Algae and Protoctist

Algae noun - Primitive chlorophyll-containing mainly aquatic eukaryotic organisms lacking true stems and roots and leaves.

Protoctist and algae are semantically related. In some cases you can use "Protoctist" instead a noun "Algae".

Protoctist noun - Any of the unicellular protists.

Algae and protoctist are semantically related. Sometimes you can replace term "Algae" with "Protoctist", this nouns are similar.