Page 103 - Algae Anatomy, Biochemistry, and Biotechnology

86 Algae: Anatomy, Biochemistry, and Biotechnology

5
Reynolds number might get up to 10 for a fish long 10 cm swimming at 1 m sec 21 . If the same
fish would be swimming at the same Reynolds number of an alga, it would be as if it was swimming
inside molasses.
Another funny thing about motion at low Reynolds number is reciprocal motion. As time does
not matter the deformation that produces the swimming must be asymmetrical. Therefore, the
pattern of flagellar beating must be three-dimensional and asymmetric, that is, the forward
stroke should be different from the reverse.
For optimum propulsive efficiency, cell body size should be 15–40 times the flagellum radius
(about 0.1 mm), and this ratio is present in many algae. When the cell body size is larger than pre-
dicted, as in Euglena, the effective radius of the flagellum is modified by simple, non-tubular hairs.
Beat patterns of most smooth flagella (i.e., without hairs) are three-dimensional, and the analy-
sis of the motion is far from straightforward. However, it is clear that the direction in which the
microorganism moves is opposite to the direction in which the waves are propagated along the
length of the flagellum, so that in almost all cases, when the cell body is to be pushed along, a
wave must be initiated at the base of the smooth flagellum. Although basal initiation is more
common than distal, both are known. The velocity of forward movement is always a small fraction
of the velocity of the wave running along the flagellum, and its propulsive efficiency depends on the
ratio of its amplitude and wavelength. Unlike smooth flagella, the propulsive force generated by a
flagellum bearing tubular hairs is in the same direction as the direction of wave propagation. These
stiff hairs remain perpendicular to the axis of the flagellum as it bends (Figure 2.58). A wave
moving away from the cell body will cause the hairs to act as oars and the overall effect will be
to propel the cell flagellum first.
Control characteristics, and thus behavioral peculiarities, are connected with the functioning of
the propelling structure of the cell. If the cell is asymmetric, it advances spinning along its axis; it
can correct its trajectory only by sudden steering obtained by changing the insertion angle of fla-
gella, or by the stiffening of internal structures. This behavior can be attributed to all heterokont
or uniflagellate algae. In the case of a symmetric cell, it can accomplish a gradual smooth correction
of its trajectory going forward without spinning (or rotating with a very long period), and displacing
the barycenter of the motor couple. This behavior can be attributed to all isokont cells.

FIGURE 2.58 Negative staining of the trailing flagellum of Ochromonas danica. (Bar: 1 mm.)