Question

Water molds are fungus-like and heterotrophic. Why are they classified as chromists rather than as protozoans or fungi?

Answer 1

Because classification today follows evolutionary (phylogenetic) relationships, not just superficial similarity. Water molds (oomycetes) look fungus‑like, but multiple lines of evidence show they belong with the chromists/stramenopiles, not with fungi or with the old, paraphyletic “protozoa.” Key reasons:

- Molecular phylogeny: DNA and protein sequence analyses (rRNA and many genes) place oomycetes with other stramenopiles (diatoms, brown algae, etc.), not with the opisthokont fungi.
- Flagella/zoospore ultrastructure: Oomycete zoospores are biflagellate and heterokont — one anterior “tinsel” flagellum with tripartite hairs (mastigonemes) and one posterior smooth (whiplash) flagellum — a diagnostic stramenopile feature. True fungi (opisthokonts) either lack flagella or have a single posterior whiplash flagellum (chytrids).
- Cell wall chemistry: Oomycetes have cellulose- or glucan-rich walls, whereas true fungi have chitinous cell walls.
- Life cycle and cytology: Oomycetes typically have a diploid-dominant lifecycle and coenocytic hyphae, differing from the haploid/dikaryotic-dominant lifecycles typical of many fungi.
- Feeding mode and morphology are convergent: Their filamentous, absorptive growth resembles fungi because of convergent evolution, not close ancestry.

Because of these structural and molecular characters, oomycetes are placed among the chromists/stramenopiles rather than with fungi or with the traditional protozoa.