The Role of Photosynthesis in the Aquarium: by Robert Paul Hudson
The Role of Photosynthesis in the Aquarium
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by Robert Paul Hudson
Glucose, a carbohydrate, is the fuel formed from photosynthesis used to build leaves, flowers, fruit, and seeds. Excess amounts are stored in the plants' roots, stems, and leaves in the form of starch that can be drawn from as a reserve. Glucose is also converted to cellulose, which is used as a structural material in the building of cell walls.
Plant photosynthesis occurs in leaves and green stems within cell structures called chloroplasts. Each leaf has tens of thousands of cells, and each cell contains 40 to 50 chloroplasts. Each individual chloroplast is sectioned by membranes into disk shaped compartments called thylakoids. Embedded in the membranes of the thylakoids are hundreds of molecules of chlorophyll, a light trapping pigment required for photosynthesis. Enzymes, which are additional light trapping pigments, are also present in the membranes.
Photosynthesis is a very complex process that is still not fully understood. In simple terms there are two stages. In the first stage, the light dependent reaction, the chloroplast traps light energy and convert it into chemical energy contained in two molecules: NADPH, nicotinamide adenine dinucleotide phosphate, and ATP, adenosiue triphosphate. In the second stage called the light-independent reaction, NADPH provides the hydrogen atoms that help form glucose, and ATP provides the energy for this and other reactions used to synthesize glucose. This is all the result of the literal meaning of the term photosynthesis, to build with light.
Two things must be present for this to happen: light and carbon dioxide. Many of the plants we use in aquariums come from a natural habitat where they grow out of the water, or have growth floating at the surface where light is more intense and carbon dioxide is taken from the atmosphere, therefore without elevated light and carbon dioxide levels these plants cannot reach a proper photosynthesis rate. Plants that grow their entire life submersed have evolved to grow in conditions where both light and carbon dioxide may be hard to come by. Some plants can absorb carbon dioxide from sediment at their roots. Sediment may be rich in carbon from decaying organic material and bacteria that goes thru a similar process releasing CO2. Another source for some plants in alkaline water is stripping the carbonic molecule in the water.
Nutrients also play a role in the plants ability to photosynthesize. For example, potassium regulates the opening and closing of stomates (the pores through which leaves exchange carbon dioxide (CO2), water vapor, and oxygen (O2)). Proper functioning of stomates is essential for photosynthesis, water and nutrient transport, and plant cooling. Sugars produced in photosynthesis must be transported through the phloem to other parts of the plant for utilization and storage. The plant's transport system uses energy in the form of ATP. If potassium is inadequate, less ATP is available, and the transport system breaks down, and the rate of photosynthesis is reduced. Another example is chlorophyll. In order for it to be present in the leaves, iron must be present. If iron is not present the leaves lose their green pigment and become yellow, and photosynthesis is interrupted.
What does this all mean for the hobbyist and the planted aquarium? By understanding the basics of how this process works, we can recognize signs of success or ways to improve conditions for better plant growth and a healthier environment.
Duplicating natural habitats in an aquarium where plants take CO2 from sediment is difficult and not fully effective, but not impossible, however not all the plants we use will respond to this. Much more favorable results are achieved by having an intense enough light source along with adding a source of carbon dioxide to the water which has immediate effect.
Very soft water is not conducive to the addition of carbon dioxide because sufficient carbonate hardness is needed as a pH buffer. The alternative source would be sediment from the substrate or gravel bed, which is achieved by allowing mulm to accumulate and not cleaning the gravel on a regular basis. While this may seem to go against what we have been taught in basic aquarium care, it can be done safely within reason. Mechanical filtration, occasional water changes, and good circulation along with a low to moderate fish load will keep the system balanced. Plants should be left undisturbed as much as possible. Constant uprooting of plants or re-arranging the substrate will release mulm and possible pathogens into the water column. At initial setup, a small amount of Sphagnum peat added to the bottom layer of the substrate will provide enough organic material that while decomposing will release small amounts of carbon dioxide.
"Pearling" is the term used to describe the plants releasing oxygen during the light hours and is an indicator of the photosynthetic rate of the growing plants. Under subdued lighting you are much less likely to see significant streams of bubbles. Increasing light intensity, (not duration) coupled with increased CO2 levels will dramatically raise pearling activity. The more intense the streams of bubbles the faster the photosynthesis rate and a sure sign that all is healthy. A CO2 level of 25 to 30ppm provides the most optimal growth.