title: Comparative studies of the graviresponses of Paramecium and Loxodes.
abstract: Gravitactic protozoa offer advantages in studying how the gravity stimulus is perceived on the cellular level. By means of a slow rotating centrifuge microscope in space the acceleration thresholds for gravitaxis of Loxodes striatus and Paramecium biaurelia were determined: < or = 0.15 x g for Loxodes and 0.3 x g for Paramecium, indicating different sensitivities of these species. Neutral-buoyant densities of  immobilized cells were determined using media of different densities, revealing densities of 1.03 to 1.035 g/cm3 for Loxodes and 1.04 g/cm3 to 1.045 g/cm3 for Paramecium. Behavioral studies revealed that gravitaxis of Loxodes persisted independent of the density of the medium. In contrast, negative gravitaxis of Paramecium was no longer measurable if the density of the medium approached the density of the cell. The results suggest that in the case of Loxodes gravity is perceived by an intracellular receptor and, in the case of Paramecium by its own mass via the pressure on the lower cell membrane.
authors.affiliation: DLR (German Aerospace Research Establishment), Institute of Aerospace Medicine, Section Biology, Koln, Germany.
authors.forename: R
authors.initials: R
authors.lastname: Hemmersbach
authors.forename: R
authors.initials: R
authors.lastname: Voormanns
authors.forename: B
authors.initials: B
authors.lastname: Bromeis
authors.forename: N
authors.initials: N
authors.lastname: Schmidt
authors.forename: H
authors.initials: H
authors.lastname: Rabien
authors.forename: K
authors.initials: K
authors.lastname: Ivanova
mesh_terms: D000818:Animals; D002498:Centrifugation; D016798:Ciliophora / Q000502:physiology*; D003470:Culture Media; D018466:Gravity Sensing / Q000502:physiology*; D018471:Hypergravity; D008124:Locomotion / Q000502:physiology*; D009949:Orientation / Q000502:physiology; D010247:Paramecium / Q000502:physiology*; D012399:Rotation; D013026:Space Flight; D013550:Swimming / Q000502:physiology; D014893:Weightlessness
chemical_list: D003470:Culture Media