The Rhizosphere: A Microscopic Ecosystem

Significant but Small

The word “ecosystem” conjures up images of green grass, lush forests, bunny rabbits, and foxes. Rarely does the image of dark, damp spaces filled with worms and bacteria come to mind. But just because an ecosystem is microscopic doesn’t mean it isn’t important. This tiny ecosystem is called the rhizosphere and it exists between plant roots and the soil. Its only requirements are water and an established homeostasis between all players. 

Microorganism Roles

The inhabitants of the rhizosphere, from smallest to largest, are bacteria, protists such as ciliates and amoebae, fungi and nematodes (roundworms, not the parasitic kind though). Protists feed on bacteria, nematodes feed on bacteria and protists, and fungi compete with bacteria. Plants purposely excrete nutrients for the bacteria via the roots. This nourishes and affects all the other microorganisms in what’s called a bottom-up effect.

The active grazing of bacteria by protists and nematodes releases organic* nitrogen and phosphorus into the soil, which are then absorbed and used by plants to build proteins. You can think of the microorganisms in the rhizosphere as plant livestock. They need to be fed, taken care of, and their products are consumed by the plant. Plants are able to adjust what they feed the bacteria to increase or decrease organic nitrogen, carbon, and phosphorus needs. 

Larger Ecosystems

Plant growth relies on the health of the rhizosphere. And as autotrophs, because plants are the foundation of all ecosystems, their ability to thrive affects all organisms up the trophic levels (another bottom-up effect). Tropical trees with large leaves provide residences and nesting spots for insects and birds alike. Grasslands provide grazing ground for bison and nesting places for small rodents. One of the problems with groundwater pollution is the destruction of the rhizosphere, which in turn can lead to plant death and ecosystem collapse. 

The rhizosphere affects the flavor of the grapes and therefore the flavor of the wine.

Rhizosphere and Wine

A line can be drawn through Europe showing the separation between beer drinkers (aka: barley growers) and wine drinkers (aka: grape growers). The taste of beer is affected by the water quality and the yeast strain. Since no water is added to wine, the taste of wine is affected by the grapes themselves (and yeast as well, but let’s just talk about the grapes for now).

So, what affects the grapes and vine growth? Why, the rhizosphere. Changes in rainfall, sunlight, and temperature are all factors that can change the rate at which microorganisms grow. These environmental factors can also put selective pressure on certain species of soil microorganisms. This bottom-up effect influences the way the vines grow and the way the grapes taste. Perhaps it is fitting that the French concept of terroir comes from the Latin root terra, meaning ground or soil.

*A Little Chemistry

I’ve mentioned the word “organic” a few times. In the context of chemistry, organic simply means molecules that are commonly found in living things; they’re mostly made of carbon, nitrogen, oxygen, and hydrogen atoms. While there is nitrogen and carbon in the atmosphere in the form of N2 and CO2 gas, these are what we call “inorganic forms”. They cannot be used directly by living organisms to build proteins (usually). The process to do so is complicated and requires a lot of energy and resources. Organic nitrogen and carbon are used to make proteins in the cell and phosphorus is used to create ATP, cellular energy. This is why available organic nitrogen, carbon, and phosphorus are all crucial for growth of an organism. 

To see how labor intensive it is to make organic carbon, we’ll use the example of plants making glucose from carbon dioxide via photosynthesis. This process involves not only a series of enzymes, but large amounts of surface area to absorb sunlight (leaves, which the plants must expend energy to grow and maintain). The process is inefficient mainly due to the enzyme RuBisCO whose job is to stick CO2 together to make glucose, a type of organic carbon. Strangely enough, instead of evolving a better enzyme, plants compensate by making a lot of RuBisCO. Since enzymes are just proteins that catalyze cellular reactions and reactions use ATP, you can see why it is beneficial for the plant to give some of its precious nutrients to the bacteria in the soil, promoting grazing by protists and increase organic nitrogen and phosphorus availability.


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