Thesis-PDF - IAP/TU Wien

is protruding to the outside and thus called the emerging flagellum, the other one
ends inside. Both take their origin in the basal bodies embedded in the base of
the reservoir (see Fig. 4.4). The emergent flagellum contains a paraxial rod, bears
mastigonemes (hairlike structures covering the flagellum) and is characterized by
a swelling that contains the photoreceptor. The other flagellum is reduced to a
short stub and its distal end approaches the emergent flagellum in the region of
the swelling.
The photoreceptor, also called paraflagellar body (PFB), is connected to the
flagellar rod and protected by a surrounding membrane. It is a highly efficient
light detector (see 4.3.3) and forms together with the stigma the visual system
of Euglena. The stigma (indicated by an arrow in the leftmost image in Fig.
4.8) consists of tiny carotenoid droplets just adverse to the photoreceptor. The
droplets shield the photoreceptor from incident light once every revolution during
swimming. This simple but complete visual system allows Euglena to orientate
itself towards a light source. [82]
Figure 4.5: Euglena in different positions relative to the light source (the
incident light comes from the left, the shaded area is drawn hatched). Swimming
direction is upwards. Image adapted from [95].
The light-orientated movement of the cell, the phototaxis, is caused by the
teamwork of the stigma and the photoreceptor. During its movement, the cell
permanently rotates and the stigma comes between the light source and the photoreceptor
(see Fig. 4.5). Euglena experiences a periodical decrease in light intensity
and changes its direction of movement until the detected light is no longer
modulated by the stigma. Then the cell is moving towards the light source.
If the light intensity is very strong, the cell flees from the light, also referred
to as negative phototaxis. Interestingly enough this process seems not to involve
the stigma - it also occurs after removal of this cell organelle. This enables E.
gracilis to either swim towards the light (if the organism receives less light than
the optimum) or away from it when too much light (especially UV-light) could
damage the cell. The changes of direction for positive phototaxis can be seen
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