Graviperception in the flagellate Euglena gracilis during a shuttle space flight

Area of Science:

• * Cellular Biology
• * Space Biology
• * Microgravity Research

Background:

• * Gravitaxis, the directional movement in response to gravity, is crucial for many microorganisms.
• * Understanding gravitaxis in unicellular organisms like Euglena gracilis provides insights into fundamental biological responses to gravity.
• * Previous studies on gravitaxis have been limited by the challenges of controlled gravity variation in space.

Purpose of the Study:

• * To investigate the gravitactic response of Euglena gracilis under varying accelerations during space flight.
• * To determine the characteristics of the gravireceptor involved in Euglena gracilis' gravitaxis.
• * To assess potential adaptation to microgravity and post-flight recovery of gravitactic behavior.

Main Methods:

• * In-flight experiments on the American shuttle Columbia utilizing a rotating centrifuge microscope (NIZEMI).
• * Controlled variation of acceleration between 0 and 1.5 x g to study gravitaxis.
• * Analysis of cell velocity distribution and response curves to determine gravitactic precision and sedimentation effects.

Main Results:

• * Euglena gracilis displayed a sigmoidal response curve for gravitaxis precision versus acceleration.
• * A physiological gravireceptor with a threshold at <= 0.16 x g and saturation at >= 1 x g was indicated.
• * No adaptation to microgravity was observed during the mission, with normal gravitactic behavior upon return.

Conclusions:

• * Euglena gracilis possesses an active physiological gravireceptor system for gravitaxis.
• * The organism's response to gravity is acceleration-dependent and does not show adaptation to microgravity.
• * Sedimentation influences swimming velocity, following Stokes' law, and Euglena gracilis does not exhibit gravikinesis.