Page 124 - Algae Anatomy, Biochemistry, and Biotechnology

Anatomy 107

Direction

In order to determine the direction of a light source, an alga must possess both a light detector and a
light screen to provide directional selectivity. Such a screen can be either an absorbing element that
prevents light coming from certain direction from reaching the detector or a refractive element that
focuses light onto the detector only from specific directions.
Femtoplankton (0.02–0.2 mm) are too small when compared with the wavelength of light to
create differential light intensity; hence, they cannot determine the direction of a light source. Still,
these microorganisms can use light, but can only measure its intensity and move in a light-intensity
gradient. In contrast, algae are large enough to determine light direction and move along a beam of
light by scanning the environment by means of their directionally sensitive receptor.

Guiding
To determine the orientation of a stimulus field, an organism has to measure the stimulus intensity
at different positions. The two fundamental alternative strategies for obtaining information are:
. Parallel sampling of the stimulus by multiple separated receptors positioned on different parts
of the organism surface. In this case the organism measures directly the spatial gradient by
comparing the intensities at the different positions (one instant mechanism) (Figure 2.71).
. Sequential sampling of the stimulus by a single receptor that moves from one place to
another. In this situation the organism measures directly a temporal gradient, and then
infers the spatial gradient from the information on the movements of the receptor, (two
instants mechanism) (Figure 2.72).

The simultaneous comparison of signals requires widely spaced receptors to detect intensity gradi-
ent, which makes large body size advantageous. On the other hand, sequential sampling requires a
coherent pattern of movement such as locomotion in a helical path. Sequential sampling also
requires some form of memory to allow the comparison with previously recorded intensities.
Another fundamental distinction is based on whether an organism is able to make turns in its
motion path, which will direct it toward its destination. Depending on the characteristic of the
stimulus and the abilities of the microorganism, guiding may be either direct in the sense of
taking a straight-line path to the destination or indirect, as in the case of a biased random walk,
to reach the vicinity of the destination.
Phototaxis is the behavior that involves orientation in response to light stimuli that carry the
information. This behavior consists in a migration oriented with respect to the stimulus direction
or gradient, which is established and maintained by direct turns (phototaxis is different from photo-
tropism. This term is frequently used for the behavior in which the organisms respond to the light
response with an oriented growth). Photokinesis is the behavior that comprises undirected
responses dependent on either the intensity or temporal changes in intensity of the stimulus.

Trajectory Control

Control characteristics, and thus behavioral peculiarities, are connected with the functioning of the
propelling structure of the algae, that is, the flagellum. If the cell is asymmetric, it advances spin-
ning along its axis; it can correct its trajectory by either sudden steering obtained by changing the
insertion angle of flagella or stiffening of the flagellum via accessory structure of the axoneme. 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 beha-
vior can be attributed to all isokont cells.