Protozoa as model systems for the study of cellular responses to altered gravity conditions

The rise of model protozoa

It is timely to evaluate the role of protozoa as model organisms given their diversity, abundance and versatility as well as the economic and ethical pressures placed on animal-based experimentation. We first define the term model organism and then examine through examples why protozoa make good models. Our examples reflect major issues including evolution, ecology, population and community biology, disease, the role of organelles, ageing, space travel, toxicity and teaching. We conclude by recognising that although protozoa may in some cases not completely mimic tissue- or whole-animal-level processes, they are extremely flexible and their use should be embraced. Finally, we offer advice on obtaining emergent model protozoa.

Graviorientation in protists and plants

Gravitaxis, gravikinesis, and gravitropism are different graviresponses found in protists and plants. The phenomena have been intensively studied under variable stimulations ranging from microgravity to hypergravity. A huge amount of information is now available, e.g. about the time course of these events, their adaptation capacity, thresholds, and interaction between gravity and other environmental stimuli. There is growing evidence that a pure physical mechanism can be excluded for orientation of protists in the gravity field. Similarly, a physiological signal transduction chain has been postulated in plants. Current investigations focus on the question whether gravity is perceived by intracellular gravireceptors (e.g. the Muller organelle of the ciliate Loxodes, barium sulfate vacuoles in Chara rhizoids or starch statoliths in higher plants) or whether the whole cell acts as a sedimenting body exerting pressure on the lower membrane. Behavioral studies in density adjusted media, effects of inhibitors of mechano-sensitive ion channels or manipulations of the proposed gravireceptor structures revealed that both mechanisms have been developed in protists and plants. The threshold values for graviresponses indicate that even 10% of the normal gravitational field can be detected, which demands a focusing and amplifying system such as the cytoskeleton and second messengers.

Influence of accelerations on the spatial orientation of Loxodes and Paramecium

The gravitactic ciliates Paramecium and Loxodes were cultivated for 15 days in space during the IML-2 spacelab mission. At dedicated times their behavioral responses to different accelerations between 10(-3) x g and 1.5 x g were investigated by using a slow rotating centrifuge microscope (NIZEMI). The threshold for gravitaxis of Paramecium was found to be at > 0.16 x g and < or = 0.3 x g. No adaptation of Paramecium to the conditions of weightlessness was observed over the duration of 15 days. Loxodes showed no graviresponses to increasing accelerations, though it demonstrated gravitaxis after return to earth.