date are members of the genera Clostridium and Acetobacterium. The habitat, the morphological and physiological properties, and the phylogenetic position of acetogenic species are presented. The electron flow of the Wood/Ljungdahl pathway as well as properties and function of enzymes involved in the acetyl-CoA pathway is shown in detail. Several biotechnological applications are described with the commercial production of acetic acid from sugars and the bioconversion of synthesis gas to acetic acid, ethanol, and other chemicals being the most important ones.