the water column which may be affected by other external factors such as light and
temperature and can be disturbed by the action of wind and waves (Piazena and
Häder 1995; Raymont 2014). Vertical migrations were even found within the sand
layer of the intertidal zone where Euglena deses shows diurnal patterns staining the
top sand layer green when the cells move to the surface (Taylor 1967).
A direct proof that Euglena and Paramecium have the capacity to use gravity for
orientation was obtained in a space experiment (Häder et al. 1990; HemmersbachKrause et al. 1993). Cell cultures were subjected to a parabolic flight on a TEXUS
rocket (technical experiments under microgravity) which provides microgravity for
about 6 min (Hemmersbach-Krause et al. 1993; Häder et al. 2010). Video downlink
allowed online observation of the swimming cells and they were found to move in
random directions, while the 1-g ground controls and post-flight analysis of flight
samples showed a precise negative gravitaxis.
It is not amazing that all investigated motile flagellate and ciliate species have
been found to respond to the gravitational field of the Earth since this force is
invariable and has affected the evolution of the biota from the very beginning.
However, the identification of the underlying mechanisms gave rise to many discussions and hypotheses. Today, by studies of gravitactic organisms on ground and
in space, we obtained new insights into the cellular physiology. Especially asking the
question, on which level of organization gravity impacts biological systems, motile
unicellular organisms allowed exciting insights into gravity-related signaling
pathways.
3.2 Gravitaxis and its Ecological Advantages
Many motile microorganisms demonstrate swimming parallel to the gravity vector
of the Earth either upwards (negative gravitaxis) or downwards (positive gravitaxis)
in order to optimize their position in the water column (Richter et al. 2007). Negative
gravitaxis brings them close to the water surface which is of an advantage for
photosynthetic organisms. Swimming downwards, e.g. into the sediment fits with
their metabolic requirements: e.g., a direct correlation between the abundance of
oxygen and gravitactic orientation has been demonstrated for the ciliate Loxodes and
the flagellate Euglena. The microaerophilic Loxodes can be found predominantly in
the sediment of lakes which it reaches by positive gravitaxis (Fenchel and Finlay
1984, 1990; Hemmersbach and Häder 1999) while the photosynthetic flagellate
Euglena gracilis swims preferentially upwards (Lebert and Häder 1996; Fig. 3.1),
thereby reaching its ecological niche.
An additional advantage of oriented movements with respect to the gravity vector
is protection from detrimental stress factors such as solar UV-B (280–315 nm)
radiation (Häder et al. 1999); when exposed to excessive visible or UV radiation
some flagellates swim actively downward (Ntefidou et al. 2002; Richter et al. 2002b)
or stop swimming and sediment passively out of the danger zone (Häder and Liu
1990; Sebastian et al. 1994). In Euglena the reversal of movement direction has
28
3 Gravitaxis in Flagellates and Ciliates
temperature and can be disturbed by the action of wind and waves (Piazena and
Häder 1995; Raymont 2014). Vertical migrations were even found within the sand
layer of the intertidal zone where Euglena deses shows diurnal patterns staining the
top sand layer green when the cells move to the surface (Taylor 1967).
A direct proof that Euglena and Paramecium have the capacity to use gravity for
orientation was obtained in a space experiment (Häder et al. 1990; HemmersbachKrause et al. 1993). Cell cultures were subjected to a parabolic flight on a TEXUS
rocket (technical experiments under microgravity) which provides microgravity for
about 6 min (Hemmersbach-Krause et al. 1993; Häder et al. 2010). Video downlink
allowed online observation of the swimming cells and they were found to move in
random directions, while the 1-g ground controls and post-flight analysis of flight
samples showed a precise negative gravitaxis.
It is not amazing that all investigated motile flagellate and ciliate species have
been found to respond to the gravitational field of the Earth since this force is
invariable and has affected the evolution of the biota from the very beginning.
However, the identification of the underlying mechanisms gave rise to many discussions and hypotheses. Today, by studies of gravitactic organisms on ground and
in space, we obtained new insights into the cellular physiology. Especially asking the
question, on which level of organization gravity impacts biological systems, motile
unicellular organisms allowed exciting insights into gravity-related signaling
pathways.
3.2 Gravitaxis and its Ecological Advantages
Many motile microorganisms demonstrate swimming parallel to the gravity vector
of the Earth either upwards (negative gravitaxis) or downwards (positive gravitaxis)
in order to optimize their position in the water column (Richter et al. 2007). Negative
gravitaxis brings them close to the water surface which is of an advantage for
photosynthetic organisms. Swimming downwards, e.g. into the sediment fits with
their metabolic requirements: e.g., a direct correlation between the abundance of
oxygen and gravitactic orientation has been demonstrated for the ciliate Loxodes and
the flagellate Euglena. The microaerophilic Loxodes can be found predominantly in
the sediment of lakes which it reaches by positive gravitaxis (Fenchel and Finlay
1984, 1990; Hemmersbach and Häder 1999) while the photosynthetic flagellate
Euglena gracilis swims preferentially upwards (Lebert and Häder 1996; Fig. 3.1),
thereby reaching its ecological niche.
An additional advantage of oriented movements with respect to the gravity vector
is protection from detrimental stress factors such as solar UV-B (280–315 nm)
radiation (Häder et al. 1999); when exposed to excessive visible or UV radiation
some flagellates swim actively downward (Ntefidou et al. 2002; Richter et al. 2002b)
or stop swimming and sediment passively out of the danger zone (Häder and Liu
1990; Sebastian et al. 1994). In Euglena the reversal of movement direction has
28
3 Gravitaxis in Flagellates and Ciliates