Though distinct behavioral responses to altered gravitational stimulation have been re-
ported, the gravireceptor candidates in Physarumremain speculative: nuclei and mito-
chondria have been proposed, both occurring in high numbers in this giant single cell.
The accumulation of zoospores near the surface of the medium raises the still-
unsolved question of whether this behaviour is guided by gravity or by oxygen (Cameron
and Carlile 1977). The gravity-induced, altered physiology of sperm cells is of high in-
terest due to possible impacts on reproduction and thus, for example, food supply during
long-term space flights (Engelmann et al. 1992; Tash and Bracho 1999). In the case of
bacteria, effects of microgravity on, e.g., growth rate, sporulation and phage productivity
have been reported (for reviews, see Mennigmann and Lange 1986; Cogoli and Gmünder
1991); however, reasons and mechanisms remain speculative. According to the model de-
rived by Klaus and coworkers (1997), it seems likely that a “cumulative effect of gravity
may have a significant impact on suspended cells via their fluid environment, where an
immediate, direct influence of gravity may otherwise be deemed negligible.”
7.4 Significance of gravitropism in single-cell systems
Among the few single-cell systems that respond gravitropically are the rhizoids of
characean green algae, which share similar functions with roots of higher plants. They
anchor the organism in the substrate by penetrating mud and soil, thereby enabling stabi-
lized upward growth of the shoots. Other gravitropically responding cell types, including
moss caulonemata and protonemata and characean protonemata, seek to grow upward in
darkness in order to find optimal ecological conditions where the plants harvest light, re-
CHAPTER 7 SINGLE-CELL GRAVITROPISM AND GRAVITAXIS 143
Figure 7.1. Unicellular systems used to study gravity sensing. A. Paramecium biaurelia (170 μm), seen in
phase contrast. B. Loxodes striatus(150 μm), bright field; arrows indicate Müller organelles (MO). C.
Euglena gracilis(50 μm), seen with differential interference contrast.