Rob is right. In addition the cultures won't grow as dense as if they are a mono-culture.
Ok...this is not my strong point but it is great to think about. Algae span several Kingdoms; Monera, Protista, Plantae . Below is a definition I took from the referenced site for anyone who wants to check it out further
(Any of various chiefly aquatic, eukaryotic, photosynthetic organisms, ranging in size from single-celled forms to the giant kelp. Algae were once considered to be plants but are now classified separately because they lack true roots, stems, leaves, and embryos...
http://www.answers.com/topic/alga) under Protista
Anyway both tetraselmis and T. Iso have flagella and have very distinct swimming behaviour. If size and color were not enough by watching them swim you could tell them apart.
