How photosynthesis works at aquarium plants

     The unique function that plant possess is the ability to obtain energy from sunlight, carbon dioxide and water, using the process of photosynthesis. Photosynthetic cells within the leaves and stems tissues contain pigments that trap light energy to break down the molecular structure of water (H2O) into hydrogen and oxygen. The hydrogen binds first to carbon dioxide and then oxygen to form glucose, which is a basic sugar and an important source of energy. Some oxygen is left over from this process and is released back into the water, where it is either used up by bacteria and animals or released into atmosphere at the water surface.

     The glucose produced from photosynthesis is water soluble and, if stored in large quantities, will absorb water and enlarge the cells that contain it. Obviously, this is undesirable for plants, so the glucose is quickly converted into an insoluble starch compound and transported to various parts of the plant for storage, in most cases to the upper root area. The starch can be easily converted back into glucose and transported around the plant when needed.

     A plant has little control over the rate of photosynthesis that occurs within its cells. Higher rates of photosynthesis will encourage faster growth, reproduction and improved plant health. Light is the most obvious environmental factor, but temperature, carbon dioxide levels and nutrient availability also affect the rate of photosynthesis.

     Light. Plants will only photosynthesize when suitable light is available to be trapped by the photosynthetic cells. At night, plants stop photosynthesizing and only start again in daylight. In nature, most tropical plants experience about 12 hours of sunlight in a 24 hour period. In the aquarium, the same duration should be employed and in most cases, a bright light source is preferable. If the light is left on for a longer period, the photosynthetic period will also increase. This may bring its own problems. It is possible that plants will oversynthesize and become damaged by wearing themselves out.

     Temperature. In plants, an increase of 10°C (18°F) will roughly double the rate of photosynthesis, assuming all other factors are favorable. If the surrounding environment becomes too warm, the plant will simply begin to die and photosynthesis will stop. An increase in temperature affects not just photosynthesis, but the whole metabolism of a plant, so it also increases the plant's requirements for nutrients, carbon dioxide and other elements. For this reason, simply increasing the temperature of an aquarium to aid plant photosynthesis and therefore plant growth, is unlikely to work.

     Carbon dioxide. Plants take up carbon dioxide from the surrounding water and substrate. If CO2 is not available in sufficient quantities, many plants have developed ways of obtaining carbon containing compounds and creating their own source of CO2. This occurs more in hardwater plants (Vallisneria and Egeria species), which experience lower CO2 levels in nature. Plats that regularly produce leaves above the surface have developed methods of utilizing CO2 from atmospheric air, where the concentrations are much higher. Floating plants have constant access to the air, so it is easier for them to obtain CO2 from the surrounding air through the leaves, in the same way as terrestrial plants.

     Nutrient availability. The photosynthetic pigments, usually chlorophyll, are produced by the plant within the cells. To do this, a number of nutrients are required (magnesium, potassium, iron and nitrogen).

     Photosynthesis and leaf color. Chlorophyll is produced in the greatest quantities in the parts of the plant that receive the most light, mainly in the leaves. The roots of plants receive virtually no light below the substrate, so do not contain chlorophyll and hence, do not appear green. In an aquarium, looking at the leaf color of a plant can help to establish what kind of light is required by the plant. A plant that produces reddish leaves may be accustomed to bright light conditions in nature and will need the same conditions in the aquarium to photosynthesize properly. Sometimes, red plants first produce green leaves, which then change to red. if they stop turning red or revert back to green, this may indicate that the intensity of light in the aquarium is not sufficiently high.

     Some plants, particularly within the cryptocoryne group, produce brown leaves. These plants are often found in shallow streams with overhanging vegetation and may have developed the use of photosynthetic pigments that are more efficient at using the green areas of the light spectrum. These may be more abundant in an environment shaded by other plants. Therefore, plants with brown leaves should do relatively well in shaded areas of the aquarium. In many cases, brown - leaved cryptocorynes will develop green leaves when kept in brightly areas of the aquarium. This color change occurs because of the change in light spectrum from the plant's natural environment.

     Above, the mottled green-and-red leaf of an Echinodorus indicates that there are two separate photosynthetic pigments. In bright light, the less efficient red pigment helps to reduce the rate of photosynthesis, while in low light, the green pigment will spread and increase the efficiency of photosynthesis.

For more info see Encyclopedia of Aquarium Plants by Peter Hiscock.