The Role of Photosynthesis in the Aquarium
by Robert Paul Hudson
from Robert's web site in Salem Oregon: Aquabotanic.com
Aquarticles
Photosynthesis is the process by which plants use the energy of light to convert carbon
dioxide and water into glucose, and the by-product released is oxygen on which most life
depends. In the absence of light, the process of respiration is the opposite of
photosynthesis. Food substances are broken down in the presence of oxygen to release
energy as heat. Carbon dioxide is produced and released as a by-product. These processes
are a vital part of the plants' growth and the introduction of high intensity light and
carbon dioxide produces a significant increase in photosynthetic activity thus creating a
boost in plant growth and vitality. Active photosynthesis is what makes the difference
between healthy aquarium plants and those that are merely surviving.
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 loose 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 affect.
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.
|