Living beings that cooperate with each other for mutual benefits abound in nature. Some live independently and only cooperate at certain times in their lives, such as many plants, with the insects that pollinate their flowers or with frugivorous animals, which help disperse seeds . In other cases, the symbiotic relationship is so complex and involves so many collaborators that it can be considered a kind of miniature ecosystem, like lichens.
However, the most common symbiotic relationship on the planet is a much more subtle, underground one, which goes unnoticed by our eyes. It is the one that occurs between the roots of most plants – up to 80% of the species, according to estimates – and many fungi. A symbiosis that receives the name of mycorrhiza .
The discovery of mycorrhizae is due to the German biologist Albert Bernhard Frank , who also coined the term ‘symbiosis’ and collaborated in the first descriptions of lichens. At first they were considered marginal or exceptional events, and it was not until the middle of the 20th century that the great importance of these relationships between fungi and plants was accepted.
The mycorrhizal phenomenon is thought to be as old as the first land plants , in the late Silurian. However, not all mycorrhizae evolved at the same time, nor do they have the same origin. In fact, these associations occur independently in different groups of plants and fungi, in a phenomenon of evolutionary convergence.
There are several types of mycorrhizae, although two stand out. The predominant and oldest type is the arbuscular mycorrhiza . In this type of association, the fungus extends its mycelium by branching its hyphae inside the root cells. These tiny branches are called arbuscules. All arbuscular mycorrhizal fungi belong to the same group, the Glomeromycota , and are usually associated with the plants of prairies, high mountain grasslands, and tropical forests.
The second type is the ectomycorrhizae . In this association, the mycelium extends between the cells of the plant roots, but does not penetrate them. Fungi of this type of mycorrhizae belong to the Ascomycota and Basidiomycota groups, and include fungi with edible and highly prized fruiting bodies, such as the milk cap or the black truffle. It is the most abundant type of mycorrhiza in temperate and Mediterranean forests.
These types are not mutually exclusive, in the same plant mycorrhizae of both types can appear simultaneously, and even of other more complex ones, such as ericoids, arbutoids or monotropoids.
In the relationship established in a mycorrhiza, plant and fungus come out notably benefited.
Mycorrhizal fungi have large networks of mycelia that extend through the subsoil, covering a much larger surface area than that occupied by the roots. Fungi, therefore, help plants with the supply of nutrients –mineral salts such as phosphorus, calcium, nitrogen and potassium–, which they capture through this mycelial network, and which they inject directly into the roots.
For its part, the plant nourishes the fungus, not only with sugars that it produces thanks to photosynthesis , but also by providing it with vitamins that it synthesizes through its secondary metabolism.
In addition, both benefit from protecting themselves against various types of environmental stress. The symbiotic relationship between trees and their associated mycorrhizal fungi is very intricate and complex, and so intimate that in many cases a mutual dependency is generated. That high complexity provides greater resilience to environmental impacts, although, paradoxically, that dependency can also be its Achilles’ heel.
If an event causes a disturbance that negatively affects one of the two components, the other may also be affected.
Mycorrhizal relationships are today considered essential for the maintenance of terrestrial ecosystems . Plants that have a mycorrhizal association grow better, obtain more nutrients and are, in general, more stable and resilient. The longevity of some plants is associated with the presence of mycorrhizae and, in some cases, such as certain species of orchids, even their own survival.
But in addition, mycorrhizae play an important role in the sequestration and subsequent storage of atmospheric carbon dioxide , the main gas responsible for the greenhouse effect that is leading us to anthropogenic climate change.
Fungi, by themselves, do not have the ability to capture carbon; in fact, they breathe like animals, so they absorb oxygen and release carbon dioxide in their metabolism.
But plants that have mycorrhizae are much more efficient in capturing carbon , and an important part passes to the fungi in the form of nutritious sugars. Since fungi live in the subsoil, the organic matter that makes up their organism remains aggregated in the soil horizons, where it is stored faster and in greater quantity than through roots or leaf litter.
Mycorrhizae are, in fact, the dominant pathway through which plants store atmospheric carbon in the soil.
Referencias:
Bast, A. et al. 2016. Does mycorrhizal inoculation improve plant survival, aggregate stability, and fine root development on a coarse-grained soil in an alpine eco-engineering field experiment? Journal of Geophysical Research: Biogeosciences, 121(8), 2158-2171. DOI: 10.1002/2016JG003422
Godbold, D. L. et al. 2006. Mycorrhizal Hyphal Turnover as a Dominant Process for Carbon Input into Soil Organic Matter. Plant and Soil, 281(1), 15-24. DOI: 10.1007/s11104-005-3701-6
Rożek, K. et al. 2020. How do monocultures of fourteen forest tree species affect arbuscular mycorrhizal fungi abundance and species richness and composition in soil? Forest Ecology and Management, 465, 118091. DOI: 10.1016/j.foreco.2020.118091
