Ecological roles of saprotrophic Peronosporales (Oomycetes, Straminipila) in natural environments

The fungus-like Peronosporales are composed of several lineages of mainly biotrophic and hemibiotrophic representatives. Saprotrophic species of Peronosporales are limited to the genera Halophytophthora and Salisapilia, and to some species in Phytopythium and Phytophthora Clades 6 and 9, which inhabit terrestrial and all types of aquatic ecosystems. The recent discovery of species of Phytophthora in marine habitats and of Halophytophthora in freshwater indicated that these genera are not only morphologically but also ecologically poorly delineated. In addition, half of these genera are not monophyletic. They play key ecological roles by upgrading nutrients to higher trophic levels through colonization of plant debris, which makes substrata more palatable for detritivores or through zoospore grazing by zooplankton, although their role as saprotrophs is still largely neglected. Some species of Phytophthora can be aggressive opportunistic pathogens of riverine forests in the presence of susceptible hosts and favourable environmental conditions and, as a consequence, most studies have focused on their role as pathogens. Identification of species is challenging due to hybridization and species complexes that harbour multiple cryptic species and, therefore, is not reliable without DNA sequencing tools.