Ability to perform photosynthesis is the main distinguishing feature between green plants and other organisms on Earth. In this chemical process, carbon dioxide and water are combined in presence of light energy to produce carbohydrate and other byproducts. This process of converting carbon dioxide to glucose with the help of radiant energy is observed in cyanobacteria (blue-green algae), some types of algae and all green plants, irrespective of the growing environment.
Photosynthesis in Aquatic Plants and Land Plants
So, is there any difference between photosynthesis in land plants and aquatic plants? Well, the process of producing food with the help of light energy remains the same for both aquatic and land plants. In addition to light, they require the basic raw materials - carbon dioxide (CO2) and water (H2O) for synthesis of glucose (C6H12O6). What is special about food production by plants under water is, deriving these raw materials and light energy from their immediate environment.
The balanced equation of photosynthesis is represented as: 6CO2 + 12H2O + Light → C6H12O6 + 6O2 + 6H2O. In case of land plants, the required gases and light energy are available easily. They absorb carbon dioxide from atmospheric air through their stomatal openings (present in upper and lower side of leaves), water from the soil through their root system, and last but not the least, radiant energy from sunlight. Hence, land plants undergo photosynthesis naturally without any special adaptations.
How do Aquatic Plants Photosynthesize?
Since water is available in more than sufficient amounts, the major challenge is to obtain carbon dioxide and light. For the same, majority of these plants show adaptations in some way or the other. Say for example, water lily (lotus) bear floating leaves with waxy surface, which have extra long petioles. The leaves on the surface of water gather light and carbon dioxide for photosynthesis, while the waxy cuticle prevents excess water absorption from the surrounding environment. It also helps in cleaning process and preventing entry of microbes.
Lotus is just one instance, and many water plants have the same feature, i.e., sending their roots at the bottom of the water body for support and leaves protruding at the surface of water. So, absorption of carbon dioxide and light is not an issue for aquatic plants having floating leaves. But, what about the remaining plants that bear submerged leaves? In this aspect, carbon dioxide is obtained from water, which is released during respiration by fish. Also, decomposition of organic matter that takes place in the water ecosystem contributes to carbon dioxide production.
Even though the rate of carbon dioxide dissolution in water is exceptionally low, traces of dissolved carbon dioxide are available to the submerged aquatic plants. As for light requirement, sunlight pass through water is harvested by plants for photosynthesis process. This is specially true for plants that thrive in shallow waters. And for the plant species found in deeper waters, they are adapted to grow under low light conditions. Of course, the rate of photosynthesis is very slow for these plants, and they rely on the dim radiation for manufacturing their own food.
Thus, aquatic plants obtain carbon dioxide and sunlight for photosynthesis process. Last but not the least, availability of usable water is not an issue for freshwater plants for photosynthesis. While for marine plants, they are adapted with waxy stems and leaves. This aids in absorbing water, while preventing the entry of salt to their system. In addition, some marine plants have specialized features to remove salt as soon as possible. All these processes help in regulating the osmotic balance, which otherwise will cause leaching of water and desiccation of plants.
This way, aquatic plants undergo photosynthesis under water. The products of photosynthesis in aquatic plants, basically carbohydrate and oxygen, are used by other organisms living in the same biotic community. And just like animals, plants do require oxygen, but in small amounts. This is obtained from the oxygen released at the time of photosynthesis.