Algae genus is a broad term that encompasses many different species of aquatic plants. Spirulina, with nine letters, is a type of blue-green algae that is often used as a dietary supplement. Diatoms, with seven letters, are single-celled algae that are found in both freshwater and marine environments. ABALONE, also with seven letters, is a type of mollusk that feeds on diatoms. DIATOM, with six letters, is another type of single-celled algae that is found in both freshwater and marine environments. AGAR, with four letters, is a type of gelatinous substance derived from certain types of algae.