Mechanisms of Gravitaxis inChlamydomonas

Abstract

The mechanisms by which many free-swimming microorganisms are able to swim preferentially upward despite the sedimenting effects of gravity (termed gravitaxis) have long been the subject of debate. Early suggestions were that upward swimming could be caused by cells either being back-heavy (1, 2, 3) or possessing fore-aft body asymmetry (4). However, there is a widespread perception that simple physical mechanisms cannot explain how physiologically active agents can influence gravitaxis, or how non-gravitactic mutants can exist; it is therefore argued that cells must contain a biological gravity sensor which somehow directs active swimming (5, 6). Published quantitative data on the unicellular biflagellated green alga Chlamydomonas reinhardtii are used here to show that shape orientation is primarily responsible for upward orientation in this organism. It is also shown that the resulting gravitational response is governed by the cells’ own swimming patterns. It is concluded that gravitaxis in Chlamydomonas is explicable in purely physical terms, and that no biosensor need be involved. Chlamydomonas cells whose flagella have been immobilized by chemical agents are found to settle in suspension with their flagella uppermost (7, 8). There are only two possible explanations. One is that the cell is back-heavy, resulting from a longitudinal density gradient that displaces the center of gravity behind the center of buoyancy, thereby producing a gravitational torque on the cell that causes orientation. The other is that the asymmetry of cell body and flagella, even when of uniform density, causes shape orientation during sedimentation because the larger cell body sediments downward faster than the flagella. Both mechanisms result in orientation towards the vertical at a rate described by the relation d dt sin (1)