Investigations carried out on the protozoan Paramecium tetraurelia and the cyanobacteria Synechococcus lividus, which were shielded against background radiation or exposed to very low doses of gamma radiation, demonstrated that radiation can stimulate the proliferation of these two single-cell organisms. Radiation hormesis depends on internal factors (age of starting cells) and external factors (lighting conditions). The stimulatory effect occurred only in a limited range of doses and disappeared for dose rates higher than 50 mGy/y.

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SUMMARYThe growth rate in glucose minimal medium and time of entry into the stationary phase in pepton cultures ware determined during the STS 42 mission of the space shuttle Dis. covery. Cells were cultured in plastic bags and growth was stopped at six different time points by lowering the temperature to 5°C, and at a single time point, by formaldehyde fixation. Based on cell number determination, the doubling time calculated for the flight samples of glucose cells was shorter (46 mini than for the ground samples (59 mini. However, a larger cell size expected for more rapidly growing cells was not observed by volume measurements with the electron'.,c particle counter, nor by eiccax)n microscopi(; measurement of cell dimensions. Only for ceils fixed in flight was a larger cell length and percentage of constricted cells found. An optical density increase -=n the peptone cultures shewed an earlier entry into the stationary phase in flight semples, but this could not be confirmed by viability counts. The single sample with cells fixed in flight showed properties indicative of gTowth stimulation. However, taking. ~H ~bservations together, we conclude that mFcrogravity has no effect on the growth rate of exponentially growing Escherichia coil cells.

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