Correspondingly, sessile organisms show growth responses called tropisms defining a
steady-state bending of an organ with respect to the direction of the stimulus source. The
kind of environmental stimulus to which the organisms respond is indicated as a prefix
to the appropriate term: for example, phototaxis or phototropism (directional response to
light; see also Chapter 4); chemotaxis and chemotropism (directional response to a chem-
ical, e.g., nutrients; Chapter 6); thermotaxis and thermotropism (response to a thermal
gradient; Chapter 6).
In this chapter we concentrate on directional responses with respect to the gravita-
tional field, which are called gravitaxis and gravitropism. In addition, a special gravity-
dependent kinetic response has been described for some protists (ciliates). These systems
have the capacity to regulate their swimming velocity depending on their swimming di-
rection. They manage to speed up during upward swimming and to decelerate during
downward swimming. Consequently, they compensate for at least part of their passive
sinking (sedimentation) rate (Machemer et al. 1991; Ooya et al. 1992; Hemmersbach-
Krause et al. 1993a). Gravikinesis is calculated by comparing the upward and downward
swimming velocities of a cell (population) with its sedimentation velocity (Machemer
and Bräucker 1992):
7.3 Occurrence and significance of gravitaxis in single-cell systems
Gravitactic behavior has been reported for several protozoan species. These unicellular
organisms are heavier than water and most of the species studied so far show negative
gravitaxis, which guides them to the surface. Well-studied examples are the negative
gravitaxis of the heterotrophic ciliates Parameciumand Tetrahymena(for review, see
Bean 1984; Häder et al. 2005; Hemmersbach and Häder 1999; Hemmersbach et al.
1999),the oxygen-dependent gravitaxis of the microaerophilic ciliate Loxodes(Fenchel
and Finlay 1986),and the light-dependent gravitaxis of the autotrophic green algae
Euglenaand Chlamydomonas(Bean 1984; Häder 1987). These examples clearly show
that the direction of gravitaxis increases the ecological fitness of the organisms.
Besides ciliates and flagellates (Figure 7.1), gravity effects have also been studied in
other unicellular organisms: amoeba, acellular slime molds, swimming reproductive
stages such as zoospores and sperm cells, and bacteria. These systems have been exposed
to different acceleration levels in order to analyze the impact of gravitational forces on
different physiological processes (behaviour, proliferation, etc.). Due to the fact that in
these cases a clear hypothesis as to the mechanism of graviperception is still missing, we
will only briefly mention these in this chapter (for more details, see Häder et al. 2005)
before describing the more thoroughly understood single-cell systems such as Euglena,
Loxodes, andChara.
The migrating plasmodium of the single-celled slime mold Physarum polycephalum
(Myxomycetes, acellular slime mold) shows gravitaxis and has been used as model sys-
tem to study the impact of gravity on actomyosin-driven movements (Block et al. 1986).
Gravikinesisupward swimming rate downw
=− aard swimming rate
2−sedimentation rate