A microscopic organism is an organism that can only be seen with the aid of a microscope. The three crossword solutions for the term 'microscopic organism' are ALGA (4 letters), AMOEBA (6 letters), and Ameba (5 letters). ALGA is a type of aquatic plant that is found in both fresh and salt water. AMOEBA is a single-celled organism that moves by extending its pseudopods. Ameba is another single-celled organism, but it is typically found in soil and freshwater.