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Water moulds (oomycetes)

Water moulds (or oomycetes) are a group of eukaryotic microorganisms that resemble microscopic fungi morphologically and by their life style. Therefore, it was previously thought that they belonged to the kingdom of Fungi. Now these organisms have been placed within their own kingdom, namely Chromista, because they differ in several molecular and phylogenetic respects from the actual fungi. For instance the following:

  1. The cell wall of oomycetes contains cellulose and generally not chitin.
  2. The cell membrane does not contain ergosterol, as the fungal cell membranes do.
  3. In the vegetative stage, the oomycetes have a diploid nucleus, while those of the fungi are haploid.

Most water moulds produce two different types of spores. The main spreading spores are asexual and they are motile by their own. They are called zoospores and can move towards or away from a chemical signal, which is called chemotaxis. The chemical signal can consist of for instance substances from potential sources of nutrition. Some water moulds produce asexual spores, which are spread in the air by the wind. Water moulds can also produce sexual spores, which are called oospores. These are translucant, double-walled and spherical structures used to survive in adverse environmental conditions.

Water moulds can have a parasitic or saprophytic lifestyle and many of them are important in aquaculture, agriculture and veterinary medicine. Among the algal fungi you will find some of the most notorious plant pathogens and in many text books, these organisms are found under "Fungus-like organisms". 

The English name water mould (Am. water mold), is inappropriate because most of the oomycetes do not live in water.

Updated: 2021-05-20.

Swedish University of Agricultural Sciences