Decay of Gravitaxis and
Gravikinesis in the Weightless Condition
Mission: Drop Tower (ZARM, Bremen)
Investigators: (1) Machemer, H.; (2) Bräucker, R.; (3)
Machemer-Röhnisch, S.; (4) Takahashi, K.; (5) Murakami, A.; (6)
Yoshimura, K.
Keywords: Gravitaxis, Gravikinesis, Microgravity, Swimming Rate,
Ciliates,
Discipline: Life Sciences
Research Area: Gravitational Physiology of Single Cells
References: H. Machemer, Bräucker, R., Takahashi, K., Murakami,
A., "Short-term microgravity Microgravity Sci. Technol. 5:
119-123 (1992).
H. Machemer, Bräucker, R., study under short-term microgravity".
Microgravity Sci. Technol. 5: 221-231 (1993).
H. Machemer & Bräucker, R., "Gravireception and graviresponses in
ciliates". Acta Protozool 31: 185-214 (1992).
U. Nagel, Watzke, D., Neugebauer, D.C., Machemer- Röhnisch,
S., Bräucker, R., Machemer, H., "Analysis of sedimentation of
immobilized cells under normal and hyper-gravity".
Microgravity Sci. Technol. 10: 41-52 (1997).
H. Machemer, Machemer-Röhnisch, S., Bräucker, R., Takahashi, K.,
"Gravikinesis in Paramecium: Theory and isolation of a
physiological response to the natural gravity vector". J. Comp.
Physiol. A 168: 1-12 (1991).
Extended Abstract
Experiment Objectives: Investigations of
gravity-induced active motor responses of free swimming cells deal
with the basic fact that gravity can induce two types of locomotion:
passive motion (= sedimentation) and active motion via stimulation
of a physiological response (gravikinesis, Delta). The goal of
isolation of the latter response implies the knowledge of three
speed parameters: (1) cellular propulsion (P) as being unaffected by
gravity, (2) the sedimentation rate (S), and (3) the observed speed
(V). While V and S are experimentally easily accessible, P is
determined under microgravity only. A drop tower provides a
step transition to µg without involving preacceleration and its
physiological consequences and is thereby principally suited for
determinations of P.
Experiment Procedure: Three species of unicellular
ciliated organisms, Paramecium, Didinium and
Loxodes, were adjusted to defined culturing state, experimental
solution, O2-supply and temperature and subjected to
step-type transition from terrestrial gravity to 4.5 s of
microgravity (near 10-4g) in the drop tower of ZARM, Bremen.
For a quantitative approach to cellular behaviour under microgravity,
four identical modules designed for video-tape recording of cellular
locomotion (velocity, orientation) were incubated at one time in the
drop capsule and operated by an integrated on-board computer.
A single module held an experimental chamber including 100 to 200
cells each.
Experiment Results: Image analysis of the data
revealed an orientational bias of vertically swimming cells prior to
transition to µg (swimming away from the center of gravity;
"negative gravitaxis";Paramecium: ro = 0.200;
Didinium: ro = 0.488). Negative gravitaxis
decayed gradually upon the onset of microgravity, but orientation
was not fully random by the end of the µg-period (Paramecium:
ro = 0.136; Didinium : ro = 0.032).
At 1 g, gravity-induced changes in locomotion speed of cells
(gravikinesis) were experimentally determined after accounting for
the sedimentation rates (S) of vertically downward and vertically
upward swimming cells (VD, VU). The null
hypothesis ("no gravikinesis exists under 1 g conditions") was
rejected because VD-VU was different from 2S
under normal gravity. Using the equation for determination of
the generalized value of gravikinesis (D): (VD-VU)/2
= S + Delta, and entering experimental data of VD (= 641
µm/s), VU (= 502 µm/s), and S (= 117 µm/s; Nagel et al.
1997), the value of gravikinesis was -48 µm/s, where the negative
sign indicates that gravikinesis acts to compensate part of the
sedimentation rate. After the onset of microgravity,
gravikinesis gradually decayed in the vertically swimming cells.
In Paramecium the differences between VD and VU
had vanished by the end of the µg-period; at this time, the median
swimming rate corresponded to the speed of horizontally swimming
cells (VH) at 1 g. We concluded that VH,
as measured under normal gravity conditions, approximates the
cellular propulsion rate (P) of Paramecium as being unaffected by
gravity. The gravikinesis of downward swimmers (DeltaD)
was calculated to be -14 µm/s, of upward swimmers (DeltaU)
-25 µm/s. Speed data obtained in the ciliate, Loxodes,
at 1 g and µg also indicated identity of VH (= 293 µm/s)
and P (=289 µm/s). In Didinium, vertical swimming
speeds converged in µg as in Paramecium, but the common value
during the final seconds of µg (= 1469 µm/s) was larger than the
horizontal speed under 1 g (1350 µm/s). These behavioural data
correspond to electrophysiological findings on the distribution of
mechanoreceptor channels in the plasma membrane of ciliates.
In Paramecium and Loxodes, a delicate balance between
gravity-induced activation of hyperpolarizing and depolarizing
channels neutralizes effects on the locomotion speed in horizontally
oriented cells. In Didinium, only a depolarizing type of
mechanoreceptors exists prevailing at the anterior cell end and
depressing the speed of horizontally and downward swimmers under
normal gravity. Absence of the gravitational input under µg
removed the depression of the locomotion speed in this cell.
The experimental data are in agreement with a previously published
model of electrophysiologically regulated gravisensory transduction
(Machemer et al. 1991).
