light which is then transformed by the cell into chemical energy needed for the synthesis of carbohydrates. In its simplest form, photosynthesis occurs as follows:

(1)   6CO₂ + 12H₂O + light  --->   C₆H₁₂0₆ + 6H₂O + 60₂

As a result of photosynthesis, there are three basic requirements for algal growth: air, water and sunlight. Elimination of any one of these three factors will hamper growth. The distribution decks and sidewalls of a cooling tower provide all three requirements and therefore represent an excellent growth environment for algae. 

For this reason, algae are all too often seen floating as free green masses on the top deck, or attached to the cooling tower structure itself.

Table below shows some common algal groups found in cooling water systems and their temperature and pH requirements for continued growth.

Because the temperature characteristics and pH considerations are both within the ranges of basic cooling water systems, it is not surprising that they often produce prolific algae blooms.

Green algae exist either as unicellular or multicellular colonies. The blue-green strains such as Oscillatoria  are filamentous strands with cells arranged end-to-end in chains; such colonies can reach a length of several feet. Diatoms  consist of two half-cells and contain a brown pigment and silica in their cell walls.

All algae produce oxygen which can depolarize the corrosion reaction and accelerate system destruction. Blue-green algae such as Nostoc, Anabaena and Calothrix can fix (extract and convert) nitrogen from the air to organic nitrogen compounds. 

These nitrogen-fixing algae are responsible for the accelerated deterioration of nitrite-base'd corrosion inhibitors. Diatoms have been known to cause silica fouling because their cell wall is impregnated with polymerized opaline silica.