Graviresponses of certain ciliates and flagellates

Abstract

Protozoa are eukaryotic cells and represent suitable model systems to study the mechanisms of gravity perception and signal transduction due to their clear gravity-induced responses (gravitaxis and gravikinesis). Among protists, parallel evolution for graviperception mechanisms have been identified: either sensing by distinct stato-organelles (e.g., the Müller vesicles of the ciliate Loxodes) or by sensing the density difference between the whole cytoplasm and the extracellular medium (as proposed for Paramecium and Euglena). These two models are supported by experiments in density-adjusted media, as the gravitaxis of Loxodes was not affected, whereas the orientation of Paramecium and Euglena was completely disturbed. Both models include the involvement of ion channels in the cell membrane. Diverse experiments gave new information on the mechanism of graviperception in unicellular systems, such as threshold values in the range of 10% of gravity, relaxation of the responses after removal of the stimulus, and no visible adaptation phenomena during exposure to hypergravity or microgravity conditions for up to 12 days.

Non-Skeletal Biomineralization by Eukaryotes: Matters of Moment and Gravity

“…By contrast, barite or celestite statocytes (or any other statocytes) are relatively rare in gravitropic ciliates and flagellates (Hemmersbach & Häder 1999;Hemmersbach et al 2005). Graviperception in the absence of statocytes is apparently a result of gravity on the cytoplasmic contents acting on stretch-sensitive ion channels in what, for the moment, is the lower part of the plasmalemma.…”

“…By increasing overall cell density magnetosomes increase the energy cost of upward movement at a given speed and decrease that of downward movement. Graviperception in the ciliates involves Müller bodies, biomineralized microconcretions that contain either strontium or barium as celestite and barite, respectively (Hemmersbach & Häder 1999;Bazylinski et al 2000;Hemmersbach et al 2005). …”

“…Celestite occurs as crystals in the cytoplasm of the flagellate swarmers of some colonial radiolarians/acatharians (Hughes et al 1989;Anderson et al 1990). Some ciliates (Alveolata) have Müller bodies containing celestite or barite that act as statocytes in graviperception of these gravitactic organisms (Hemmersbach & Häder 1999;Hemmersbach et al 2005). Finally, barite particles occur in at least two plankonic flagellate species within the Chromista (phylum Haptophyta; class Prymnesiophyceae, order Pavlovales; Fresnel et al 1979, Gayal and.…”

“…If denser components of the cell are at one peripheral part of the cell then this part of the cell will orient at the bottom; this can cause positive gravitaxis if the placement of the ballast relative to the flagella and their direction of functioning causes downward swimming, or vice versa for negative gravitaxis (Hemmersbach & Häder 1999;Hemmersbach et al 2005). This passive orientation does not involve movement of the denser components relative to the intracellular components of the cell, thereby differing from the statocyte mechanism.…”

Mechanomics and Physicomics in Gravisensing

“…Although physical processes, and therefore biological systems, are behaving differently under (near) weightlessness (Albrecht-Buehler 1991;Todd 1989;van Loon 2007c) including studies under hypergravity c.q. hyper-weight conditions provide information for the entire gravity spectrum (Hatton et al 2003;Hemmersbach and Haeder 1999;Thoumine et al 1996;Searby et al 2005;Tabony 2004;van Loon et al 1999). It was at first argued from a purely physical point of view, that non-specialized single cells might not be susceptible to small forces such as gravity (Pollard 1965;Went 1968), many studies have shown otherwise.…”