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Anatomy
and/or electrical information, but also as amplifiers and eventually as transmitters. Hence, photoreceptors can be considered algal “eyes,” with many similarities with the complex vision systems of higher
organisms, since they do possess optics, photoreceptors, and signal transduction chain components.
The proper functioning of these basic visual systems relies on some necessary and universal
structural, behavioral, and physiological features, such as the following:
• Shading device (structural)
• Photoreceptor and photoreceptive proteins (structural)
• Sampling strategies (behavioral)
• Trajectory control (behavioral)
• Signal transmission (physiological)
The shading device and the photoreceptor are the structural elements used to screen the environment and provide information about the intensity and directionality of the incident light. These two
elements are always in close association. The shading device, termed eyespot, is usually an absorbing element that prevents light coming from certain direction from reaching the detector.
In flagellate algae, eyespot position is usually strictly defined with respect to the plane of beat
of the flagella, though variation exists; however, along the longitudinal cell axis, the eyespot being
located either more anteriorly or more posteriorly. The position of the eyespot gives the algae distinctive left–right asymmetry. The typical organization of the eyespot consists of a single layer of
closely packed globules containing mainly carotenoids that can play the shading role, thanks to their
strong absorbance in the 400–500-nm range.
The most common (and primitive) type of photoreceptor consists of extensive two-dimensional
(2D) patches of photosensitive proteins, embedded in the plasma membrane in close association
with the eyespot; the proteins have an ordered disposition in order to maximize light absorption. For
detecting the direction of light of a specific spectral range, a photoreceptor demands a high packing density of chromophore molecules organized in a lattice structure, with high absorption crosssection of the chromophore, that is, high probability of photon absorption by the chromophore, and
very low dark noise. The dark noise is the noise inherent in a receptor, constant, and independent
of light level, which arises from the random thermal motions of the molecules. For detecting patterns of light, the number and location of photoreceptors, having fixed size and exposure time, must
be viewed according to the pattern of motion of algae; therefore, the design of the photoreceptive
apparatus in conjunction with the helical movement of the cell produces a highly directional optical
device allowing effective tracking of the light direction. The function of photoreception is inseparable from the presence of photoreceptive proteins, but not necessarily from the presence of an
eyespot; hence, when the eyespot is absent, its function must be performed by the whole algal body.
Only few photoreceptors can be identified by optical microscopy (Figure 1.56), whereas the eyespot is always easily recognizable because of its size and colour, usually orange-red (Figure 2.68).
FIGURE 2.68 Apical eyespot of Tetraflagellochloris mauritanica. Scale bar, 5 μm.
Anatomy
and/or electrical information, but also as amplifiers and eventually as transmitters. Hence, photoreceptors can be considered algal “eyes,” with many similarities with the complex vision systems of higher
organisms, since they do possess optics, photoreceptors, and signal transduction chain components.
The proper functioning of these basic visual systems relies on some necessary and universal
structural, behavioral, and physiological features, such as the following:
• Shading device (structural)
• Photoreceptor and photoreceptive proteins (structural)
• Sampling strategies (behavioral)
• Trajectory control (behavioral)
• Signal transmission (physiological)
The shading device and the photoreceptor are the structural elements used to screen the environment and provide information about the intensity and directionality of the incident light. These two
elements are always in close association. The shading device, termed eyespot, is usually an absorbing element that prevents light coming from certain direction from reaching the detector.
In flagellate algae, eyespot position is usually strictly defined with respect to the plane of beat
of the flagella, though variation exists; however, along the longitudinal cell axis, the eyespot being
located either more anteriorly or more posteriorly. The position of the eyespot gives the algae distinctive left–right asymmetry. The typical organization of the eyespot consists of a single layer of
closely packed globules containing mainly carotenoids that can play the shading role, thanks to their
strong absorbance in the 400–500-nm range.
The most common (and primitive) type of photoreceptor consists of extensive two-dimensional
(2D) patches of photosensitive proteins, embedded in the plasma membrane in close association
with the eyespot; the proteins have an ordered disposition in order to maximize light absorption. For
detecting the direction of light of a specific spectral range, a photoreceptor demands a high packing density of chromophore molecules organized in a lattice structure, with high absorption crosssection of the chromophore, that is, high probability of photon absorption by the chromophore, and
very low dark noise. The dark noise is the noise inherent in a receptor, constant, and independent
of light level, which arises from the random thermal motions of the molecules. For detecting patterns of light, the number and location of photoreceptors, having fixed size and exposure time, must
be viewed according to the pattern of motion of algae; therefore, the design of the photoreceptive
apparatus in conjunction with the helical movement of the cell produces a highly directional optical
device allowing effective tracking of the light direction. The function of photoreception is inseparable from the presence of photoreceptive proteins, but not necessarily from the presence of an
eyespot; hence, when the eyespot is absent, its function must be performed by the whole algal body.
Only few photoreceptors can be identified by optical microscopy (Figure 1.56), whereas the eyespot is always easily recognizable because of its size and colour, usually orange-red (Figure 2.68).
FIGURE 2.68 Apical eyespot of Tetraflagellochloris mauritanica. Scale bar, 5 μm.