1.3 Gravity Responses of Motile Microorganisms
There has been a historical long-lasting debate as to which level of organization
gravity interferes with living organisms and influence their behavior, physiology and
functionality. Motile microorganisms sense and respond to a plethora of environmental physical and chemical stimuli to adjust the position in their habitat for optimal
growth and reproduction (Häder et al. 2005). Prokaryotic and eukaryotic protists
respond to the intensity and direction of light and show positive and/or negative
phototaxis (movement toward or away from the light source) (Fraenkel and Gunn
1961; Häder 1991; Fiedler et al. 2005; Häder and Iseki 2017) as well as a dependence
of their swimming speed on the light intensity (photokinesis) (Nultsch 1975; Zhenan
and Shouyu 1983). Many organisms sense and respond to chemical stimuli by
swimming up a gradient of attractants or down a gradient of repellents (positive
and negative chemotaxis) which has been studied extensively in bacteria (Dusenbery
1985; Clegg et al. 2003; Wadhams and Armitage 2004). Flagellates and ciliates
exploit oxygen gradients to control their vertical migrations in the water column
(Finlay et al. 1986; Finlay and Fenchel 1986; Hemmersbach-Krause et al. 1991a;
Porterfield 1997). The magnetic field of the Earth is exploited by many organisms
(magnetotaxis) from bacteria to mammals (Lohmann et al. 2004; Simmons et al.
2004; Lin et al. 2017), and others detect electric fields or currents (galvanotaxis)
(Votta and Jahn 1972; Kim 2013). The ambient temperature has many effects on
biochemical and physiological reactions in all organisms. Therefore, it is no wonder
that motile organisms utilize thermal gradients to reach habitats of favorable temperatures (Kamykowski and Zentara 1977; Kushner 1985; Steidinger and Tangen 1996;
Maree et al. 1999). Some organisms have been found to be capable of detecting
amazingly small temperatures on the order of less than 0.01
C cm
À1 (Poff and
Skokut 1977; Whitaker and Poff 1980; Fontana and Poff 1984).
Due to the ability of microorganisms to respond to a variety of environmental
stimuli, the question arose whether organisms have also developed sensors for gravity
and respond to the gravitational vector perpendicular to the Earth surface even if the
reactions may not be visible or have not yet been revealed. Only very small prokaryotic organisms such as bacteria may not be able to respond to gravity since their
motile behavior is governed by Brownian motion (Todd 2007; Li et al. 2008;
Buttinoni et al. 2012). However, recent studies showing that even the fluidity of
membranes is altered by gravity shed new light on this consideration and might
change the view (Kohn et al. 2017). In microorganisms, movement parallel to the
gravity vector of the Earth is called gravitaxis (see Chaps. 2 and 3). Initially the term
“geotaxis” (Bean 1984; Fenchel and Finlay 1984) was used but replaced by
“gravitaxis” since the organisms do not only respond to the gravity field of the
Earth but also to that of other celestial bodies or artificial accelerations. Some
organisms move up against the gravity vector (negative gravitaxis) (HemmersbachKrause and Häder 1990; Häder et al. 1995), others swim downward (positive
gravitaxis; Bechert 2009) or are capable of doing both at different developmental
phases such as the photosynthetic flagellate Euglena (Häder and Hemmersbach
4
1 Gravity Sensing, Graviorientation and Microgravity
There has been a historical long-lasting debate as to which level of organization
gravity interferes with living organisms and influence their behavior, physiology and
functionality. Motile microorganisms sense and respond to a plethora of environmental physical and chemical stimuli to adjust the position in their habitat for optimal
growth and reproduction (Häder et al. 2005). Prokaryotic and eukaryotic protists
respond to the intensity and direction of light and show positive and/or negative
phototaxis (movement toward or away from the light source) (Fraenkel and Gunn
1961; Häder 1991; Fiedler et al. 2005; Häder and Iseki 2017) as well as a dependence
of their swimming speed on the light intensity (photokinesis) (Nultsch 1975; Zhenan
and Shouyu 1983). Many organisms sense and respond to chemical stimuli by
swimming up a gradient of attractants or down a gradient of repellents (positive
and negative chemotaxis) which has been studied extensively in bacteria (Dusenbery
1985; Clegg et al. 2003; Wadhams and Armitage 2004). Flagellates and ciliates
exploit oxygen gradients to control their vertical migrations in the water column
(Finlay et al. 1986; Finlay and Fenchel 1986; Hemmersbach-Krause et al. 1991a;
Porterfield 1997). The magnetic field of the Earth is exploited by many organisms
(magnetotaxis) from bacteria to mammals (Lohmann et al. 2004; Simmons et al.
2004; Lin et al. 2017), and others detect electric fields or currents (galvanotaxis)
(Votta and Jahn 1972; Kim 2013). The ambient temperature has many effects on
biochemical and physiological reactions in all organisms. Therefore, it is no wonder
that motile organisms utilize thermal gradients to reach habitats of favorable temperatures (Kamykowski and Zentara 1977; Kushner 1985; Steidinger and Tangen 1996;
Maree et al. 1999). Some organisms have been found to be capable of detecting
amazingly small temperatures on the order of less than 0.01
C cm
À1 (Poff and
Skokut 1977; Whitaker and Poff 1980; Fontana and Poff 1984).
Due to the ability of microorganisms to respond to a variety of environmental
stimuli, the question arose whether organisms have also developed sensors for gravity
and respond to the gravitational vector perpendicular to the Earth surface even if the
reactions may not be visible or have not yet been revealed. Only very small prokaryotic organisms such as bacteria may not be able to respond to gravity since their
motile behavior is governed by Brownian motion (Todd 2007; Li et al. 2008;
Buttinoni et al. 2012). However, recent studies showing that even the fluidity of
membranes is altered by gravity shed new light on this consideration and might
change the view (Kohn et al. 2017). In microorganisms, movement parallel to the
gravity vector of the Earth is called gravitaxis (see Chaps. 2 and 3). Initially the term
“geotaxis” (Bean 1984; Fenchel and Finlay 1984) was used but replaced by
“gravitaxis” since the organisms do not only respond to the gravity field of the
Earth but also to that of other celestial bodies or artificial accelerations. Some
organisms move up against the gravity vector (negative gravitaxis) (HemmersbachKrause and Häder 1990; Häder et al. 1995), others swim downward (positive
gravitaxis; Bechert 2009) or are capable of doing both at different developmental
phases such as the photosynthetic flagellate Euglena (Häder and Hemmersbach
4
1 Gravity Sensing, Graviorientation and Microgravity