Valves of Coscinodiscus wailesii diatoms, monocellular micro-algae characterized by a diameter between 100 and 200 μm, show regular pores patterns which confine light in a spot of few μm2. This effect can be ascribed to the superposition of diffracted wave fronts coming from the pores on the valve surface. We studied the transmission of partially coherent light, at different wavelengths, through single valves of Coscinodiscus wailesii diatoms. The spatial distribution of transmitted light strongly depends on the wavelength of the incident radiation. Numerical simulations help to demonstrate how this effect is not present in the ultraviolet region of the light spectrum, showing one of the possible evolutionary advantages represented by the regular pores patterns of the valves.