Aquatic plants obtain carbon dioxide (CO2) primarily through direct absorption from the surrounding water and, in some cases, from the atmosphere.
Mechanisms for CO2 Acquisition in Aquatic Plants
Aquatic plants have evolved various strategies to access the carbon dioxide they need for photosynthesis, since CO2 diffuses much slower in water than in air.
1. Direct Absorption from Water
- Dissolved CO2: Plants directly absorb dissolved CO2 from the water surrounding their leaves and stems. This is the most common method for submerged aquatic plants. The concentration of dissolved CO2 in water depends on several factors including pH, temperature, and the presence of other organisms.
- Bicarbonate (HCO3-) Use: Some aquatic plants can utilize bicarbonate, a more abundant form of inorganic carbon in many aquatic environments, as a CO2 source. They have specialized enzymes (e.g., carbonic anhydrase) that convert bicarbonate into CO2 at the leaf surface.
2. Accessing Atmospheric CO2
- Floating Leaves: Plants like water lilies have floating leaves with stomata, similar to terrestrial plants. These stomata allow them to directly absorb CO2 from the atmosphere.
- Emergent Leaves: Some aquatic plants have leaves that extend above the water surface. These emergent leaves also have stomata that facilitate the uptake of CO2 from the air.
3. CO2 Uptake from Sediments
- Rooted Plants: Some submerged aquatic plants also get CO2 from the substrate in which they are rooted. This occurs when the plant transfers CO2 from the roots to the leaves.
Factors Influencing CO2 Availability
The availability of CO2 for aquatic plants is influenced by several factors:
- Water pH: Higher pH levels tend to decrease the amount of dissolved CO2 and increase the proportion of bicarbonate.
- Water Temperature: Warmer water holds less dissolved CO2.
- Water Movement: Water circulation helps to replenish CO2 around the plant surfaces.
- Photosynthetic Activity of Other Organisms: The photosynthesis of algae and other aquatic plants can deplete the CO2 concentration in the surrounding water.
- Decomposition: Decomposition of organic matter releases CO2 in aquatic ecosystems.
In summary, aquatic plants have adapted to obtain carbon dioxide through various mechanisms, including direct absorption from water, utilization of bicarbonate, and access to atmospheric CO2 via floating or emergent leaves. The availability of CO2 depends on water chemistry and biological activity in the aquatic environment.
