Page 102 - Algae Anatomy, Biochemistry, and Biotechnology

Anatomy 85

HOW ALGAE MOVE
Cytoplasm, cell walls, and skeletons of algae have a density greater than the medium these organ-
isms dwell in. The density of fresh water is 1.0 g cm 23 and that of sea water ranges from 1.021 to
1.028 g cm 23 , but most cytoplasmic components have a density between 1.03 and 1.10 g cm 23 , the
silica forming the diatom frustule and the scales of Chrysophyceae have a density of 2.6 g cm 23 ,
and both calcite and aragonite of Haptophyta coccoliths reach an even higher value of
23
2.7 g cm . With this density values, algae must inevitably sink. Therefore, one of the problems
facing planktonic organisms (organisms that wander in the water or are carried about by the move-
ments of the water rather than by their own ability to swim) is how to keep afloat in a suitable atti-
tude between whatever levels are suitable for their life. The phytoplankton must obviously remain
floating quite close to the surface because only here is there a sufficient illumination for photosyn-
thesis. There are broadly two solutions by which algae can keep afloat and regulate their orientation
and depth: a dynamic solution, obtaining lift by swimming; and a static solution, by buoyancy
control, or through adaptations reducing sinking rates. In many cases the two solutions function
together.

Swimming

What does it mean to swim? It means that an organism immersed in a liquid environment is allowed
to deform its body in the same manner. The algae are all good movers or better good swimmers.
They swim more or less continuously and control their level chiefly by this means. For example
dinoflagellates, which can achieve speeds of 200–500 mm sec 21 , 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 fla-
gella, which are used as 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.
Forward movement of a swimming alga is resisted by two things: the inertial resistance of the
fluid that must be displaced, which depends on the density of the fluid and the viscous drag experi-
enced by the moving organism, that is, the rearward force exerted on the organism by the fluid mol-
ecules adhering to its surface when it passes through the viscous fluid. The ratio of inertial and
viscous forces is the Reynolds number (R), which depends on the size of the organism (related
to the linear dimension, (l)), its velocity (u), and to the density (r) and viscosity (h) of the fluid
medium according to the equation:

R ¼ (Inertial forces) (Viscous forces) 1 ¼ l u r h 1 (2:1)

2
1
As the ratio between the viscosity and the density is the kinematic viscosity (n in cm sec ),
Equation (2.1) can be written as:
R ¼ l u n 1 (2:2)
2
In water the kinematic viscosity is 10 22 cm sec 21 .
An algae 50 mm long swimming at 10 mm sec 21 has the minuscule Reynold’s number of
4
5 10 , hence inertial effects are vanishing 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.