Life is essentially about information, how information is perceived, how it is stored, passed on and used by organisms as they live and reproduce. In the world of photosynthetic microorganisms, where virtually all life depends on solar energy, light becomes also a source of information, used to orient microorganisms spatially and to guide their movements or growth. Responses using light as a sensory stimulus for orientation towards areas that best match their individual irradiation requirements are thus a virtually universal behavior among algae. The full exploitation of light information necessitates proper perceiving devices, able to change the small signal represented by the light falling upon them in a larger signal and response of an entirely different physical nature, that is, these devices, termed photoreceptors, must perform perception, transduction,amplification, and transmission.
The processing of a photic stimulus and its transformation into an oriented movement can be considered the “vision” phenomenon of motile algae. True vision involves production of a focused image of the external world, and the optical requirements for an eye probably cannot be satisfied by algae, requiring true multicellularity with cell specialization and division of labor.
Still, algal “eyes” have many similarities with the complex vision systems of higher organisms, because they do possess optics, photoreceptors, and signal transduction chain components. The essential elements of these basic visual systems are the shading device(s), for example, the eyespot and the detector, that is, the true photoreceptor(s). When the eyespot is absent its function is performed by the whole algal body.
The eyespot is a sort of roundish shield, inwardly or outwardly concave, made up of one or more layers of lipidic globules closely packed. These globules contain mainly carotenoids that can play the shading role due to their strong absorbance in the 400–500 nm range. The most common type of photoreceptor consists of extensive two-dimensional patches of photosensitive proteins, present in the plasma membrane in close association with the eyespot. Very often the photoreceptor cannot be identified by optical microscopy, while the eyespot can be seen easily because of its size and color, usually orange-red. This is one of the main reasons for the plethora of data present in the literature on the morphology, composition, and ultrastructure of algal eyespots with respect to the few data available on their photoreceptors.
Ultrastructural studies over the past 40 years have shown that photosensory systems in algae have certain characteristics in common, and distinct types of these apparatus can be identified. On the assumption that photoreception is inseparable from the presence of photoreceptive proteins but not necessarily from the presence of an eyespot, we will group photoreceptive systems into three main types. The description will include also those algae with well documented eyespot but only presumptive photoreceptor localization.