Oomycetes, commonly called water molds, are a group of heterotrophic, fungus-like organisms with a filamentous thallus. Unlike fungi, they have diplontic life cycles and cellulose in their cell walls....Oomycetes, commonly called water molds, are a group of heterotrophic, fungus-like organisms with a filamentous thallus. Unlike fungi, they have diplontic life cycles and cellulose in their cell walls. They are important decomposers in aquatic ecosystems and plant parasites in terrestrial ones.