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.