Rhodospirillum rubrum is a Gram-negative bacterium, with a size of 0.8-1 μm. It is phylogenetically classified as a purple nonsulfur bacterium. It is a facultative anaerobe with a large set of possible metabolisms depending on the conditions of its environment. It's capable of using oxygen for aerobic respiration under aerobic conditions, or an alternative terminal electron acceptor for anaerobic respiration under anaerobic conditions. Alternative terminal electron acceptors for R. rubrum include dimethyl sulfoxide or trimethylamine oxide.