According to this formula the shorter the photon wavelength, the higher its energy; for example, the energy of one photon of 300 nm light is 6.63 x 10-19 J, the energy of one photon of 400 nm light is 4.97 x 10-19 J, the energy of one photon of 700 nm light is 2.84 x 10-19 J, and the energy of one photon of 4000 nm light is 0.49 x 10-19 J. The energy of photons can also be expressed in terms of electron volts (eV). Absorption of a photon can lead to excitation of an electron and hence of a molecule. This excited electron acquires potential energy (capacity of producing chemical work) measured in eV. An electron volt is the potential energy of 1V gained by the excited electron, which is equal to 1.60 x 10-19 J. Thus the energy of one photon of 300 nm light is equal to 4.14 eV, the energy of one photon of 400 nm light is equal to 3.11 eV, the energy of one photon of 700 nm light is equal to 1.77 eV, and the energy of one photon of 4000 nm light is equal to 0.30 eV.

The average intensity of the total solar radiation reaching the upper atmosphere is about 1.4 kW m^-2 (UV 8%, visible light 41%, and infrared radiation 51%). The amount of this energy that reaches any one “spot” on the Earth’s surface will vary according to atmospheric and meteorological (weather) conditions, the latitude and altitude of the spot, and local landscape features that may block the Sun at different times of the day. In fact, as sunlight passes through the atmosphere, some of it is absorbed, scattered, and reflected by air molecules, water vapor, clouds, dust, and pollutants from power plants, forest fires, and volcanoes. Atmospheric conditions can reduce solar radiation by 10% on clear, dry days, and by 100% during periods of thick clouds. At sea level, in an ordinary clear day, the average intensity of solar radiation is less than 1.0 kW m^-2, (UV 3%, visible light 42%, infrared radiation 55%). Penetrating water, much of the incident light is reflected from the water surface, more light being reflected from a ruffled surface than a calm one and reflection increases as the Sun descends in the sky (Table 3.1). As light travels through the water column, it undergoes a decrease in its intensity (attenuation) and a narrowing of the radiation band is caused by the combined absorption and scattering of everything in the water column including water. In fact, different wavelengths of light do not penetrate equally, infrared 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,

Iz = I0 x e-kz                   (3:1)

where I_z is the intensity of light at depth z, I₀ 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:

Both types of processes depend on the absorption of photons by electrons of chromophore molecules 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 molecular 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 photoisomerization 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 Å . 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 electrochemical work, avoiding its dissipation as heat or fluorescence. The ability to perform electrochemical work for each electron that is transferred is termed redox potential; a negative redox potential indicates a reducing capability of the system (the system possesses available electrons), while a positive redox potential indicates an oxidizing capability of the system (the system lacks available electrons).

Photosynthetic activity of algae, which roughly accounts for more than 50% of global photosynthesis, make it possible to convert the energy of PAR into biologically usable energy, by means of reduction and oxidation reactions; hence, photosynthesis and respiration must be regarded as complex redox processes.

As shown in Equation (3.2), during photosynthesis, carbon is converted from its maximally oxidized state (+4 in CO₂) to strongly reduced compounds (0 in carbohydrates, [CH₂O]n) using the light energy.

(3:2)

In this equation, light is specified as a substrate, chlorophyll a is a requisite catalytic agent, and (CH₂O)n represents organic matter reduced to the level of carbohydrate. These reduced compounds may be reoxidized to CO₂ during respiration, liberating energy. The process of photosynthetic electron transport takes place between +0.82 eV (redox potential of the H₂O/O₂ couple) and 20.42 eV (redox potential of the CO₂/CH₂O couple).

Approximately half of the incident light intensity impinging on the Earth’s surface (0.42 kW m^-2) belongs to PAR. In the water, as explained earlier, the useful energy for photobiochemical processes is even lower and distributed within a narrower wavelength range. About 95% of the PAR impinging on algal cell is mainly lost due to the absorption by components other than chloroplasts and the ineffectiveness of the transduction of light energy into chemical energy. Only 5% of the PAR is used by photosynthetic processes. Despite this high energy waste, photosynthetic energy transformation is the basic energy-supplying process for algae.