Chlorophyll a chemical energy but is not the only pigment that can be used for photosynthesis. All oxygenic photosynthetic organisms use chlorophyll "a", but differ in accessory pigments like chlorophylls "b". Chlorophyll "z" can also be found in very small quantities in the green sulfur bacteria, an anaerobic photoautotroph. These organisms use bacteriochlorophyll and some chlorophyll "a" but do not produce oxygen. Anoxygenic photosynthesis is the term applied to this process, unlike oxygenic photosynthesis where oxygen is produced during the light reactions of photosynthesis. The molecular structure of chlorophyll "a" consists of a chlorin ring, whose four nitrogen atoms surround a central
Anoxygenic photosynthesis Anoxygenic photosynthesis The pigments used to carry out anaerobic photosynthesis are similar to chlorophyll but differ in molecular detail and peak wavelength of light absorbed. Bacteriochlorophylls "a" through "g" absorb electromagnetic photons maximally in the near-infrared within their natural membrane milieu. This differs from chlorophyll a, the predominant plant and cyanobacteria pigment, which has peak absorption wavelength approximately 100 nanometers shorter (in the red portion of the visible spectrum). Some archaea (e.g. "Halobacterium") capture light energy for metabolic function and are thus phototrophic but none are known to "fix" carbon (i.e. be photosynthetic). Instead of a chlorophyll-type receptor and electron