Have you seen the green, pebble-like blobs in the lake?
Kitty, a Volunteer Park Ranger describes it as “Ophrydium versatile”, a colonial freshwater organism.
To form the balls they group together and live in symbiotic association with “chlorella” (green algae), where they both benefit from the interaction. The algae photosynthesise and share some of the sugar products with the “Ophrydium”; in return, the Ophrydium gives it nitrogen/other nutrients.
In short, the green blobs are a “ciliate protist combined with green algae”, much like how a lichen is a partnership of algae and fungi.