Submerged aquatic plants obtain carbon dioxide from the water around them. CO₂ dissolves into freshwater and seawater from the atmosphere and is also released by fish, microbes and other organisms through respiration. Dissolved carbon can enter thin aquatic leaves and be used in photosynthesis, while oxygen produced by the plant can move back into the water.
If plants are underwater, where does their carbon dioxide come from?
Water exchanges gases with the atmosphere. Respiration and decomposition also add carbon dioxide to water. In addition, dissolved inorganic carbon can occur as carbonic acid and bicarbonate. Different aquatic species have different abilities to use these carbon forms. Not all underwater plants absorb bicarbonate efficiently; some depend mainly on free dissolved CO₂.
How do leaves absorb the dissolved carbon?
Many submerged plants have thin leaves that expose lots of surface area directly to the surrounding water. Gases and dissolved carbon species move across tissues through a combination of diffusion and, in some species, specialized uptake processes. Unlike a typical terrestrial leaf, a submerged leaf does not necessarily rely on stomata taking carbon dioxide directly from the air.
Why does underwater light matter?
Plants still require useful light energy to turn carbon dioxide and water into sugars. Water depth, suspended sediment, algae and surface shading all reduce light that reaches submerged leaves. An underwater plant can have enough dissolved carbon but still grow poorly if the water is too murky or the habitat too deep.
Do aquatic plants create those tiny oxygen bubbles?
In bright light, a submerged leaf may release oxygen more quickly than it can dissolve, forming visible bubbles often called pearling by aquarium hobbyists. Bubbles alone do not quantify the whole rate of photosynthesis. Water flow, temperature, existing dissolved gases and other conditions affect whether bubbles appear.
Can some submerged plants use carbon at night?
Yes. Researchers have documented a variant of crassulacean acid metabolism, or CAM, in certain aquatic plants. Those species may capture carbon at night when dissolved CO₂ is more available and use the stored carbon later. This is an adaptation of particular species, not a universal rule for all aquatic plants.
Frequently asked questions
Do aquatic plants release oxygen into water?
Yes, when they photosynthesize. Oxygen can dissolve into the surrounding water and supports aquatic ecosystems.
Can plants photosynthesize in dark water?
Only if enough usable light reaches their leaves. Little or no light makes sustained net photosynthesis difficult or impossible.
Do all aquatic plants have the same adaptations?
No. Rooted emergent plants, floating-leaved plants and fully submerged plants obtain gases in different ways.
Scientific sources
USGS: ventilation systems in wetland plants and USGS: CAM photosynthesis in submerged aquatic plants.
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