Cyanothece the process of fixing nitrogen became unfavorable, and natural selection eliminated some of the necessary genes for the nitrogenase protein complex to increase evolutionary fitness. Cyanobacteria turn energy from the sun into chemical energy through oxygenic photosynthesis. Their light-harvesting complex that captures the photons usually includes the pigments chlorophyll a and phycocyanin. A cyanobacterium’s typical blue-green color is a result of the combination of these two pigments. Three "Cyanothece" strains, sp. PCC 7424, 7822 and 8801, have the additional pigment phycoerythrin, which expands the wavelengths of light these species use for energy. Phycoerythrin also gives these three species a brownish-green
Thylakoid centers, each consisting of an antenna complex that uses chlorophylls and accessory photosynthetic pigments such as carotenoids and phycobiliproteins to harvest light at a variety of wavelengths. Each antenna complex has between 250 and 400 pigment molecules and the energy they absorb is shuttled by resonance energy transfer to a specialized chlorophyll "a" at the reaction center of each photosystem. When either of the two chlorophyll "a" molecules at the reaction center absorbs energy, an electron is excited and transferred to an electron-acceptor molecule. Photosystem I contains a pair of chlorophyll "a" molecules, designated P700, at its reaction center that