The following blog post is third in our series on Soil Health.
"Living roots release many types of organic materials into the rhizosphere within six inches of the surface of the soil. The rhizosphere typically contains 1,000 to 2,000 times more microorganisms than the typical soil without roots."
- James J. Hoorman, Ohio State University Extension
Protozoa play a key role in nutrient cycling between plants, soil minerals, and soil organisms. W. Foissner, in a 2014 reference module wrote “about 1,600 species have been recorded from soil, many of which have special adaptations to the soil environment.” He went on to write that the estimate of protozoa species is more than 4,000. Whatever type of soil you have there are thousands of soil protozoa; eating bacteria, other protozoa, and even some fungi.
Why Protozoa are Good for Your Soil
Bacteria are tiny one-celled microorganisms that are major decomposers of organic matter in your soil. That’s a good thing, except that the bacteria retain the soil nutrients in their bodies (called immobilization) until they die or are eaten by predators. Enter the one-celled, but much larger than bacteria, protozoa. They are predators of bacteria, primarily.
The protozoan doesn’t need as many nutrients as a bacterium, so when it eats a bacterium it excretes the excess nutrients in plant-available form. Both plants and other microorganisms use these nutrients, particularly nitrogen. So, there is a constant cycling of nutrients between prey, predator, and plant.
Where Most Protozoa Hang Out
Of course, predators congregate where there is prey. If you’re a protozoan you want to be sure you’re around a lot of bacteria. Most bacteria are located where they have access to their food source, the exudates from plant roots. The highest percentage of protozoa and bacteria will be found in a plant’s rhizosphere.
One-celled microorganisms don’t move very far or very fast. Bacteria, as well as fungi, stay near plant roots and have mutualistic relationships with plants. Bacteria convert organic matter and soil minerals into plant-available forms in exchange for those root exudates. Bacteria, even if they aren’t gobbled up by a predator – don’t have a very long life. It’s been estimated that the average lifespan of a bacterium is around 12 hours.
Most bacteria get gobbled up by protozoa long before their life would be over. It’s only after the bacteria are eaten by the protozoa that nutrients are released to the plant. Remember, protozoa don’t need as many nutrients and excrete the nutrients the bacteria were hoarding.
Protozoa are water dwelling microorganisms. That means not only are they found in streams and lakes, but they also move in the water film, coating soil particles. That water film is also used by bacteria and is what agronomists call the soil water, or water in the pores between soil particles. Plant roots use that water in the soil pores and it is especially critical in drought. A lot of biological activity occurs in the space between soil particles!
Protozoa Found in Average Agricultural Soil
The thousands of species of protozoa are broken down into three main subgroups; ciliates, flagellates, and amoebae. They are grouped mainly based on their means of locomotion and these subgroups have numerous species. But the one thing they all have in common is they prey on bacteria. Additionally, their presence do indicate soil environmental conditions.
The Ciliates Subgroup
They move by tiny hairs, cilia, that surround the cell, moving like oars. They are facultative organisms which mean that they live in low oxygen conditions (below 6 ppm Oxygen) and are often an indicator that conditions are going anaerobic or were anaerobic. According to James Hoorman, from Ohio State University, like all protozoa, the "ciliates may consume as many as ten thousand bacteria per day." The cilia move bacteria to the ciliates feeding area, cytostome, by creating tiny water currents. They, like other protozoa, can develop hard shells, or cysts, to protect them until soil conditions are more favorable.
The Flagellates Group
These are the most abundant protozoa and play a significant role in nutrient cycling. Bacteria are their principal prey and they generate a great deal of soil ammonium, the plant-available form of nitrogen, through their grazing.
They agitate their flagella, a whip-like structure, to move through the water film on soil particles. They are very small, with a very flexible body, so they can access pore spaces larger protozoa can’t reach to feed on bacteria.
Flagellates are true aerobes and their presence in soil indicates oxygen conditions are equal to or above 6 ppm.
The Amoebae Group
We’ve all seen or read about “shape shifters” and amoebae are just that. There are two types of amoeba protozoa; the naked and the testate. They move by creating “pseudopods” or foot like projections that move them in the direction of the nearest group of bacteria.
The naked are adept at changing shape and exploring very tiny spaces where they sense bacterial activity. This quality makes them extremely valuable for nutrient cycling. There are some soils, such as clay, that are distinguished by tiny pore structure. In these soils naked amoebae are important and necessary for plant and soil health.
Testate amoebae are recognized by their testate, a hard shell that they produce. They are more abundant in sand and silt soils where the soil pores are larger.
How Protozoa Cycle Nutrients
Protozoa help maintain an ecological balance in the soil. When they graze on bacteria the number of bacteria increases. More bacteria mean faster decomposition of organic matter. But there can be too much of a good thing. Selective eating by protozoa of certain species of bacteria keeps the soil food web in balance.
(img source: https://www.soilfoodweb.com/)
Soil, plant, and microorganism health is dependent on that balance. If a specific species of protozoa, or any other microorganism, becomes overwhelming nature has a solution. There are predators. For protozoa those are nematodes, other protozoa, and arthropods.
Plant rhizospheres have teaming microbial communities. The rhizosphere is also where plant pathogens would like to set up shop. With a healthy beneficial protozoan community, it’s difficult for pathogens to survive. Protozoa prefer bacteria, including pathogenic bacteria, but they also eat other protozoa, including pathogenic protozoa.
The types of protozoa and the effectiveness of nutrient cycling depends on the type of soil and how the soil is managed. Agricultural production creates disturbed soils which are dominated by bacteria. In no-till and undisturbed soils, such as a forest, fungi and bacteria co-exist so there is usually a greater diversity of microorganisms.
The soil structure also determines the type of protozoa, small pores-small protozoa, large pores-large protozoa.
Protozoa and the Poop-Loop
No matter what type of protozoa is in your soil, as they consume bacteria and excrete excess nitrogen they are completing what Dr. Elaine Ingham coined as the "poop-loop". That loop consists of the relationships between the different trophic levels from the plant’s photosynthesis and its release of exudates at its roots, the bacterial consumption of these exudates and the soil organic matter, to the protozoa and other predators preying on the bacteria and releasing the excess nutrients as plant-available nitrogen.
The poop-loop does not stop at the protozoa level but continues up the food chain. The poop-loop cannot exist without the photosynthesis of plants. Plants are dependent on microorganisms and must enter into a symbiotic relationship with them in order to survive. Plants may give up to 70% of their total fixed carbon to microorganisms in exchange for water and nutrients.
Although protozoa are small as individuals, their combined biomass quantity in healthy soils is greater than all other soil microorganisms. The effects of protozoa and other microorganisms on plant health cannot be overstated. Nutrient cycling is just the tip of the iceberg.
Future Discoveries About Protozoa and the Soil Food Web
Plants are not passive participants in the poop-loop but are a controlling factor. Scientists are beginning to look at signaling pathways between plants and the environmental factors that contribute to certain microbial abundance.
The term 'rhizosphere control points' has been coined to explain the concept of information exchange between plants and microorganisms for the health of all parties. Plant root structures are affected by bacteria and the role of protozoa on bacterial species is being studied. And do protozoa have a hormonal effect on plant structure? How does the soil food web affect a plant’s ability to turn on its defense mechanisms against plant diseases or above ground predation?
These are open questions and areas of scientific debate. Soil is an incredibly complicated space and the plant rhizosphere even more so. Scientists all agree we haven’t identified all the components to healthy soil. Some components are so small scientists know of them only by their effect.
You can read the full series here: