Heliobacteria

The heliobacteria are phototrophic: they convert light energy into chemical energy by photosynthesis and they use a type I reaction center. The primary pigment involved is bacteriochlorophyll g, which is unique to the group and has a unique absorption spectrum; this gives the heliobacteria their own environmental niche. Phototrophy takes place at the cell membrane, which does not form folds or compartments as it does in purple bacteria. Though heliobacteria are phototrophic, they can grow without light by fermentation of pyruvate.

RNA trees place the heliobacteria among the Firmicutes, but they do not stain Gram-positively. They have no outer membrane and like certain other firmicutes (Clostridia), they form heat-resistant endospores, which contain high levels of calcium and dipicolinic acid. Heliobacteria are the only firmicutes known to conduct photosynthesis.

Heliobacteria are photoheterotrophic, requiring organic carbon sources, and they are exclusively anaerobic. Chlorophyll g is inactivated by the presence of oxygen, making them obligate anaerobes (they cannot survive in aerobic conditions). So far, heliobacteria have only been found in soils, and are apparently widespread in the waterlogged soils of paddy fields. They are avid nitrogen fixers, so are probably important in the fertility of paddy fields.