Phytochromes are photoreceptors that were first discovered in plants, but homologues have since been identified in bacteria, cyanobacteria and fungi. Phytochromes reversibly isomerise upon illumination in red or far-red light, making them uniquely adapted to regulating antenna pigments and photosystems in photosynthetic organisms. Acaryochloris marina, a unicellular oxygenic photosynthetic cyanobacterium, lives in infrared light enriched environments. It has adapted to these light conditions by evolving a red-shifted chlorophyll, chlorophyll d. Since A. marina has adapted to infrared light enriched light conditions, phytochrome may be an important photoreceptor for this organism. To understand red light perception in A. marina, the newly sequenced genome of this cyanobacterium was searched for phytochrome homologues. Interestingly, A. marina contains a gene for a putative red-shifted phytochrome, denoted AmrBphP. AmrBphP was included in a phylogenetic analysis of the phytochrome superfamily and found to be more closely related to bacteriophytochromes than typical cyanobacterial phytochromes. The red-shifted absorption spectrum of a bacteriophytochrome would better overlap the spectrum of Chl d then the spectrum of a cyanobacterial phytochrome. A. marina may have evolved or retained a bacteriophytochrome to adapt to the infra red light-enriched environment it inhabits.