107
Anatomy
In Cyanobacteria, phototactic orientation has been described in A. variabilis, Pseudoanabaena
sp., and Phormidium, although no defined structure for light sensing have been detected so far. The
first identification of a complex photoreceptive system leading to the evidence of a photoreceptive
protein was performed in Leptolyngbya sp. This deep red cyanobacterium lives in Roman hypogea
at extremely low light intensity (10 13 photons m −2 s −1 ). It possesses an orange eyespot at the tip of
the apical cell of the trichome. Electron microscopy revealed that this eyespot is characterized by
osmiophilic globules of about 100 nm in diameter arranged in a peripheral cap extending 2–3 μm
from the apex and with a possible layered pattern (Figure 2.2). Microspectrophotometric analysis of
the tip of the apical cell of Leptolyngbya trichomes revealed a complex absorption spectrum with
two main bands. The band centered at 456 nm is due to the absorption of the carotenoid present
in the eyespot, whereas the band centered at 504 nm can be assigned to rhodopsin-like molecules
packed in the plasma membrane of the tip of the apical cell.
In Ochrophyta, data exist indicating that the photoreceptor molecules are present inside the cell
membrane of zygotes of the fucoid brown algae, Fucus sp. and Silvetia compressa. Experimental
work confirmed this localization in S. compressa (Figure 2.70a), where a rhodopsin-like protein was
identified in the zygote membrane (Figure 2.70b). Since no eyespot can be detected in these algal
stages, the shading function is assigned to the whole-cell body.
In the Haptophyta, the eyespot is present only in some species of the order Pavlovales. It consists of a single layer of globules situated at the anterior end of one of the chloroplasts, beneath the
posteriorly directed flagellum, at the level of its emersion from the cell. In these algae, the photoreceptor has not yet been localized, but we can presume it is positioned inside the membrane in close
association with the eyespot.
In the Cryptophyta, the presence of an eyespot is limited to a small number of species belonging
to the genus Chroomonas. The eyespot is situated at the center of the cell, within a conical lobe of
the chloroplast. It consists of a single layer of about 35 closely packed globules, attached to the chloroplast envelope and the endoplasmic reticulum. Also in these algae, the photoreceptive proteins
should be located inside the plasma membrane overlaying the eyespot.
In the Dinophyceae, the eyespot is chloroplastic in Peridiniun sp., consisting of a layer of globules under the chloroplast envelope, situated behind the longitudinal sulcus, and truly extraplastidic
in Woloszynskia coronata, where it consists of an irregular cluster of globules located beneath the
sulcus, and immediately adjacent to the subthecal microtubules. In the latter, neither a connection
with the chloroplast nor membranes surrounding the eyespot are present. Glenodinium foliaceum and
Peridinium balticum possess another type of eyespot. It is roughly a triangular body situated behind
the sulcus and is an independent structure bounded by a three-membrane envelope. Basically, there
are two layers of pigmented globules, separated by a vesicle of granular material. The eyespot can fold
FIGURE 2.70 (a) A Silvetia compressa zygote in bright-field microscopy and (b) under fluorescence microscopy labeled with antirhodopsin antibody. The two arrows point to the cell membrane layer in which photoreceptive protein are located. Scale bar, 2 μm.
Anatomy
In Cyanobacteria, phototactic orientation has been described in A. variabilis, Pseudoanabaena
sp., and Phormidium, although no defined structure for light sensing have been detected so far. The
first identification of a complex photoreceptive system leading to the evidence of a photoreceptive
protein was performed in Leptolyngbya sp. This deep red cyanobacterium lives in Roman hypogea
at extremely low light intensity (10 13 photons m −2 s −1 ). It possesses an orange eyespot at the tip of
the apical cell of the trichome. Electron microscopy revealed that this eyespot is characterized by
osmiophilic globules of about 100 nm in diameter arranged in a peripheral cap extending 2–3 μm
from the apex and with a possible layered pattern (Figure 2.2). Microspectrophotometric analysis of
the tip of the apical cell of Leptolyngbya trichomes revealed a complex absorption spectrum with
two main bands. The band centered at 456 nm is due to the absorption of the carotenoid present
in the eyespot, whereas the band centered at 504 nm can be assigned to rhodopsin-like molecules
packed in the plasma membrane of the tip of the apical cell.
In Ochrophyta, data exist indicating that the photoreceptor molecules are present inside the cell
membrane of zygotes of the fucoid brown algae, Fucus sp. and Silvetia compressa. Experimental
work confirmed this localization in S. compressa (Figure 2.70a), where a rhodopsin-like protein was
identified in the zygote membrane (Figure 2.70b). Since no eyespot can be detected in these algal
stages, the shading function is assigned to the whole-cell body.
In the Haptophyta, the eyespot is present only in some species of the order Pavlovales. It consists of a single layer of globules situated at the anterior end of one of the chloroplasts, beneath the
posteriorly directed flagellum, at the level of its emersion from the cell. In these algae, the photoreceptor has not yet been localized, but we can presume it is positioned inside the membrane in close
association with the eyespot.
In the Cryptophyta, the presence of an eyespot is limited to a small number of species belonging
to the genus Chroomonas. The eyespot is situated at the center of the cell, within a conical lobe of
the chloroplast. It consists of a single layer of about 35 closely packed globules, attached to the chloroplast envelope and the endoplasmic reticulum. Also in these algae, the photoreceptive proteins
should be located inside the plasma membrane overlaying the eyespot.
In the Dinophyceae, the eyespot is chloroplastic in Peridiniun sp., consisting of a layer of globules under the chloroplast envelope, situated behind the longitudinal sulcus, and truly extraplastidic
in Woloszynskia coronata, where it consists of an irregular cluster of globules located beneath the
sulcus, and immediately adjacent to the subthecal microtubules. In the latter, neither a connection
with the chloroplast nor membranes surrounding the eyespot are present. Glenodinium foliaceum and
Peridinium balticum possess another type of eyespot. It is roughly a triangular body situated behind
the sulcus and is an independent structure bounded by a three-membrane envelope. Basically, there
are two layers of pigmented globules, separated by a vesicle of granular material. The eyespot can fold
FIGURE 2.70 (a) A Silvetia compressa zygote in bright-field microscopy and (b) under fluorescence microscopy labeled with antirhodopsin antibody. The two arrows point to the cell membrane layer in which photoreceptive protein are located. Scale bar, 2 μm.