A PHOTOSYSTEM-I REACTION-CENTER DRIVEN BY CHLOROPHYLL-D IN OXYGENIC PHOTOSYNTHESIS

A far-red type of oxygenic photosynthesis was discovered in Acaryochlo ris marina, a recently found marine prokaryote that produces an atypic al 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 absorp tion changes of reaction center Chi d that was named P740 after its pe ak wavelength. A midpoint oxidation reduction potential of P740 was de termined to be +335 mV. P740 uses light of significantly low quantum e nergy (740 nm = 1.68 eV) but generates a reducing power almost equival ent to that produced by a special pair of Chi a (P700) that absorbs re d light at 700 nm (1.77 eV) in photosystem I of plants and cyanobacter ia. The oxygenic photosynthesis based on Chi d might either be an accl imation to the far-red light environments or an evolutionary intermedi ate between the red-absorbing oxygenic and the far-red absorbing anoxy genic photosynthesis that uses bacteriochlorophylls.