Historical background

The phenomenon that some free-swimming unicellular organisms tend to swim to the top of a tube and gather there — independent of whether the tube is open or closed — has been observed more than 100 years ago. This behavior was termed geotaxis (orientation with respect to the gravity vector of the Earth) — negative geotaxis if the organisms orient upward and positive geotaxis if they swim downward (cf. Section 1.2). Nowadays, this term has been replaced by gravitaxis. Many early and detailed studies between 1880–1920 provided descriptive observations limited by optical and analytical means. This led to the establishment of various hypotheses that have been reviewed by different authors (Bean, 1984; Davenport, 1908; Dryl, 1974; Haupt, 1962b; Hemmersbach et al., 1999b; Jennings, 1906; Kuznicki, 1968; Machemer & Bräucker, 1992).

The results were rather conflicting and led to controversial interpretations. While Stahl (1880) stated that Euglena and Chlamydomonas do not orient with respect to gravity, Schwarz (1884) concluded from his observations that Euglena moves upward by an active orientational movement and is not passively driven (e.g., by currents in the water or attracted by oxygen at the surface). He found that the force of gravity could be replaced by centrifugal force and that Euglena could move upward against forces of up to 8.5 × g. The author also concluded that Euglena belongs to the negative geotactic organisms.