Final answer:
Aquatic plants utilize carbon dioxide dissolved in water for photosynthesis because it is a necessary carbon source for autotrophic organisms, and in an aquatic environment, it is most readily available in this form for the plants to use in the photosynthetic process.
Step-by-step explanation:
Aquatic plants use carbon dioxide dissolved in water to prepare their food during photosynthesis primarily because carbon dioxide is the necessary source of carbon for autotrophic organisms. For all autotrophic organisms, carbon is acquired as carbon dioxide, either from the atmosphere or dissolved in water. In aquatic ecosystems, light may be limited, especially in deeper waters which can influence the distribution of photosynthetic organisms that rely on photosynthesis for their energy and must stay in upper levels or near the surface. However, the question why they use carbon dioxide specifically from water as opposed to another source is because in an aquatic environment, the carbon dioxide is more readily available in dissolved form.
While oxygen is also dissolved in water and available for respiration in aquatic organisms, it is not the primary reason for aquatic plants to use carbon dioxide from water during photosynthesis. Additionally, water doesn't act as a catalyst in photosynthesis; rather, it's involved as a reactant. The abundance of dissolved carbon dioxide available to water-bound plants is crucial for their survival and ability to perform photosynthesis effectively.