Sigrun Machemer-Röhnisch

Gravikinesis in Paramecium: Theory and isolation of a physiological response to the natural gravity vector

Summary1.We have investigated a physiological component of the gravitaxis of Paramecium using established mechanisms of ciliate mechanosensitivity. The horizontal, up and down swimming rates of cells, and the sedimentation of immobilized specimens were determined. Weak DC voltage gradients were applied to predetermine the Paramecium swimming direction.2.An observed steady swimming rate is the vector sum of active propulsion (P), a possible gravity-dependent change in swimming rate (Δ), and rate of sedimentation (S). We approximated P from horizontal swimming. S was measured after cell immobilization.3.Theory predicts that the difference between the down and up swimming rates, divided by two, equals the sum of S and Δ. Δ is supposed to be the arithmetic mean of two subcomponents, Δa and Δp, from gravistimulation of the anterior and posterior cell ends, respectively.4.A negative value of Δ (0.038 mm/s) was isolated with Δa(0.070 mm/s) subtracting from downward swimming, and Δp(0.005 mm/s) adding to upward propulsion. The data agree with one out of three possible ways of gravisensory transduction: outward deformation of the mechanically sensitive ‘lower’ soma membrane. We call the response a negative gravikinesis because both Δa and Δp antagonize sedimentation.

Neutral gravitaxis of gliding Loxodes exposed to normal and raised gravity

SummaryIn the statocystoid-bearing, flat ciliate Loxodes, the peculiar steady locomotion on submersed substrates (called “gliding”) was investigated between 1 g and 5.4 g under controlled environmental conditions in a centrifuge microscope. Videorecordings of the movements of large cell populations were processed with an automated analysis procedure. At 1 g, possible sedimentation was fully compensated, and vertical shifts of the population were neutralized because upward and downward orientations of the cells occurred at equal proportions (“neutral gravitaxis”). With rising gravity the resultant velocity of upward-gliding cells remained unchanged, whereas the velocity of downward-gliding cells increased continuously. Long-term exposure to hypergravity did not generate detectable signs of adaptation. The bipolar orientation of Loxodes persisted even under fivefold normal gravity, but the axis of orientation rotated from the gravity axis in the counterclockwise direction. The data suggest that both gravikinesis and graviorientation of gliding Loxodes are instrumental in perfect neutralization of sedimentation at terrestrial conditions.

Electric-field effects on gravikinesis in Paramecium

Equilibrated Paramecium caudatum cells exposed to a constant DC gradient reorient with their depolarized anterior ends toward the cathode (galvanotaxis). Voltage gradients were applied to cells swimming either horizontally or vertically. Their velocity and orientation were recorded and compared to unstimulated cells. The DC field increased the horizontal velocity (= reference) up to 175% (galvanokinesis). Swimming velocities saturated after 1 min and were unchanged during the following 4 min. The upward and downward swimming velocities of stimulated cells were below those of horizontal swimmers. The difference in vertical rates (determining gravikinesis) was independent of variations in absolute velocity. Normalization of vertical velocities to horizontal velocities (= 100%) separated DC-field dependent changes from gravity-induced changes in velocities. A weak voltage gradient (0.3 V/cm) was most effective in raising downward gravikinesis up to threefold (-202 μm/s) above the unstimulated reference (-66 μm/s) and to change sign of gravikinesis in upward swimmers (-43 μm/s →+33 μm/s). We conclude that DC-field stimulation is equivalent to a depolarizing bias on gravikinetic responses of Paramecium. The stimulation does not directly interfere with mechanoreception, but modulates somatic Ca2+ entry to induce contraction of the cell soma. This presumably affects the gating of gravisensory transduction channels.

A gravity-induced regulation of swimming speed in Euglena gracilis

Abstract We investigated the autotrophic flagellate Euglena gracilis for gravity-induced modulation of the speed of swimming as previously documented for larger protozoan cells. Methods of video-tracking of swimming and sedimenting cells under 1 g and hypergravity up to 2 g, and computer-assisted data processing were applied. The vertical and horizontal swimming speed, and sedimentation rates of immobilized cells, were found to be linear functions of acceleration. Accounting for sedimentation in the observed upward and downward movements of Euglena, the active component of speed (propulsion) rose in proportion to acceleration. No saturation of gravikinesis was seen within the g-range tested. Gravity-dependent augmentation of speed was maximal in upward swimmers and decreased continuously over horizontal to downward swimmers. Linear extrapolations of the data to zero-g conditions suggest the absence of a threshold of gravikinesis in Euglena. Energetic considerations indicate a high sensitivity of gravitransduction near the level of Brownian molecular motion.

Evidence of central and peripheral gravireception in the ciliate Loxodes striatus

Abstract Effects of the density of the external medium on gravireception in Loxodes striatus were investigated using Percoll solutions. With increasing density, the swimming rates changed from prevailing in the downward direction to prevailing in the upward direction. A cellular density of 1.036 g cm−3 was determined measuring direction and speed of sedimenting immobilized cells at different accelerations and medium densities. Viscosity increases by Percoll were measured and taken into account. At 30% air saturation Loxodes maintained a negative gravikinesis of approximately −27 μm s−1 at external densities corresponding to cellular density (±0.02 g cm−3). Negative gravikinesis decreased gradually to −9 μm s−1 with the density difference rising from 0.020 to 0.036 g cm−3 (=normal). The data indicate the existence of central gravireception, presumably by the Müller organelle, to generate in swimming Loxodes a constant value of gravikinesis and a bimodal gravitaxis. Peripheral gravireception occurs, in addition to central gravireception, when the transmembrane density difference exceeds 0.02 g cm−3. Peripheral gravireception can neutralize, in part, gravikinesis as raised by the central gravireceptor. We hypothesize that both central and peripheral gravireception of Loxodes guide vertical locomotion in gliding and swimming cells.

Velocity and Graviresponses in Paramecium during Adaptation and Varied Oxygen Concentrations

Summary Populations of Paramecium caudatum adapted to new O 2- and chemical environments within at least 6 hours show two subgroups of cells which are distinguished by their median swimming velocity. “Fast swimmers” prevailed at the beginning, and “normal swimmers” toward the end of the adaptation period. The median swimming rate of these two groups was largely constant throughout 6 hours, apart from initial disturbances following transfer into a closed environment. The degree of air saturation of the experimental solution (5% to 100%) was monitored continuously in a new flow-through chamber. Changes in the O 2 level did not have a consistent effect on the swimming velocity. Negative gravitactic orientation in normal swimmers gradually decreased with incubation time. Gravitaxis was absent in the fast swimmers. Variation in O 2 tension did not produce perceptible effects on gravitaxis. At all air saturations and incubation times negative gravikinesis was found, both in normal and in fast swimmers, centering near −40 µm/s. Gravikinesis correlated with swimming velocity, but not with O 2 tension. No correlation was found between the kinesis and orientation coefficients. The data suggest that gravikinesis in Paramecium is largely independent of O 2 levels and adaptation time. Graviorientation declines with adaptation, but is not a direct function of O 2 saturation of the medium.

A Ca paradox: Electric and behavioural responses of Paramecium following changes in external ion concentration

Summary 1. Raising [Ca2+]o depolarized the membrane but caused augmented forward swimming which is characteristic of hyperpolarizing stimulation. 2. Lowering [Ca2+]o hyperpolarized the membrane but induced backward swimming or reduction in forward swimming; these motor responses are known to follow depolarizing stimulation. 3. The apparent inconsistencies in responses (“Ca paradox”) were suspended when changes in Ca2+ concentration had been compensated by equivalent amounts of Mg2+. 4. Mechanical disturbance of the cells during solution transfer affected the early electrical and behavioural responses of Paramecium for up to 5 min. 5. External Ca2+ modulated the rates of forward swimming, but not the frequency rates of reversals. Swimming was elevated in high-Ca solutions; peak rates of reversals occurred in solutions of high Mg2+ plus low Ca2+. 6. The analysis of the data suggests that ionically-induced changes of observed membrane potentials do not always reflect changes of the transmembrane potential which controls the behaviour via modulation of voltage-sensitive membrane channels. 7. The data are discussed using a model [22] which resolves the Ca paradox by incorporation of three instantaneous effects on the membrane potential of [Ca2+]o (and other divalent cations): (1) external surface charge neutralization, (2) shifts in equilibrium potential, and (3) changes in ion conductance. 8. Three observed parameters: Ca-dependent shifts in current-voltage relation, changes in input resistance, and behavioural adaptation suggest that the transmembrane potential accommodates (i.e. returns to rest in a time-dependent manner)

Graviresponses of gliding and swimming Loxodes using step transition to weightlessness.

Cells of Loxodes striatus were adjusted to defined culturing, experimental solution, O2‐supply, temperature, and state of equilibration to be subjected to step‐type transition of acceleration from normal gravity (1 g) to the weightless condition (μg) during free fall in a 500‐m drop shaft. Cellular locomotion inside a vertical experimental chamber was recorded preceding transition and during 10 s of μg. Cell tracks from video records were used to separate cells gliding along a solid surface from free swimmers, and to determine gravitaxis and gravikinesis of gliding and swimming cells. With O2 concentrations ≥ 40% air saturation, gliders and swimmers showed a positive gravitaxis. In μg gravitaxis of gliders relaxed within 5 s, whereas gravitaxis relaxation of swimmers was not completed even after 10 s. Rates of horizontal gliders (319 μm/s) exceeded those of horizontal swimmers (275 μm/s). Relaxation of gravikinesis was incomplete after 10 s of μg. Analysis of the locomotion rates during the g‐step transition revealed that gliders sediment more slowly than swimmers (14 versus 45 μm/s). The gravikinesis of gliders cancelled sedimentation effects during upward and downward locomotion tending to maintain cells at a predetermined level inside sediments of a freshwater habitat. At ≥ 40% air saturation, gravikinesis of swimmers augmented the speed of the majority of cells during gravitaxis, which favours fast vertical migrations of Loxodes.

Is gravikinesis in Paramecium affected by swimming velocity

Summary Negative gravikinesis is an acceleration-activated modulation of vertical swimming velocity which can neutralize the passive sedimentation of ciliates. This paper investigates effects of swimming velocity levels on gravikinesis. Evaluation of a large number of data of free swimming cells with velocities ranging from 500 to 2000 μ m/s show that the ratio of vertically upward (and downward) swimming cells over horizontally swimming cells is represented by linear functions with slopes of 1. This is equivalent to a constant value of gravikinesis ( Δ ) in upward swimming cells ( Δ U ) and downward swimming cells ( Δ D ) irrespective of the swimming rate. Mechanoresponses due to velocity-dependent membrane deformations (rheokinesis) do not affect determinations of gravikinesis and may not exist at all. DC-field stimulation interferes with the slopes of swimming rates depressing vertical velocities as referred to horizontal velocity. The reduction of slopes (