Threshold for Acceleration Sensitivity in Physarum.

Freeliving cells often use the gravity ("g") vector for their spatial orientation and also show distinct gravisensitivities. Plasmodia of the ameboid Myxomycete (acellular slime mold) Physarum polycephalum offer with the rhythmic contractions of their protoplasmic strands a sensitive parameter which can be modified by external stimuli. One approach to determine the gravity influence on cell functions is the signal deprivation. Near weightlessness "0 g" conditions are provided in space. Space experiments and ground-based 0 g-simulation experiments had established that the contraction frequency transiently increases after a transition from 1 g to 0 g with a back-regulating process starting after 30 min. In the experimental approach to determine the acceleration-sensitivity threshold a slow-rotating centrifuge microscope (NIZEMI-Niedergeschwindigkeits-Zentrifugen-Mikroskop) was used, providing accelerations from 0 g to 1.5 g. A stepwise acceleration increase revealed that the lowest acceleration levels capable to induce a response are between 0.2 and 0.4 g. This range for an accelerating-sensitivity threshold was recently found also in Paramecia, Euglena and jellyfish. The Physarum experiments also showed that acceleration increase and stimulus deprivation induce opposite responses (decrease and increase in contraction frequency, respectively). However, the time schedule of the responses and back-regulating process seems to be fixed (the latter can not be delayed by a further stimulus increase), suggesting that every acceleration being above the threshold is able to induce the complete response-regulation process.