[2] They are a key player in symbiotic nitrogen fixation alongside plants, and use a type I reaction center like green-sulfur bacteria.

[5] They have no outer membrane and like certain other Bacillota (Clostridia), they form heat-resistant endospores, which contain high levels of calcium and dipicolinic acid.

[5] Phototrophic processes take place at the cell membrane, which does not form folds or compartments as it does in purple bacteria.

[5] Bacteriochlorophyll g is inactivated by the presence of oxygen, making them obligate anaerobes (they cannot survive in aerobic conditions).

[9] Heliobacteria are mainly terrestrial phototrophs, contrary to the multitudes of others that are aquatic, and often form mutualistic relationships with the plants near them.