By Nicole Masters
The protists do not fall into the bacteria, animal, plant or fungal categories. These microscopic organisms are loosely grouped by the presence of a nucleus and membrane. In evolutionary terms, these organisms represent the original ancestors of all living cells; your great, great, great…grandma or pa, which morphed out the depths around 2 billion years ago as oxygen levels rose.
In a teaspoon of healthy vibrant soils, there may be as many as a million of these creatures building the corridors, living spaces and highways through the soil environment. Single-celled and mobile, protists have diverse and vital roles in both soil and plant health. They speed up nutrient cycling, control bacterial populations and stimulate plant defence.
There are 3 major groups in soil: flagellates, amoebae and ciliates. Flagellates are the smallest, named for the one or 2 whip-like flagella they use to zip around soil. Many of the amoebae are much slower moving, using a pseudopod, or false foot, to ooze through the soil moisture, absorbing all who fall into their path. Amoebae can consume a variety of food sources, some herbivorous species resemble tiny bloated dairy cows, peacefully grazing on algae and dead organic materials. Testate amoebae, who fashion themselves in hardened shells for protection, have been observed hunting together as a pack, to track down and attack bacteria-feeding nematodes. While the ‘vampire amoebae’ (Vampyrella) drill into algae and fungi and then suck away their life essence. The larger ciliates (up to 500um), named for the hair-like cilia which cover their bodies, zip through soil feeding upon bacteria, fungi, other protists and nematodes. Their bodies also provide tasty morsels for hungry micro-arthropods.
Protists have an important role in plant growth and are one of the main groups responsible for nutrient cycling to the plant. They release plant growth hormones such as auxins and cytokinins, stimulating an increase in root length and branching. These voracious animals may eat as much as 10,000 bacteria in a single day, excreting plant-available nutrients in their wake. As they feed upon bacteria and fungi, in a dynamic relationship with the plant, they selectively graze upon bacteria or fungi to maintain the bacteria: fungi (B: F) ratios in the soil. The valuable activity from protists grazing on bacteria can contribute to 75% of plant-available nitrogen. As they are important predators for bacteria, a deficit in protists can lead to a boom in bacterial populations, creating a cascade of impacts throughout the soil foodweb.
All the microbes in the soil foodweb are interconnected, in part why it’s called a “web.” The term first arose in the 1960s with the realisation that all organisms are limited by food and are connected to each other. These relationships are often complex and unpredictable. Take protists for instance, in the presence of disease organisms, they stimulate bacteria to produce signaling molecules, which aid the plant in self-defense. A study in Maize (Zea Mays) showed that grazing protists reduced stress metabolites in the plants, and increased yield.
In a process called a “microbial loop,” plants create conditions more conducive for growth. By releasing root exudates, plants stimulate bacterial populations, which in turn attracts more protists and nematodes. These predators then release the nutrients bound in the bodies of bacteria in plant-available forms back to the plant.
What is concerning now is that under intensive breeding programs, many modern plant cultivars have lost their ability to communicate with many beneficial soil organisms, from protists to their fungal partners, the mycorrhizae.
Creating the ground for the protists:
- Protists are feeding on fungi and bacteria, so a program that builds biological biomass will build protists.
- Increasing carbon energy flows, from root exudates or carbon-based fertilisers, stimulate protist biomass and diversity.
- Reduce practices known to be harmful; such as overgrazing, synthetic fertilisers and pesticides.
- Ruminant saliva is a hotbed for protists. While livestock manure is a rich source protists, including earthworm poop!
You can always trial making a protozoa tea to see if protozoa are low in your soil ecosystems. Soak 5 kg of high-quality unsprayed lucerne (alfalfa) hay for 3 days in 200 litres fresh water. If it smells or froths up, then more water is needed. Spray 50 litres/Ha diluted or apply more concentrated to test plots. If you are low in protist biomass the plots will flush green like you’ve just applied nitrogen.
Rossmann et al (2020). Multitrophic interactions in the rhizosphere microbiome of wheat: from bacteria and fungi to protists, FEMS Microbiology Ecology, Volume 96, Issue 4, April.
Xiong et al. Soil protist communities form a dynamic hub in the soil microbiome. ISME J 12, 634–638 (2018).