The family heliobacteriaceae

Abstract

Heliobacteria are anoxygenic phototrophic bacteria of the phylum Firmicutes and are distinct from all other anoxygenic phototrophs in many ways. These include their phylogeny, synthesis of the unique photopigment bacteriochlorophyll., production of heat-resistant endospores, and their primarily soil habitat. Five genera of heliobacteria have been described, including a total of 11 species. Heliobacteria are obligate anaerobes, and most species are capable of both phototrophic and chemotrophic growth. Two distinct phylogenetic clades of heliobacteria exist, including a group that inhabits neutral pH soils and a group that inhabits alkaline soils and soda lake ecosystems. As a group, heliobacteria are distant relatives of endospore-forming bacteria of the Bacillaceae and Clostridiaceae. The genome of the thermophile Heliobacterium modesticaldum lacks genes for autotrophy but contains genes encoding key endospore-specific proteins and nitrogenase; the heliobacterial photosynthesis gene cluster encodes the most streamlined photosystem of any known anoxygenic phototroph. Heliobacteria are widespread in paddy soils where their strong nitrogen-fixing capacities may benefit rice plants. The photoheterotrophic lifestyle of the heliobacteria may also benefit from such associations by receiving organic carbon from plant exudates.