were fed with iron-particles to increase the density of the cytoplasm.
The swimming speed and orientation of iron-fed and iron-free control
cells were analyzed under terrestrial gravity and raised acceleration
up to 6 g in a centrifuge. Iron-fed cells sedimented at increased
rates (133 µm s-1 per g unit) as compared to controls (118 µm s-1 per g unit). Gravikinesis
increased in iron-fed cells with rising acceleration at a rate of
67 µm s-1 per g unit as compared to 46 µm s-1 per g unit in control cells. In particular, the gravikinesis
of downward swimming iron-fed cells was strongly enhanced, thereby
compensating for the sedimentation rate. This was not the case in
upward swimming cells, where ingested iron depressed the gravikinetic
response. The negative gravitaxis of Paramecium, as being represented by the cell orientation coefficient,
was much pronounced in iron-fed cells at 1 g (roc = 0.36; controls: roc = 0.13). At 4 g, the orientation coefficient of iron-fed
to 0.80 (controls: 0.54). The effect of artificially raised cytoplasmic
density on gravikinesis is explained at the basis of the mechanoreceptor
organization of Paramecium. The effect on gravitaxis continues to be uncertain.