A far-red type of oxygenic photosynthesis was discovered in Acaryochloris marina, a recently found marine prokaryote that produces an atypical pigment chlorophyll d (Chl d), The purified photosystem I reaction center complex of A. marina contained 180 Chl d per 1 Chl a with PsaA-F, -L, -K, and two extra polypeptides. Laser excitation induced absorption changes of reaction center Chi d that was named P740 after its peak wavelength. A midpoint oxidation reduction potential of P740 was determined to be +335 mV. P740 uses light of significantly low quantum energy (740 nm = 1.68 eV) but generates a reducing power almost equivalent to that produced by a special pair of Chi a (P700) that absorbs red light at 700 nm (1.77 eV) in photosystem I of plants and cyanobacteria. The oxygenic photosynthesis based on Chi d might either be an acclimation to the far-red light environments or an evolutionary intermediate between the red-absorbing oxygenic and the far-red absorbing anoxygenic photosynthesis that uses bacteriochlorophylls.