Light – the driving force for growth of aquatic plants

However, plants are able to use visible light only from wavelengths of about 400 to 700 nm and this range is termed Photosynthetically Active Radiation (PAR). Primarily, red and blue light are used in photosynthesis while green light is reflected or transmitted and thus, plants appear green because green light is not absorbed by plant pigments. Plants capture light by means of pigments, which absorb light of different colors, depending on the pigment in question (Figure 1). All plants have Chlorophyll-A, many have Chlorophyll-B while only few have Chlorophyll-C. The three chlorophylls have very different absorption spectra, i.e., they absorb light of different colors and thus, they may complement each other in the process of light harvesting. Carotenoids is a group of pigments that can absorb blue-green light where chlorophylls are inefficient (carotenes are orange as we know them from carrots where they don’t play any role in the light absorption). Not all higher plants have carotenoids, whereas most algae do and thus, algae may become a nuisance if the light source over the aquarium contains too much green and yellow-green light. In that case, this additional light only benefits the algae.

Light quantity
Light intensity is an expression of how much light (energy) reaches a given surface and in natural sciences, light intensity is measured in µmol photons per square meter per second (µmol m-2 s-1). In the aquarium hobby, Lux has traditionally been used to measure light because quantum sensors are extremely expensive devices, whereas lux can be measured by an old-fashioned light meter used in photography. As a rule of thumb, 1 µmol m-2 s-1 is equivalent to 55 Lux in the PAR spectrum but this conversion is not accurate since the Lux scale has been developed to suit the eye’s sensitivity and thus, it is not the same for all color combinations.

Facts Box Light is the energy source in photosynthesis where water and inorganic carbon is transformed to energy rich sugar and oxygen: 6H2O + 6 CO2 + light → 6C6H12O6 + 6O2 In this process, light energy is transformed and fixed into chemical energy, which later may be used in other metabolic processes in the plant. Consequently, the sugar is primarily used directly to synthesize cellulose and starch, which are important elements in plant growth. The chloroplasts contain stacks of membranes that are termed thylakoid membranes. These stacks are usually reduced in aquatic leaves to achieve a better light efficiency at low light intensities. In the thylakoids, light is captured by pigments (mainly chlorophylls), which are placed like antenna on an energy center termed the light reaction center. The antenna and the light reaction center are called, collectively, a light harvesting complex. The number of antenna containing chlorophyll may vary from about 300 units to more than 1000 units and measurements have shown that light harvesting increases linearly up to about 1000 units. Once a pigment has captured a photon (the light particle), the energy is channeled to the light reaction center and when enough energy is present, an electron is released and channeled through a series of energy rich compounds (illustrated by the tilted “z” on the figure). The energy is used to split water into hydrogen and oxygen and to melt 6 carbon dioxide molecules into a sugar molecule. The Z-scheme, the antenna and the light reaction center is termed the photosynthetic unit.

In nature, many aquatic plants are found in places where they receive direct sunlight (2000 µmol m-2 s-1) at least part of the day. Not even plants growing in shade receive less than about 200 µmol m-2 s-1 at noon. In comparison, a very well illuminated aquarium receives about 80-100 µmol m-2 s-1. This is a dramatic reduction in energy supply, but it is nevertheless what most aquarium plants face when they are transferred from the well-illuminated nursery, where most plants are grown emergent, to the low light environment in the aquarium. As a consequence, many plants lose their terrestrial leaves and new ones are formed. These new leaves are much better adapted to light harvesting under low light in the aquarium, where it becomes important to capture every single photon that reaches the leaf surface.

Various sources of light may be used over the planted aquarium and it is a science on its own and beyond the scope of this article to analyze each and every possible solution. We will focus on fluorescent tubes because they are the most economic and efficient in terms of useful light per Watt consumed. Unfortunately, a large proportion of the light that is emitted by the fluorescent tubes never reaches the plants. Light is emitted in all directions and only beams that hit the water surface almost perpendicular penetrate the water surface, while the remaining light is reflected (Figure 3, upper panel). Use of cover glass causes an even larger proportion of light to be wasted. However, an ideal reflector increases the proportion of light that penetrates the water surface considerably, because the reflector collects all beams from the sides and above and reflects them in parallel bundles (Figure 3, lower panel).

Once the light has successfully penetrated the surface, the depth of the tank is the most important factor controlling how much light reaches the bottom. The light intensity decreases dramatically with distance from the lamp. For example, if 50% of the light reaches an area at a depth of 10”, then only 25% will reach that area at a depth of 20”. Most of this reduction is due to the fact that the light beams are not totally parallel and thus, much light is scattered on its way to the bottom. Another part is absorbed by colored substances such as humic acids dissolved in the water and by particles suspended in the water (mostly microscopic algae and detritus). In conclusion, much of the useful wavelengths have been filtered before the light reaches the bottom of the aquarium.

The temperature inside the fluorescent tubes is of great importance for the amount of light emitted. Above a certain temperature, less light is actually emitted. The optimum temperature is about 38 ºC (100 ºF) and at 60 ºC, most fluorescent tubes emit 25% less light compared to optimum operating temperature. Equally important is the type of fluorescent tubes. Basically, the old T8 type is much less efficient compared to the newer T5. A T5 tube may emit at least 50% more light per Watt consumed and part of it is due to the fact that the temperature is much lower in those tubes.

Illumination time
Most aquarium plants come from the tropics with a typical day length of 10 to 14 hours. The plants follow this rhythm, which can be observed in for example Cabomba, which folds in the leaves in the shoot apex at the time just prior to switching off the light. It is probably even more important to respect that plants need a dark period to “rest.” If they are not given this dark period, they develop symptoms of stress or they may develop a non-desirable architecture. They are using the dark period to transform energy-rich compounds formed in photosynthesis to more complicated molecules that eventually leads to new growth.

The optimum illumination time is approximately 12 hours for most plants. Any additional light does not really benefit the higher plants, whereas algae are always able to capitalize on the extra energy provided. On the other hand, a significantly short period of illumination has an adverse effect on the plants. They simply do not get enough energy and they start losing leaves, particularly the lower ones (Figure 4). However, because of the relatively large starch deposits in higher plants they are able to withstand short periods of really low light. This fact is often used in the battle against algae, where a prolonged period of darkness may kill the algae, because they have few energy stores, whereas the higher plants survive.