Passage III

Paramacrobiotus richtersi (a species of microscopic

invertebrate) are often found living on moist leaf litter and mosses. When their surround.ing environment dries out, active P. richtersi become desiccated (dried out) and may go dormant. Compared to active P. richtersi, dormant P. richtersi have a greatly reduced metabolic rate and can survive for years without food or water. Dormant P. richtersi can become active again once they are rehydrated.

An experiment was conducted to determine whether exposure to the high levels of ionizing radiation experienced in Earth orbit affects the survival of dormant P. richtersi.

Experiment

Four groups (Groups 1-4) were prepared with 8 sterilized petri dishes each. Then, a substrate—either 5.5 g of dry leaf litter or sterilized filter paper—was placed in dishes in a group. Next, 100 dormant P. richtersi were added to each dish. Groups 1 and 3 were transported into Earth orbit for 12 days, while Groups 2 and 4 remained on Earth (see Table 1). All four groups were kept at the same temperature and humidity throughout the experiment.

Once Groups 1 and 3 were returned to Earth, the P. richtersi in all four groups were rehydrated and the average percent survival (percent of the P. richtersi that were alive) was determined for each group (see Figure 1).