What triggers pfiesteria change from a harmless algae feeder to a toxin producing fish killer?

Researchers in their study of Pfiesteria found out that the organism can have up to 24 different life stages and the life stage that it exhibits depend on the environment where it found itself. 
Under normal condition, pfiesteria feed on algae but in a situation where there is plenty of algae and fishes available, pfiesteria will change its mode of nutrition and start secreting toxin which are poisonous to the available fish population.
Presence of excess nutrients trigger pfiesteria to produce the toxin.

They both determine the exact age of rocks.