The color of photosynthetic apparatus can be used for inferring the process of evolutionary selection of photosynthetic pigments and as possible signs of life on distant habitable exoplanets. The absorption spectra of photosynthetic apparatus have close relationships with the spectra and intensity of incident radiation. Most terrestrial plants use specific light-harvesting chlorophylls and carotenoids for photosynthesis and have pale green chloroplasts. However in aquatic ecosystems, there are phototrophs with various colors having different photosynthetic pigments. Oxygenic photosynthesis uses visible light, and far-red photons are not used for this process. While some phototrophic bacteria are able to use far-red photons for their life, they do not generate O2. Other aspect of light is the harmful effect of light. Although efficient light absorption is important for photosynthesis, UV and excess light absorption damages photosynthetic apparatus. In terrestrial environments, portion of incident solar radiation reaches to the surface, which are called direct radiation (PARdir), while the other are optically altered by the Earth’s atmosphere, scattered by the sky and clouds, which are called diffuse radiation (PARdiff). The photosynthetic systems of terrestrial plants are fine-tuned to reduce the energy absorption of PARdir. The safe use of PARdir and the efficient use of PARdiff are achieved in light-harvesting complexes of terrestrial plants. In addition to the type of central star, the optical properties of the atmosphere of the planet may have significant effects on the evolution of photosynthetic systems and photoreceptors.
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