Diversity Of Soil Organisms – Part 7


Protists Continued…


Stramenopiles are unicellular organisms with two cilia that beat in different directions: a front one that includes tiny hairs (visible on electron microscope images) that pulls the cell, and a trailing one that pushes the cell. In some groups, however, the trailing cilium is missing. Other groups are usually filamentous and only the dispersal cell is ciliated. Terrestrial species form resting cysts in the soil, and in some sexual species dispersal spores are produced after sexual reproduction.

This supergroup includes the brown algae and several groups previously thought to be fungi, such as Hyphochytriales and Peronosporomycetes, which are commonly found in soils. Some species of true brown algae occur in alpine soils. Most terrestrial species have lost the ability to photosynthesise and appear colourless. They absorb nutrients from the living or decomposing tissues into which they grow.

Hyphochytriales are found in moist soil environments. They absorb dissolved nutrients with a network of filaments that extend from the cell. Terrestrial species of Peronosporomycetes are decomposers of organic matter or live as plant parasites. They feed by extending filaments into plant tissues.

They are economically important because they include species that cause some of the most damaging plant diseases, such as Pythium (which causes the damping-off disease in greenhouses), downy mildews and white blister rusts.

Diatomea are typically aquatic species that can be found in riparian or regularly flooded soils, and sometimes inside rotting tree logs.

Diversity, abundance and biomass
Only approximately 25 genera of Hyphochytriales are known to science, but many still remain to be described. Fewer than 700 species of Peronosporomycetes are described, but there are likely to be 1 000 – 10 000 species.


The Irish Potato Famine


  • The Irish Potato Famine, a period of mass starvation, disease and emigration in Ireland between 1845 and 1852, was caused by Phytophthora infestans, a Peronosporomycetes.
  • Originally from the Toluca Valley in Mexico, once introduced through infected potatoes, it spread rapidly to much of northern and central Europe.
  • Because prior to 1980 they were considered to be fungi, we still lack an effective chemical compound to treat stramenopile parasites since fungicides (aiming to disrupt fungi) do not work.



Ref: Global Soil Biodiversity Atlas p39