Page 122 - Algae Anatomy, Biochemistry, and Biotechnology

Anatomy 105

Sensitivity

The ability to perceive and adapt to changing light conditions is critical to the life and growth of
photosynthetic microorganisms. Light quality and quantity varies diurnally, seasonally, and with
latitude, and is influenced by cloud conditions and atmospheric absorption (e.g., pollution).
Competition for light in aquatic environment may be particularly fierce because of shading
among the different organisms and the rapid absorption of light in the water column. Illumination
of the surface layers varies with place, time, and conditions depending on the intensity of light
penetrating the surface and upon the transparency of the water. Hence, detecting light as low as
possible (i.e., a single photon) becomes an adaptive advantage, because a photosynthetic organism
in dim light can obtain more metabolic energy if it is able to move to more lighted and suitable
areas.
For detecting the direction of light of a specific spectral range, a photoreceptor demands a high
packing density of chromophore molecules organized in a lattice structure, with high absorption
cross-section of the chromophore, that is, high probability of photon absorption by the chromo-
phore, and very low dark noise (see later). For detecting patterns of light, the number and location
of photoreceptors having fixed size and exposure time must be viewed according to the pattern of
motion of algae. For transmitting the detected signal a photoreceptor must generate a potential
difference, or a current.
The most investigated photoreception system is that of Chlamydomonas. It consists of a patch
of rhodopsin-like proteins in the plasma membrane (Type I). The packing density of these
molecules appears to be about 20,000–30,000 mm 22 of membrane, with a molar absorption coeffi-
cient 1 of 40,000–60,000 M 21 cm 21 and a dark noise (see later) approximately equal to zero. The
number of embedded molecules per square micrometer of membrane, the absorption cross-section,
and the dark noise are at the best of theoretical limits. Nevertheless, the fraction of photon absorbed
from a single layer of these molecules is less than 0.05% (each layer contributes approximately to
0.005 OD).
Let us calculate how many photons this simplest but real photoreceptive system can absorb
(Type I). In a sunny day about 10 18 photons per square meter per second per nanometer are
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emitted by the sun (in a cloudy day the number of photons lowers to 10 ). As algae dwell in an
aquatic environment we have to consider absorption and reflection effects of the water, and we
lower this figure to 10 17 photons per second per nanometer (in a cloudy day the number of
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photons lowers to 10 ). This means that a photoreceptor of 1 mm can catch at most 10 photons
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per second per nanometer, that is, 10 photons in its 100 nm absorbance window in the sunniest
day. As only 0.05% of the incident radiation will be effectively absorbed by a single-layer photo-
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receptor, the amount of photons lowers to about 10 photons per second. The true signal that the
algae should discriminate results from the difference between the number of photons absorbed by
the photoreceptor when it is illuminated (for about 400 msec) and the number of photons absorbed
by the photoreceptor when it is shaded by a screen such as the eyespot (for about 600 msec).
In the case of the sunniest day the highest signal is lower than 100 photons per second, which
lowers to about 10 photons per second in a cloudy day. Even in the sunniest day the number of
photons absorbed for detecting light direction is very low. As this photoreceptor does not form
any image, but detects only light intensity, this amount of photons is enough; however, this photo-
receptor must posses a very low threshold and a very low or negligible dark noise (see later).
It has been demonstrated that not only single photons induce transient direction changes but
also fluence rates as low as 1 photon cell 21 sec 21 can actually lead to a persistent orientation in
Chlamydomonas (Chlorophyceae).
Strategies have been evolved to increase the sensitivity of a photoreceptor, as stacking-up many
pigment-containing membranes in the direction of the light path (Euglena gracilis is a wonderful
example of this solution) or exploiting the reflecting properties of the eyespot as in Chlamydomonas
and in different species of dinoflagellates. The effectiveness of the multilayer strategy has been