Final answer:
Diatoms are unicellular photosynthetic protists with intricate glassy cell walls made of silicon dioxide, playing crucial roles in freshwater and marine ecosystems.
Explanation:
Diatoms are unicellular photosynthetic protists encased in intricately patterned, glassy cell walls made of silicon dioxide. They are essential components of freshwater and marine plankton, contributing significantly to ecosystems. Their unique characteristics, such as silica frustules and diverse species ranging up to millions, make them a significant group in the biological world.
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