Abstract: A polymer composition having a modified absorption and transmission characteristic particularly suited for photoreactors or photobioreactors made of plastic molded parts and exposed to sunlight or suitable artificial light sources, the polymer selectively comprises the following substances or a combination thereof in addition to the conventional standard additives: an inorganic or organic near infrared absorber for absorbing long-wavelength radiation, an inorganic or organic reflector for reflecting ultraviolet radiation, an inorganic or organic reflector for reflecting visible, near infrared, or infrared radiation, an optical brightener or fluorescent dye for converting the absorbed ultraviolet radiation into visible light or fluorescent light, a photochromic dye for modifying the transmission characteristic of the plastic molded part as a function of light intensity, and an antimicrobial additive for preventing or reducing organic deposits in the photobioreactor.