Protists are defined as unicellular eukaryotes. Many form filaments (such as some fungi), are colonial or aggregate into larger clusters of cells. They are divided into the Archaeplastida (green algae, red algae and ancestors of higher plants), the Amoebozoa (many amoeboid species), the Opisthokonta (collar cells, fungi and ancestors of animals), Stramenopiles, Alveolata, Rhizaria and Excavata. Typically, they have one nucleus and soil species have a contractile vacuole for regulating water and ion concentrations. Many species have a swimming dispersal stage with one or more cilia. Cysts form in sub-optimum living conditions or when prey are scarce. Although many protists can be identified under the microscope to family or genus level, species identification is made through DNA sequence analysis.
Cells typically produce very thin hair-like extensions called filopodia that can branch and merge together again, forming a complex network in some species. They tend to grow flat on surfaces and their filopodia can extend into small crevices in the soil searching for bacteria. When detached from surfaces, they swim with two cilia. They can also move by amoeboid locomotion or gliding on surfaces. Soil species form resting cysts that enable them to survive adverse environmental conditions. There are many variations of this basic morphology as it is a diverse group.
This supergroup has one major soil lineage: the Cercozoa. The Cercozoa (common name cercomonads) consist of a diverse variety of species of small bacterial-feeding unicells less than 10 μm in size. One subgroup common in soils is the Silicofilosea that secrete silica scales on their surface. The Silicofilosea also include the Euglyphida that form vase-shaped protective layers (known as tests) outside the cell. Other Cercozoa include Vampyrellida that feed on fungal hyphae, the Phytomyxea that are parasites of plants and Stramenopiles and Ascetospora that are parasites on soil invertebrates.
Rhizaria live on the surfaces of soil and organic matter particles where they select bacteria to ingest. Species may have depth preferences in the soil. Some prefer organic matter and litter on the surface of the forest floor. Others, such as Vampyrellida, prefer to penetrate fungal hyphae or spores. Those species with cilia can explore their habitat by swimming. The filopodia can extend into very small crevices (< 1 μm) to search for bacterial prey.
Diversity, abundance and biomass
There are hundreds or even thousands of soil Cercozoa species that cannot be distinguished by microscopy and, therefore, many genera remain to be described. These are usually the most common active protists in soils, and abundances vary with moisture as well as with the abundance of bacteria or other prey. Densities may reach more than one million cells per gramme of soil but are usually 103 – 105 per gramme.
Ref: Global Soil Biodiversity Atlas p38