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Algae
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. Trajectory control characteristics, and thus behavioral peculiarities, are connected with
both the shape of the cell and the functioning of the propelling structure of the algae, that is, the
flagellum. If the cell is asymmetric, that is, if it possesses recognizable dorsal and ventral sides, then
it advances spinning along its axis. It can correct its trajectory by either sudden steering obtained by
changing the insertion angle of flagella, as, for example, Ochromonas danica, or stiffening of the
flagellum via accessory structure of the axoneme, as Euglena gracilis. This behavior can be attributed to all heterokont and 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, as Dunaliella salina. This behavior
can be attributed to all isokont cells.
Signal Transmission
Signal transmission in algae is still a poorly investigated topic. Algae are aneural organisms, lacking
any system for the transmission of the stimuli received from the outside. The information carried
by light has to be translated into an organism-specific swimming control mechanism that allows
orientation to the light with high fidelity. Hence, the light signal will be first transduced in an electric signal by means of electron or ion flux, and this electric signal will be transmitted to the algae
motor apparatus, that is, the flagellum/flagella. It has been demonstrated that in Chlamydomonas
reinhardtii, light absorbed by sensory rhodopsins initiates local photocurrents (a fast photocurrent
and a slow photocurrent) in the eyespot region, presumably in the plasma membrane right above the
eyespot, where the photosensitive proteins are located. At low intensities, sensory rhodopsins trigger a highly efficient biochemical amplification reaction which involves activation of yet unknown
downstream elements and control of some diffusible messenger, a process analogous to vision in
animals. At high light intensities, the second function of sensory rhodopsins, namely direct channel
activity, begins to contribute to depolarization of the plasma membrane, which eventually allows
motility responses. Biphasic photocurrents have been shown to exist also in other algae, such as
Volvox carteri (Chlorophyta) and Cryptomonas sp. (Cryptophyta).
an example: phoTorecepTor and phoTorecepTion in EuglEna
As we have described in the previous section, in order for phototaxis (photoreception) to occur,
proper perceiving devices (structural features) satisfying essential requirements are necessary
together with specific behavioral and physiological features. In this section, we will use Euglena
as an example to analyze how these structural, behavioral, and physiological aspects combine to
achieve a concerted response to light stimuli. Euglena dwells in natural shallow ponds and uses
sunlight as source of energy and information. Its chloroplasts are the energy-supplying devices,
whereas the simple but sophisticated photoreceptive system already described is used as a light
detector.
The photoreceptor ciliary line of evolution, which had its climax in the elaborate and remarkably
complex vertebrate eye, originated from the photoreceptor of Euglena. According to the definition
of the Swiss genetist Walter Gehring, the prototypical eye, which was presumably the common
ancestor of all eyes, is a combination of a photoreceptor cell and a pigment cell, which achieves
some directional selectivity by using screening pigment to block light coming from certain directions. The photoreceptor cell is located close to the effector and transmits the information conveyed
by the light directly to it (without an intervening information-processing organelle). In the case of
Euglena, the “eye” is formed within a single cell by the assembly of pigmented and photoreceptive
molecules within that cell in two distinct organelles, namely the eyespot and the photoreceptor. In
the following, we will describe and analyze the different components of this primitive eye to demonstrate how its simple design fits Gehring’s definition.
Algae
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. Trajectory control characteristics, and thus behavioral peculiarities, are connected with
both the shape of the cell and the functioning of the propelling structure of the algae, that is, the
flagellum. If the cell is asymmetric, that is, if it possesses recognizable dorsal and ventral sides, then
it advances spinning along its axis. It can correct its trajectory by either sudden steering obtained by
changing the insertion angle of flagella, as, for example, Ochromonas danica, or stiffening of the
flagellum via accessory structure of the axoneme, as Euglena gracilis. This behavior can be attributed to all heterokont and 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, as Dunaliella salina. This behavior
can be attributed to all isokont cells.
Signal Transmission
Signal transmission in algae is still a poorly investigated topic. Algae are aneural organisms, lacking
any system for the transmission of the stimuli received from the outside. The information carried
by light has to be translated into an organism-specific swimming control mechanism that allows
orientation to the light with high fidelity. Hence, the light signal will be first transduced in an electric signal by means of electron or ion flux, and this electric signal will be transmitted to the algae
motor apparatus, that is, the flagellum/flagella. It has been demonstrated that in Chlamydomonas
reinhardtii, light absorbed by sensory rhodopsins initiates local photocurrents (a fast photocurrent
and a slow photocurrent) in the eyespot region, presumably in the plasma membrane right above the
eyespot, where the photosensitive proteins are located. At low intensities, sensory rhodopsins trigger a highly efficient biochemical amplification reaction which involves activation of yet unknown
downstream elements and control of some diffusible messenger, a process analogous to vision in
animals. At high light intensities, the second function of sensory rhodopsins, namely direct channel
activity, begins to contribute to depolarization of the plasma membrane, which eventually allows
motility responses. Biphasic photocurrents have been shown to exist also in other algae, such as
Volvox carteri (Chlorophyta) and Cryptomonas sp. (Cryptophyta).
an example: phoTorecepTor and phoTorecepTion in EuglEna
As we have described in the previous section, in order for phototaxis (photoreception) to occur,
proper perceiving devices (structural features) satisfying essential requirements are necessary
together with specific behavioral and physiological features. In this section, we will use Euglena
as an example to analyze how these structural, behavioral, and physiological aspects combine to
achieve a concerted response to light stimuli. Euglena dwells in natural shallow ponds and uses
sunlight as source of energy and information. Its chloroplasts are the energy-supplying devices,
whereas the simple but sophisticated photoreceptive system already described is used as a light
detector.
The photoreceptor ciliary line of evolution, which had its climax in the elaborate and remarkably
complex vertebrate eye, originated from the photoreceptor of Euglena. According to the definition
of the Swiss genetist Walter Gehring, the prototypical eye, which was presumably the common
ancestor of all eyes, is a combination of a photoreceptor cell and a pigment cell, which achieves
some directional selectivity by using screening pigment to block light coming from certain directions. The photoreceptor cell is located close to the effector and transmits the information conveyed
by the light directly to it (without an intervening information-processing organelle). In the case of
Euglena, the “eye” is formed within a single cell by the assembly of pigmented and photoreceptive
molecules within that cell in two distinct organelles, namely the eyespot and the photoreceptor. In
the following, we will describe and analyze the different components of this primitive eye to demonstrate how its simple design fits Gehring’s definition.