Photosynthesis is the amazing process by which plants combine sunlight, carbon dioxide and water to create the energy they need to live. While most people think that photosynthesis is conducted by green plants living on the ground, it is achieved by a variety of bacteria, algae and underwater plants. Aquatic plants have plenty of water to work with, so their main challenge is getting enough sunlight and air.
Aquatic plants still need sunlight to perform photosynthesis, but fortunately sunlight can pass through the water easily enough. This is why many aquatic plans may have stems that reach down hundreds of feet, but most of the plant floats near the surface, where it can absorb the sunlight. Aquatic plants are also usually green like topside plants, to absorb the most of the sunlight spectrum that enters the atmosphere.
However, the sunlight that enters the water is affected by more variables. Not only do aquatic plants have to deal with cloudy days, but also with cloudy water. Silt and mud stirred up in active water can create a barrier between plants and sunlight. The color of the water, which is affected by plankton and minerals, can also have an effect on how much sunlight can make it through to the plants.
Carbon Dioxide and Oxygen
Plants in the air can simply absorb carbon dioxide through their roots and pores, but plants underwater have a much more difficult process to go through, since carbon dioxide levels are often lower in water and the gas moves much more slowly. To compensate, many aquatic plants grow leaves with a thin cuticle that protects them from the elements, but also has enough pores to allow carbon to enter very easily, more easily than most kinds of plants. Aquatic plants do not usually have stomata to absorb air, because there is no air to absorb--although some varieties will grow leaves above the surface of the water to take in carbon dioxide.
Plants also need a certain amount of oxygen, though not much. The oxygen they produce and release into the water is almost always sufficient for their needs, and is absorbed back into the plant as it is needed. The problem is that many other aquatic creatures also need a certain amount of oxygen to survive. At night, when photosynthesis is no longer possible, the plants continue to take in oxygen but no longer exude any, which can make it difficult for other organisms to survive.
Water is not usually a problem for freshwater plants, but saltwater plants face a curious paradox. The more water they take in, the more salt they take in. Salt will actually leech water back out of the plant faster than the plant can absorb it, so a normal plant would die of dehydration or salt poisoning trying to absorb seawater. Aquatic plants that live in salty conditions combat this by growing very thick, waxy leaves and stems that seal the plant against salt penetration. They also have specific metabolism functions that get rid of salt quickly while preserving the water for their processes.