95
Anatomy
In water, at 20°C the kinematic viscosity is 10 −2 cm 2 s −1 .
A 50-μm long alga swimming at 10 μm s −1 has the minuscule Reynold’s number of 5 × 10 −4 ,
hence inertial effects are vanishing small and the major constraint is the viscous drag; this means
that what an algae is doing at the moment is entirely determined by the forces that are exerted on it
at that moment, and by nothing in the past. Therefore, when the flagellum stops, forward movement
of the cell will cease abruptly without gradual deceleration.
Swimming strategies employed by larger organisms that operate at high Reynolds number, such
as fish, birds, or insects, are ineffective at the small scale. Reynolds number might rise up to 10 5
for a fish that is 10 cm long and swims at 1 m s −1 . 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. Any attempt to
move by imparting momentum to the fluid, as is done in paddling, will be foiled by the large viscous
damping. Therefore, microorganisms have evolved propulsion strategies that successfully overcome
and exploit drag.
Another funny thing about motion at low Reynolds number is reciprocal motion. Since time does
not matter, the deformation that produces the swimming must be asymmetrical. Therefore, the pattern of flagellar beating must be 3D 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 μm) and this ratio is present in many algae. When the cell body size is larger than predicted, as in Euglena, the effective radius of the flagellum is modified by simple, nontubular hairs.
Beat patterns of most smooth flagella (i.e., without hairs) are three-dimensional, and the analysis 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 of wave propagation. These stiff hairs
remain perpendicular to the axis of the flagellum as it bends (Figure 2.60). 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 flagella 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.
Examples of main swimming patterns among algae will be described in the following.
In Ochromonas sp. (Ochrophyta), only the flagellum bearing hairs seems to be active during
swimming (Figure 2.60). It is directed forward and executes simultaneous undulatory and helical
waves that travel from its base to the tip. The resulting flagellum movements cause the whole body
of the cell to rotate as it moves forwards. The shorter flagellum trails backward passively, lying
against the cell; it is capable of acting as a rudder to steer the cells. The two rows of stiff hairs cause
a reversal of the flagellum thrust. Water is propelled along the flagellum from the tip to the base, so
that the cell is towed forward in the direction of the flagellum (Figure 2.61).
In desmokont dinoflagellates such as Prorocentrum sp., the longitudinal flagellum, which extends
apically, beats with an anterior-to-posterior whipping action, generating a wave in a tip-to-base
mode. The second flagellum, perpendicular to the first, is coiled and attached to the cell body except
for the tip, which beats in a whiplash motion, while the attached part undulates (Figure 2.62). In
Anatomy
In water, at 20°C the kinematic viscosity is 10 −2 cm 2 s −1 .
A 50-μm long alga swimming at 10 μm s −1 has the minuscule Reynold’s number of 5 × 10 −4 ,
hence inertial effects are vanishing small and the major constraint is the viscous drag; this means
that what an algae is doing at the moment is entirely determined by the forces that are exerted on it
at that moment, and by nothing in the past. Therefore, when the flagellum stops, forward movement
of the cell will cease abruptly without gradual deceleration.
Swimming strategies employed by larger organisms that operate at high Reynolds number, such
as fish, birds, or insects, are ineffective at the small scale. Reynolds number might rise up to 10 5
for a fish that is 10 cm long and swims at 1 m s −1 . 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. Any attempt to
move by imparting momentum to the fluid, as is done in paddling, will be foiled by the large viscous
damping. Therefore, microorganisms have evolved propulsion strategies that successfully overcome
and exploit drag.
Another funny thing about motion at low Reynolds number is reciprocal motion. Since time does
not matter, the deformation that produces the swimming must be asymmetrical. Therefore, the pattern of flagellar beating must be 3D 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 μm) and this ratio is present in many algae. When the cell body size is larger than predicted, as in Euglena, the effective radius of the flagellum is modified by simple, nontubular hairs.
Beat patterns of most smooth flagella (i.e., without hairs) are three-dimensional, and the analysis 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 of wave propagation. These stiff hairs
remain perpendicular to the axis of the flagellum as it bends (Figure 2.60). 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 flagella 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.
Examples of main swimming patterns among algae will be described in the following.
In Ochromonas sp. (Ochrophyta), only the flagellum bearing hairs seems to be active during
swimming (Figure 2.60). It is directed forward and executes simultaneous undulatory and helical
waves that travel from its base to the tip. The resulting flagellum movements cause the whole body
of the cell to rotate as it moves forwards. The shorter flagellum trails backward passively, lying
against the cell; it is capable of acting as a rudder to steer the cells. The two rows of stiff hairs cause
a reversal of the flagellum thrust. Water is propelled along the flagellum from the tip to the base, so
that the cell is towed forward in the direction of the flagellum (Figure 2.61).
In desmokont dinoflagellates such as Prorocentrum sp., the longitudinal flagellum, which extends
apically, beats with an anterior-to-posterior whipping action, generating a wave in a tip-to-base
mode. The second flagellum, perpendicular to the first, is coiled and attached to the cell body except
for the tip, which beats in a whiplash motion, while the attached part undulates (Figure 2.62). In