Taxonomy, phylogeny, and ecology of the heliobacteria

Abstract

Heliobacteria are a recently discovered group of anoxygenic phototrophic bacteria, first described in 1983. Heliobacteria contain bacteriochlorophyll g, a pigment unique to species of this group, and synthesize the simplest photosynthetic complexes of all known phototrophs. Also, unlike all other phototrophs, heliobacteria lack a mechanism for autotrophy and produce endospores. Four genera of heliobacteria containing a total of 10 species are known. Species of the genera Heliobacterium, Heliobacillus, and Heliophilum grow best at neutral pH, whereas species of Heliorestis are alkaliphilic. Heliobacterium, Heliobacillus, and Heliophilum species form one phylogenetic clade of heliobacteria, while Heliorestis species form a second within the phylum Firmicutes of the domain Bacteria. Heliobacteria have a unique ecology, being primarily terrestrial rather than aquatic phototrophs, and may have evolved a mutualistic relationship with plants, in particular, rice plants. The genome sequence of the thermophile Heliobacterium modesticaldum supports the hypothesis that heliobacteria are "minimalist phototrophs" and that they may have played a key role in the evolution of phototrophic bacteria.

Carbon Flow of Heliobacteria Is Related More to Clostridia than to the Green Sulfur Bacteria

“…All of the cultured heliobacteria require organic carbon for anoxygenic growth, and several of the species can fix nitrogen (1,2). Heliobacteria are the only cultured Gram-positive photosynthetic bacteria and are phylogenetically related to the bacterial phylum Firmicutes, such as the aerobic Bacillus and anaerobic Clostridia (1).…”

“…Heliobacteria are the only cultured Gram-positive photosynthetic bacteria and are phylogenetically related to the bacterial phylum Firmicutes, such as the aerobic Bacillus and anaerobic Clostridia (1). Among 10 cultured heliobacteria (2), Heliobacterium modesticaldum is the only one that can grow at temperatures of Ͼ50°C. Madigan and co-workers (1,3) reported that pyruvate, lactate, acetate (ϩHCO 3 Ϫ ), or yeast extract can support the phototrophic growth of H. modesticaldum, and our recent studies demonstrated that D-sugars can also support the growth of H. modesticaldum (4).…”

Early Evolution of Photosynthesis

“…The GSB use the reverse tricarboxylic acid cycle, and many of the FAPs use the 3-hydroxypropionate cycle (Zarzycki et al, 2009). The Gram-positive heliobacteria lack any known autotrophic carbon fixation pathway and usually grow photoheterotrophically (Asao and Madigan, 2010). Similarly, the aerobic anoxygenic phototrophs, which are closely related to the purple bacteria, lack any apparent ability to fix inorganic carbon.…”

The Core- and Pan-Genomes of Photosynthetic Prokaryotes

“…The previous examination of several other Heliobacteria species for sporulating genes has indicated that sporulation gene presence may be universal within the heliobacteria (KimbleLong & Madigan, 2001). It should be noted that a set of genes involved in bacteriochlorophyll (Bchl) g biosynthesis, not found in other phototrophs, were frequently reported in other heliobacteria species (reviewed in Asao & Madigan, 2010). These enzymes were not clustered into the core-genome due to their absence in the non-phototrophic Firmicutes.…”