Page 153 - Algae Anatomy, Biochemistry, and Biotechnology

136 Algae: Anatomy, Biochemistry, and Biotechnology

TABLE 3.1
Sun Light Reflected by Sea Surface

Angle between Sun rays and zenith 08 108 208 308 408 508 608 708 808 908
Percentage of reflected light 2 2 2.1 2.1 2.5 3.4 6 13.4 34.8 100

water column including water. In fact, different wavelengths of light do not penetrate equally, infra-
red light (700–4000 nm) penetrates least, being almost entirely absorbed within the top 2 m, and
ultraviolet light (300–400 nm) is also rapidly absorbed. Within the visible spectrum (400–
700 nm), red light is absorbed first, much of it within the first 5 m. In clear water the greatest
penetration is by the blue-green region of the spectrum (450–550 nm), while under more turbid
conditions the penetration of blue rays is often reduced to a greater extent than that of the
yellow-red wavelengths (550–700 nm). Depending on the conditions about 3–50% of incident
light is usually reflected, and Beer’s law can describe mathematically the way the light decreases
as function of depth,

I z ¼ I 0 e kz (3:1)

where I z is the intensity of light at depth z, I 0 is the intensity of light at depth 0, that is, at the surface,
and k is the attenuation coefficient, which describes how quickly light attenuates in a particular
body of water. Algae use the light eventually available in two main ways:

. As information in sensing processes, supported by the photoreceptors systems, which has
been already explained in Chapter 2
. As energy in transduction processes, supported by chloroplasts in photosynthesis

Both types of processes depend on the absorption of photons by electrons of chromophore mol-
ecules with extensive systems of conjugated double bonds. These conjugated double bonds create a
distribution of delocalized pi electrons over the plane of the molecule. Pi electrons are characterized
by an available electronic “excited state” (an unoccupied orbital of higher energy, higher meaning
the electron is less tightly bound) to which they can be driven upon absorption of a photon in the
range of 400–700 nm, that is, the photosynthetic active radiation (PAR). Only absorption of a
photon in this range can lead to excitation of the electron and hence of the molecule, because
the lower energy of an infrared photon could be confused with the energy derived by molecular
collisions, eventually increasing the noise of the system and not its information. The higher
energy of an UV photon could dislodge the electron from the electronic cloud and destroy the mol-
ecular bonds of the chromophore. Charge separation is produced in the chromophore molecule
elevated to the excited state by the absorption of a photon, which increases the capability of the
molecule to perform work. In sensing processes, charge separation is produced by the photoisome-
rization of the chromophore around a double bond, thus storing electrostatic energy, which triggers
a chain of conformational changes in the protein that induces the signal transduction cascade. In
photosynthesis, a charge separation is produced between a photo-excited molecule of a special
chlorophyll (electron donor) and an electron-deficient molecule (electron acceptor) located
˚
within van der Waals distance, that is, a few A. The electron acceptor in turn becomes a donor
for a second acceptor and so on; this chain ends in an electron-deficient trap. In this way, the
free energy of the photon absorbed by the chlorophyll can thereby be used to carry out useful elec-
trochemical work, avoiding its dissipation as heat or fluorescence. The ability to perform electro-
chemical work for each electron that is transferred is termed redox potential; a negative redox