Page 123 - Algae Anatomy, Biochemistry, and Biotechnology

106 Algae: Anatomy, Biochemistry, and Biotechnology

experimentally tested by Robert R. Birge of the W.M. Keck Center of the Syracuse University (per-
sonal communication). The absorption value recorded on a preparation of precisely ordered rho-
dopsin-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.

Noise
A signal consists of a true signal component and a noise component. Hence the ability to detect a
signal is limited by noise. Organisms must deal with noise of various kinds:

. The dark noise is the noise inherent in a receptor, constant and independent of light level. It
arises from the random thermal motions of the molecules.
. The photon noise (also known as shot noise) is due to the quantic nature of the light stimu-
lus and to the resulting statistical fluctuation in the capture of a photon. This noise can be
accurately estimated as the square root of the number of photons captured.
. The response noise leads to random variations in the locomotory response of the microor-
ganism. Response noise can be further divided into motor noise, in which variations occur
in locomotion from one time to another in an individual; and developmental noise, in which
variations occur between individuals, as a bias to turn toward one side.
. The environmental noise consists in extraneous signal arising from sources other than those
of the light signal.

The general problem is to decide whether a true signal is indeed present when a given signal is
observed. In the simplest case the problem becomes that of determining the threshold intensity
upon which to make the decision, that is, if the signal intensity is higher than the threshold intensity,
a true signal is detected; if the observed intensity is below the threshold it can be concluded that a
true signal is not present. Usually, if a situation is not well defined optimal threshold intensity can
be chosen, as the true signal intensity and noise intensity distributions overlap. The degree of over-
lapping is measured by the signal-to-noise (S/N) ratio.
In the absence of true signal (i.e., the signal is only the noise component and S/N ¼ 1) the only
strategy available is pure guessing, but the detector could guess in same way, for example, the
signal is present or the signal is absent, or choose some other combinations. If a unequivocal
true signal is available (S/N . 1000), a correct decision can always be made. For ambiguous
signals (S/N 10), right decisions are approximately 70%.
After all this theoretical stuff, let us calculate if the numbers of photons that reach the algal
photoreceptors are enough to be detected or can generate false alarms. These photoreceptors
have only to discriminate between two intensity levels (dark and light).
The light stimulus (S) that maximizes information transmission is intrinsically random and has
1/2
a root-mean-square deviation of standard deviation S . As we have seen before, 100 photons are
the true light signal in the sunniest day for the photoreceptor and 10 photons are the standard devi-
ation of this signal (ten and three, respectively, in a cloudy day). So the light intensity level can
oscillate from 110 to 90 photons (13 and 7 in a cloudy day). We can assume that the dark noise
1/2
level (N, with a standard deviation of N ) is about zero when a rhodopsin-like protein, with the
chemical–physical properties described earlier, is used as photoreceptive molecule. So the dark
intensity level is about zero. Detection theory states that the two intensity levels must differ by
2N 1/2 to be distinguishable with the 95% reliability.
On the basis of what we have said and, once calculated, the number of photons in the sunniest and
cloudy days, we can state that the simplest photoreceptor system of most algae could recognize light
from dark even in a cloudy day. Theory allows us to understand how one photon cell 21 sec 21 can actu-
ally lead to a persistent orientation in Chlamydomonas photon and is able to elicit an algae response.