Diversity Of Soil Organisms – Part 6

Protists Continued…

Amoebozoa

Morphology
The Amoebozoa is another group of unicellular organisms whose cells are covered by a very thin protein layer with or without microscales.

Taxonomy
The Amoebozoa is a supergroup that contains bacterial-feeding amoeboid species. Several lineages contain mostly aggregative species referred to as ‘social amoebae’, such as the Myxogastria and the Dictyostelia, but aggregative species occur in other protists as well. In Arcellinida the cell is inside a vase- or helmet-shaped structure made of protein, sometimes amended with soil particles bound together by proteins.

Microhabitat
Amoeboid species occur on moist surfaces and live in water microfilms where they forage for palatable bacteria or other prey. Some species prefer wet conditions, others occur in drier conditions, some have depth and litter preferences, and some are known colonisers and occur in disturbed soils where other species are absent. Amoeba are very effective at scouring surfaces for bacteria. A small number feed on fungal hyphae or prey on protists or microinvertebrates.

Diversity, abundance and biomass
Although most genera have probably been described, and about 3 000 species have been identified, many species still remain to be discovered. When active, there can be as many as 100 000 cells per gramme of soil, but more typically numbers are 103 – 104, depending on the ecosystem.

Alveolata

Morphology
The Alveolata is a group of protists characterised by folded membranes underneath their cell membranes (called alveoli). Most species have rows of cilia that beat in a coordinated manner, and a specialised funnel structure for capturing and ingesting prey. They also often have specific defensive or aggressive structures, called ejectosomes. These are made of mucus that is ejected from the cell. A complex network of vacuoles inside the cell regulates the digestion of food and the water balance.

Taxonomy
There are three main supergroups in the Alveolata (Apicomplexa, Dinoflagellata and Ciliophora), but only Ciliophora (ciliates) are found free-living in the soil. Most ciliates ingest bacteria, but some ingest other protists or are specialised symbionts or parasites.

Microhabitat
As soil dries, the ciliates’ habitat becomes restricted to water films on surfaces. They detect prey by chemical-sensing and swim toward the signal, or away from toxic molecules. Their dispersal is by water infiltration through soil pores, or in the air if dry soil is disturbed.

Diversity, abundance and biomass
More than 1 500 species of soil ciliates have been described, but many more remain undescribed so far. Temperate soils typically hold 20 – 30 species per gramme of soil, but most are inactive. In moist soils with plenty of bacteria or prey, there can be 10 000 active cells per gramme declining to none in very dry soils. Although the biomass of ciliates per gramme of soil is very low, when active they can ingest several hundred bacterial cells per minute.

 

Ref: Global Soil Biodiversity Atlas p38