Photosynthesizers

Algae form the base of the food web. They can get out of hand and cause trouble, and they're so important in the aquarium that they get their own "Algae" section. Complementing algae are Euglæna and its kin, and diatoms and dinoflagellates, all of them photosynthesizing single-cell protists that are neither plant nor animal.
 
Photosynthesis doesn't fuel all the food web, though. Also down at the base of the food web are bacteria and fungi—especially the ascomycetes— water molds like Saprolegnia, and the yeasts.