Heterotrophic Protists

Heterotrophic organisms must consume organic matter to obtain energy. Under the umbrella of protists, there are several heterotrophic groups. Some of these are closely related to photosynthetic lineages of protists that have obtained chloroplasts via endosymbiotic events, such as Oomycota and Phaeophyta. However, due to the relationship between nutritional strategies and ecological roles, it often makes more sense to discuss these lineages in groups of heterotrophs and autotrophs, rather than evolutionary relatedness.

This chapter will introduce slime molds and oomycetes, as these are groups commonly discussed in botany courses. However, it is important to note that there is a vast diversity of heterotrophic protist lineages not covered here with important and fascinating ecological roles.

• Topic hierarchy

2.4.2.2: Oomycota 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.

2.4.2.1: Slime Molds Slime molds represent several different lineages: the cellular slime molds (Dictyostelids), Protostelids, and plasmodial slime molds (Myxomycetes). These organisms move about as amoebae consuming bacteria until conditions become unfavorable, at which point they form spores. They can be found in damp substrates with ample bacteria and are most frequently found on decaying logs and forest duff.