Carbon Metabolism in Green Bacteria

Abstract

SummaryThe only common feature of the two families of green bacteria, Chlorobiaceae (green sulfur bacteria) and Chloroflexaceae (green gliding bacteria), is their type of light harvesting chlorophyll and the organization of these pigments into chlorosomes. In most other respects, including metabolism, photosynthetic apparatus and phylogeny, they are very different and far apart. Each of the two most studied genera possesses a unique pathway for autotrophic fixation of CO2: the reductive tricarboxylic acid cycle used by Chlorobium and the newly discovered 3-hydroxypropionate cycle used by Chloroflexus.The two pathways are described in detail and the experimental evidence for their existence is given. Likewise are the experimental approaches used to elucidate these pathways discussed and evaluated. The biochemical reactions participating in the pathways are considered in relation to the nature of the photosynthetic apparatus of the organisms and their ecology.The current knowledge of the metabolism of organic compounds in Chlorobium and Chloroflexus is reviewed, and the accumulation of glycogen and its breakdown in Chlorobium is discussed in detail.Assuming that life originated in a primordial sea of abiotically produced organic molecules, and taking into account phylogenetic relationships based on 16S rRNA analysis, the possible development by retroevolution of the metabolic pathways used by the green sulfur bacteria and the green gliding bacteria is discussed.