Several environmental factors influence the precision of graviorientation mechanism. The precision of gravitaxis follows a circadian rhythm entrained by a light/
dark rhythm (Lebert et al. 1999a; Nasir et al. 2014), which cannot be explained by a
pure passive mechanism. In the dark phase the cells show a minimum in the
precision of gravitactic orientation, an increase before the beginning of the light
phase and a peak in the early afternoon (Häder and Lebert 2001). The rhythm
persists even when the culture is transferred to constant light conditions (Lebert
et al. 1999a). In parallel, the cells are more elongated during daytime while they are
more rounded at night. An interesting phenomenon is that young Euglena after cell
division show positive gravitaxis while older cells in the stationary phase display a
pronounced negative gravitaxis (Stallwitz and Häder 1994). One explanation could
be that the paramylon concentration in young cells is smaller than in older ones and
thus the cells have a lower specific density. However, it was surprising when we
found that the precise downward swimming of young cells could be reversed into an
upward swimming upon application of heavy metal ions such as cadmium, copper,
mercury or lead.
Application of a red background light strongly increases the precision of gravitaxis
in the flagellate Chlamydomonas (Sineshchekov et al. 2000). The action spectrum of
this response indicates the involvement of chlamy-rhodopsin which is the presumed
receptor for phototaxis in this organism (Kianianmomeni and Hallmann 2014).
Reducing the concentration of calcium ions in the medium even further enhances
gravitaxis.
Ciliates have been called “swimming nerve cells” due to the electromotoric
coupling of the membrane potential to ciliary beating. Like Euglena, Paramecium
uses it’s whole body mass for graviperception which sediments onto the lower cell
membrane and activates mechanosensitive ion channels. Electrophysiological studies have revealed a polar distribution of mechanosensitive calcium and potassium
ion channels, which upon stimulation either de- or hyperpolarize the cell membrane
and in turn determine the ciliary activity and thus orientation and swimming speed of
the cell. This polar distribution is a prerequisite for another gravity-related response
of ciliates. Besides for orientation, gravity can also be used to modify the swimming
speed. Speeding up during upward swimming (activation of potassium channels
resulting in hyperpolarization) and decreasing during downward swimming (activation of calcium channels resulting in depolarization) enables the cells to compensate
sedimentation (Machemer 1994; Gebauer et al. 1999).
A true gravireceptor potential was found in the ciliate Stylonychia mytilus proving
that the above hypothesis is valid for the gravitactic transduction chain. Membrane
potential changes of 4 mV after cell reorientation in the gravity field by turning the
cell upside down identified the existence of gravireceptor potentials and thus support
the hypothesis that the cytoplasm can exert the pressure onto a gravireceptor (Krause
et al. 2010). The polar distribution of mechanosensitive ion channels and the
signaling cascade between their stimulation and ciliary beating are prerequisites
for using gravity for active speed control.
Representatives of the family Loxodidae kindled the interest in the search
of gravisensing mechanisms in single cells. So-called Müller organelles, of which
34
3 Gravitaxis in Flagellates and Ciliates
dark rhythm (Lebert et al. 1999a; Nasir et al. 2014), which cannot be explained by a
pure passive mechanism. In the dark phase the cells show a minimum in the
precision of gravitactic orientation, an increase before the beginning of the light
phase and a peak in the early afternoon (Häder and Lebert 2001). The rhythm
persists even when the culture is transferred to constant light conditions (Lebert
et al. 1999a). In parallel, the cells are more elongated during daytime while they are
more rounded at night. An interesting phenomenon is that young Euglena after cell
division show positive gravitaxis while older cells in the stationary phase display a
pronounced negative gravitaxis (Stallwitz and Häder 1994). One explanation could
be that the paramylon concentration in young cells is smaller than in older ones and
thus the cells have a lower specific density. However, it was surprising when we
found that the precise downward swimming of young cells could be reversed into an
upward swimming upon application of heavy metal ions such as cadmium, copper,
mercury or lead.
Application of a red background light strongly increases the precision of gravitaxis
in the flagellate Chlamydomonas (Sineshchekov et al. 2000). The action spectrum of
this response indicates the involvement of chlamy-rhodopsin which is the presumed
receptor for phototaxis in this organism (Kianianmomeni and Hallmann 2014).
Reducing the concentration of calcium ions in the medium even further enhances
gravitaxis.
Ciliates have been called “swimming nerve cells” due to the electromotoric
coupling of the membrane potential to ciliary beating. Like Euglena, Paramecium
uses it’s whole body mass for graviperception which sediments onto the lower cell
membrane and activates mechanosensitive ion channels. Electrophysiological studies have revealed a polar distribution of mechanosensitive calcium and potassium
ion channels, which upon stimulation either de- or hyperpolarize the cell membrane
and in turn determine the ciliary activity and thus orientation and swimming speed of
the cell. This polar distribution is a prerequisite for another gravity-related response
of ciliates. Besides for orientation, gravity can also be used to modify the swimming
speed. Speeding up during upward swimming (activation of potassium channels
resulting in hyperpolarization) and decreasing during downward swimming (activation of calcium channels resulting in depolarization) enables the cells to compensate
sedimentation (Machemer 1994; Gebauer et al. 1999).
A true gravireceptor potential was found in the ciliate Stylonychia mytilus proving
that the above hypothesis is valid for the gravitactic transduction chain. Membrane
potential changes of 4 mV after cell reorientation in the gravity field by turning the
cell upside down identified the existence of gravireceptor potentials and thus support
the hypothesis that the cytoplasm can exert the pressure onto a gravireceptor (Krause
et al. 2010). The polar distribution of mechanosensitive ion channels and the
signaling cascade between their stimulation and ciliary beating are prerequisites
for using gravity for active speed control.
Representatives of the family Loxodidae kindled the interest in the search
of gravisensing mechanisms in single cells. So-called Müller organelles, of which
34
3 Gravitaxis in Flagellates and Ciliates