Energy Conservation In Heliobacteria: Photosynthesis And Central Carbon Metabolism

Heliobacteria are a group of anoxygenic phototrophic bacteria that use a unique pigment, bacteriochlorophyll g, for photosynthetic energy conversion within a type I homodimeric reaction center. Like their nonphotosynthetic relatives the clostridia, heliobacteria have a gram-positive cell structure and can form heat-resistant endospores. Heliobacteria are also unusual in that they are the only anaerobic anoxygenic phototrophs that lack a mechanism for autotrophic growth. Growth of heliobacteria is therefore dependent upon the presence of usable organic carbon sources and occurs either photoheterotrophically or chemotrophically (via pyruvate fermentation). While knowledge of heliobacterial photosynthesis and physiology has steadily increased since the relatively recent discovery of these phototrophs in the 1980s, high-resolution structural data pertaining to features of the heliobacterial photosynthetic apparatus are not yet available. This chapter summarizes our current understanding of energy conservation in heliobacteria as it relates to central carbon metabolism (in both light and dark conditions), electron transport, and light harvesting and photochemistry within the reaction center.