Plants – Bacteria – Fungi

Symbiosis

It is important to understand that symbiosis is an ancient form of collaboration between various forms of life and key in an ecosystem construction. Only by imagining the long genesis of two interrelated organisms and the tools of interconnection they have developed to collaborate we may start to consider the importance of such mechanisms in a complex ecosystem. Scientists think that plants / mycorrhizae symbiosis started 450 million years ago, when plants migrated to land. Legumes / rhizobia (nitrogen fixing bacteria) symbiosis is more recent; 60 million years old and may have evolved from plant fungi symbiosis mechanism.

In recent studies we learn that fungi and bacteria use a mechanical and chemical stimulation to start a symbiotic connection with a plant.

After reception of the signal by a fungus the plant builds a canal. Once the path is completed inside the plant the fungus is informed, penetrates the plant at canal level and starts to grow inside the plant to form an anchor for resources exchanges.

Such a “civic” processes presuppose an essential and productive collaboration between the 2 organisms and the importance of promoting this type of interconnection in modern agriculture for better yield.

In the case of symbiosis with nitrogen fixing bacteria the plant goes even further and the root hair of the plant when receiving the bacteria signal will trap the colony of bacteria curling the hair root around it.

Plant / Bacteria association

Plants attract bacteria to their root zones by producing “exudates” (sugar = soluble carbohydrates). Bacteria feed on these exudates.

Bacteria are the most protein dense organisms and the main nutrient material of the soil food web. They attract predators; protozoa and nematodes mainly, which feed on them and reject nitrogen and other minerals in surplus.

The minerals are then used by the plant as nutrients. The loop is closed.

In addition to this process 2 important mechanisms play a role in plants and bacteria association;
– Plant symbiosis with nitrogen fixing bacteria (it is the case for leguminous trees and mycorrhizae). In a depleted system (poor in nitrogen) an important ratio of leguminous species is necessary to help plants growing.
– Competition between plants and bacteria to access nitrogen. Here biodiversity is important so that bacteria predators can control this competition.

Plant / Fungi association

The symbiosis with fungi (mycorrhizal), another building block of soil food web, uses as well exudates, this time to trade fungi capacity to extend the plant root system and route minerals, water and phytochemicals over long distances. Fungi have much smaller diameter than plants root which allow them to penetrate the soil more effectively and reach nutrients and water over long distances.

Not tilling allows to respect the fragile network of fungi and increase plant nutrients uptake.

Organic nitrogen, phosphorus, sulfur, potassium, calcium, magnesium, iron and essential trace elements such as zinc, manganese and copper are returned to plant hosts in exchange for carbohydrates.

Fungi plays as well a direct role in Carbon storage and Nitrogen uptake;

There are 2 types of mycorrhizal fungi; ecto and ericoid mycorrhizal fungi, abbreviated as EEMF or EEM, and arbuscular mycorrhizal fungi, abbreviated as AMF or AM.

– Plants absorb carbon dioxide from the atmosphere and release part of the carbon into the soil; this carbon is taken up principally by the AMF, which processes it rapidly and act as a buffer, releasing the carbon gradually to other microorganisms in the soil over time.

– EEMF produces nitrogen-degrading enzymes, which allows them to extract more nitrogen from the soil than the AMF type and gives a competitive advantage to the plants against microorganisms for nitrogen availability. The resulting effect of a better nitrogen uptake is the reduction of carbohydrate decomposition and a higher ratio of carbon per unit of nitrogen.

Fungus and bacteria symbiosis with plant have been identified but many other symbiosis take place in the soil food web, for example nitrogen-fixing bacteria have been found inside ectomycorrhizae mantle.

Flow exchanges in the food web

Plants play 4 essential roles: harvest energy from the sun, fix carbon from the atmosphere, store the energy in the form of carbohydrates and feed the soil ecosystem.
Bacteria play 3 essential roles; store plant energy into a stable organic form (themselves) inside the soil, fix nitrogen from the air and provide food for the ecosystem (themselves).
Fungus play 3 essential roles: store carbon from the plant into the soil, extend plant root system access to nutrients, water and other resources and help nitrogen uptake by the plant

These 3 organisms have as well another important role; degrade matter for nutrients uptake (rocks, carbohydrate, lignin, …)

Bigger the plants (mostly trees and perennials) more energy is harvested from the sun and more influential is the orchestration of the soil life and the resulting fertility of the soil.

Source: Colin Averill, College of Natural Sciences at University of Texas in Austin.
Source: Proceedings of the National Academy of Sciences (PNAS)
Source: Jean-Michel Ané – University of Wisconsin-Madison

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