Final answer:
The most prominent organelle in algal cells responsible for photosynthesis is the chloroplast, which contains structures like thylakoids vital for this process.
Step-by-step explanation:
The most conspicuous organelle in the algal cell is the chloroplast. Chloroplasts are essential for algae as they perform photosynthesis, a process that is vital for converting light energy into chemical energy, which in turn forms the foundation of food chains in most ecosystems. Unlike animal cells, algal cells have one or more chloroplasts with a structure called thylakoids, where the photosynthesis takes place.
The thylakoid membranes provide a large surface area to accommodate the proteins and pigments necessary for this process. Additionally, algae can have complex cell structures including vacuoles, nuclei, and sometimes pyrenoids within the chloroplasts for synthesizing and storing starch, indicative of their role in photosynthesis and energy storage within the cell.