94
Algae
in the range 200–500 μm s −1 , are said to maintain themselves near the surface by repeated bursts of
upward swimming, alternating with short intervals of rest during which they slowly sink. Motility is
present in unicellular algae or colonies that are propelled by flagella; in some classes, it is confined
only to gametes and asexual zoospores provided with flagella, which are used as a motor system for
their displacement in the fluid medium.
In order to move through a fluid, the swimming cell must use its motor system to push a portion
of the fluid medium in the direction opposite to that in which the movement is to take place.
The physics governing swimming at the micrometer scale experienced by algae is different from
the physics of swimming at the macroscopic scale. Forward movement of a swimming alga is resisted
by two things: the inertial resistance of the fluid that must be displaced, which depends upon the density of the fluid, and the viscous drag experienced by the moving organism, that is, the rearward force
exerted on the organism by the fluid molecules adhering to its surface when it passes through the viscous fluid. The world of microorganisms is the world of low “Reynolds number,” a world where inertia
plays little role and viscous damping is paramount. The Reynolds number (R) is defined as the ratio of
inertial and viscous forces; it depends on the size of the organism (related to the linear dimension, l),
its velocity (u), and to the density (ρ) and viscosity (η) of the fluid medium according to the equation:
R
l u
=
=
−
−
(
) (
)
.
inertial forces viscous forces
1
1
rh
(2.1)
Since the ratio between the viscosity and the density is the kinematic viscosity (ν = ηρ −1 in
cm 2 s −1 ), Equation 2.1 can be written as
R = luv −1
(2.2)
FIGURE 2.59 Root system of Euglenophyceae. F1, F2: flagella; DR: dorsal root; IR: intermediate root; VR:
ventral root; TSC: transversely striated connective.
Algae
in the range 200–500 μm s −1 , are said to maintain themselves near the surface by repeated bursts of
upward swimming, alternating with short intervals of rest during which they slowly sink. Motility is
present in unicellular algae or colonies that are propelled by flagella; in some classes, it is confined
only to gametes and asexual zoospores provided with flagella, which are used as a motor system for
their displacement in the fluid medium.
In order to move through a fluid, the swimming cell must use its motor system to push a portion
of the fluid medium in the direction opposite to that in which the movement is to take place.
The physics governing swimming at the micrometer scale experienced by algae is different from
the physics of swimming at the macroscopic scale. Forward movement of a swimming alga is resisted
by two things: the inertial resistance of the fluid that must be displaced, which depends upon the density of the fluid, and the viscous drag experienced by the moving organism, that is, the rearward force
exerted on the organism by the fluid molecules adhering to its surface when it passes through the viscous fluid. The world of microorganisms is the world of low “Reynolds number,” a world where inertia
plays little role and viscous damping is paramount. The Reynolds number (R) is defined as the ratio of
inertial and viscous forces; it depends on the size of the organism (related to the linear dimension, l),
its velocity (u), and to the density (ρ) and viscosity (η) of the fluid medium according to the equation:
R
l u
=
=
−
−
(
) (
)
.
inertial forces viscous forces
1
1
rh
(2.1)
Since the ratio between the viscosity and the density is the kinematic viscosity (ν = ηρ −1 in
cm 2 s −1 ), Equation 2.1 can be written as
R = luv −1
(2.2)
FIGURE 2.59 Root system of Euglenophyceae. F1, F2: flagella; DR: dorsal root; IR: intermediate root; VR:
ventral root; TSC: transversely striated connective.