Photosynthesis and Respiration in Aquatic Plants




Before we discuss the techniques and equipment necessary to maintain healthy aquarium plants, let us look briefly at the fundamental processes that take place in living plants.

Plants range in structure from simple unicellular algae to complex higher plants composed of many different types of cells adapted to perform different functions. Thus, the roots, stems, branches, leaves, flowers and fruits all play their part in maintaining the whole plant. The two most vital processes that occur in all green plants, whether simple or complex in form, are photosynthesis and respiration.


As the term suggest, this is a building up process that is 'powered' by the energy of light. During photosynthesis, carbon dioxide is absorbed and oxygen liberated. These gases enter and leave the plant throught tiny pores called stomata. Inside the plant cells - principally in the leaves - carbon dioxide and water are chemically combined in the presence of light and the green pigment chlorophyll to produce simple sugars, such as glucose. Although still imperfectly understood, the process can be simply expressed in the following equation:

            6CO2 + 6H2O  --------->  C6H12O6 + 6O2

The formation of simple sugars is quickly followed by other reactions that convert sugar into starch. Oxygen is produced as a by-product.

Photosynthesis Illustration

Photosynthesis is most active in the blue and red portions of the light spectrum, a factor to bear in mind when providing effective aquarium lighting. It is the chlorophyll that absorbs these wavelengths to energize photosynthesis, although only about 3% of the light falling on a leaf is absorbed and used in this way.

Chlorophyll is really a mixture of two green pigments: chlorophyll a and chlorophyll b. Other plant pigments, such as carotene (orange) and xanthopyll (yellow), are masked by chloropyll in green plants and are responsible for the striking patterns of variegated ones. They take no part in photosynthesis. A plant grown in the dark loses its chlorophyll, allowing other pigments to give it a sickly yellow 'etoliated' appearance.

For chlorophyll to form in the leaves a plant must have access to iron, hence the importance of providing sufficient iron in the substrate. Plants lacking iron - said to be 'chlorotic' - assume a similar yellow or pale green appearance to etoliated plants, although the causes are quite different in the two instances.

The intensity of illumination, the supply of carbon dioxide and the temperature of the surroundings all affect the rate of photosynthesis. Between 0oC and 25oC (32-77oF), a rise of 10oC (18oF) effectively doubles the rate of photosynthesis. Similarly, a high concentration of carbon dioxide coupled with a high light intensity also produces a significant increase in photosynthetic activity. Thus, introducing carbon dioxide into the aquarium in carefully controlled amounts can boost plant growth in well-lit tanks.


In simple terms, respiration is the reverse of photosynthesis. It is the process by which food substances are broken down in the presence of oxygen to liberate energy, principally as heat. Carbon dioxide is produced as a by-product. Respiration takes place in all plant cells and continues irrespective of light. Thus, during darkness - when photosynthesis ceases - respiration accounts for the net absorption of oxygen and the liberation of carbon dioxide from the plant.

Respiration Diagram

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