How Algae Use Light Information
No physical quantity regulates and stimulates the developments of algae as strongly as light. Light
is an electromagnetic radiation characterized by its quality (different wavelengths) and intensity. To
detect light and to measure both parameters and react to them, algae photoreceptor systems have to
satisfy five main requirements:
- They should possess a photocycling protein
- They should possess high sensitivity
- They should be characterized by a low noise level
- They should detect either spatial or temporal patterns of light
- They should transmit the detected signal in order to modify the cell behavior
Upon absorption of a photon, the photocycling protein undergoes a series of conformational
changes generating intermediate state(s); one of these states is the “active” state that will start signal transmission. The last intermediate state is driven back to the original state of the protein, by either a thermal process, or a second absorbed photon of different wavelength. The primary event in the photoreceptive process is the structural change of the chromophore (isomerization)
to which the protein adapts. It occurs within a few picoseconds after the absorption of a photon,
and this is one of the fastest biological processes in nature. The whole photocycle is very fast
(order of microseconds or less), hence the intracellular response is immediately reset so that the
system is prepared for a new light signal, and algae must respond rapidly on a time scale of milliseconds to seconds as environmental conditions change or as they change position relative to their
static surroundings. A photoreceptor protein capable of photocycling is mandatory for algae whose
photoreceptive systems are an integral part of the cell body. This localization would not allow the continuous recovery of the exhausted photoreceptive proteins without interfering with a continuous and immediate response of the alga cell to the light.
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
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 chromophore,
and very low dark noise. 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 µm-2 of membrane, with a molar absorption coefficient 1 of 40,000–60,000 M-1 cm-1 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 1018 photons per square meter per second per nanometer are
emitted by the sun (in a cloudy day the number of photons lowers to 1017). As algae dwell in an aquatic environment we have to consider absorption and reflection effects of the water, and we
lower this figure to 1017 photons per second per nanometer (in a cloudy day the number of
photons lowers to 1016). This means that a photoreceptor of 1 µm2 can catch at most 105 photons per second per nanometer, that is, 107 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 photoreceptor, the amount of photons lowers to about 103 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 photoreceptor must posses a very low threshold and a very low or negligible dark noise.
It has been demonstrated that not only single photons induce transient direction changes but
also fluence rates as low as 1 photon cell-1 sec-1 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 experimentally tested by Robert R. Birge of the W.M. Keck Center of the Syracuse University (personal communication). The absorption value recorded on a preparation of precisely ordered rhodopsin-like proteins multilayers was about 95% of the incident light, very close to the
absorption value recorded on the photoreceptor crystal of Euglena, which consists of more than
100 layers of proteins.