The Secret Kingdom: Beneath the Soil

Getting to know what you can see is one thing, but have you ever taken the time to uncover a hidden gem? As a nature enthusiast and a scientist, I spent years searching for my passion—something that would ignite my curiosity and drive me forward. The journey left me with a mind hungry for knowledge yet overwhelmed by its vastness. Eventually, I realized that true discovery often lies beneath the surface, in the parts we overlook. This insight led me to the fascinating world of fungi, where the most important aspects are often hidden from view.

Figure 1. Fungal fruiting bodies

When people think of fungi, the first image that comes to mind is usually a mushroom—the visible, above-ground part of the organism. However, just as my journey taught me to look beyond the obvious, understanding fungi requires us to look beyond the 'mushroom'. The mushroom is merely the fruiting body of the fungus, focused on reproduction. It produces spores, the fungal equivalent of seeds, which disperse to new environments, allowing the organism to propagate. But beneath the surface lies the true powerhouse of the fungus: the mycelium, a vast network of thread-like structures called hyphae (McGinnis et al., 1996). For instance, Armillaria ostoyae, the world's largest single living organism, owes its massive size not to its mushrooms but to this hidden network! This hidden aspect of fungi is crucial to understanding the organism as a whole. Just as not all fungi produce mushrooms, many aspects of life require us to look deeper, beyond the visible and immediate. This is why, while all mushrooms are fungi, not all fungi are considered mushrooms (Feeney et al., 2014).

Figure 2. Diversity of fruiting bodies of fungi

Interestingly, the mushroom is just one of many types of fruiting bodies that fungi produce. Fungi have evolved a remarkable diversity of structures to carry out their reproductive functions. For example, puffballs are spherical fruiting bodies that release clouds of spores when disturbed, while morels have a honeycomb-like structure, which maximizes spore dispersal. Another type, the bracket fungi, form shelf-like structures on trees and logs, often persisting for years as they continuously release spores. These various fruiting bodies are all specialized adaptations to different environments and modes of spore dispersal. Importantly, not all fungi produce fruiting bodies (Virágh et al., 2022). Some, like arbuscular mycorrhizal fungi, lack these structures entirely, they rely on their symbiotic relationships with plants to help disperse their spores. These fungi form intimate associations with plant roots, where they exchange nutrients and, in return, benefit from the plants' growth and interactions with other organisms (Schüßler et al., 2007) . In these relationships, the fungi rely on the plant's life cycle and the activities of soil organisms or animals that are found in the environment for spore dispersal, rather than producing a distinct fruiting body.

Figure 3. Underground partners

But why is there even a mycelium? The mycelium is the primary structure through which fungi interact with their environment, absorbing nutrients and breaking down organic matter. It functions as the organism's digestive system, secreting enzymes that decompose complex materials into simpler forms that can be absorbed and used for growth. Mycelium also plays a crucial role in nutrient cycling within ecosystems, contributing to the health of forests and other natural habitats (Islam et al., 2017). Understanding these hidden aspects of fungi—the mycelium and its diverse roles—reveals that the true essence of these organisms lies not in the mushrooms we see, but in the complex, unseen networks that sustain life beneath our feet. Just as not all fungi produce mushrooms, many aspects of life require us to look deeper, beyond the visible and immediate, to truly appreciate the connections that shape our world.

Fungi have adapted to different ecological niches, leading to the evolution of two main types of mycelium: mycorrhizal and saprophytic. Mycorrhizal fungi form symbiotic relationships with plants, with the mycelium extending into plant roots and soil, allowing for an exchange of nutrients. The fungi supply plants with essential minerals like phosphorus, while the plants provide the fungi with carbohydrates produced through photosynthesis (Huey et al., 2020). This mutualistic relationship is vital for the growth of many plants, including those in agriculture and natural ecosystems. In contrast, saprophytic fungi derive their nutrients from decaying organic matter. These fungi play a key role in breaking down dead plants, wood, and other organic materials, recycling nutrients back into the environment. Unlike mycorrhizal fungi, saprophytes do not form partnerships with living plants but are essential decomposers in ecosystems, maintaining the balance of nutrient cycles.

Figure 4. Structure of a fungus

Figure 5. Mycorrhiza

One concept that is worth mentioning when it comes to mycelium is the "Wood Wide Web" that describes the network of mycelium in forests, functioning as a sort of underground internet connecting trees and plants. This network, primarily formed by mycorrhizal fungi, allows plants to communicate and share resources with one another, trees can transfer nutrients like carbon, nitrogen, and water to other plants, especially those that are younger, weaker, or shaded from the sun (Giovannetti, 2006). This exchange helps maintain the health and stability of the entire forest ecosystem. The idea of the Wood Wide Web also suggests that trees might "warn" each other of dangers, such as pests or diseases, by sending chemical signals through the fungal network (Rhodes, 2017). This underground system challenges the traditional view of plants as solitary organisms, presenting a picture of forests as interconnected communities where cooperation can be as vital as competition.

However, the concept of the Wood Wide Web has sparked debate among scientists. While the idea of trees communicating and supporting each other through fungal networks is compelling, some researchers argue that the evidence for such complex interactions is not yet conclusive. Critics point out that while nutrient transfer through mycorrhizal networks has been documented, the extent to which this is a deliberate or beneficial process for the recipient plants remains unclear. Some skeptics suggest that what appears to be cooperation might simply be a byproduct of the fungi's efforts to expand their networks and obtain more nutrients from their plant hosts. Additionally, the notion that plants might "warn" each other through these networks is still under investigation. Some scientists argue that the transfer of chemical signals might not be as intentional or sophisticated as the Wood Wide Web theory suggests, and more research is needed to fully understand these interactions.

Despite the skepticism, the Wood Wide Web remains an exciting area of study that has transformed our understanding of how forests function. It highlights the complexity of natural systems and encourages us to consider the possibility that plants and fungi are far more interconnected and interdependent than we once thought. As research continues, we may discover even more about the hidden, underground world that sustains life above ground.

References

Giovannetti, M., Avio, L., Fortuna, P., Pellegrino, E., Sbrana, C., & Strani, P. (2006). At the root of the wood wide web: self recognition and non-self incompatibility in mycorrhizal networks. Plant signaling & behavior, 1(1), 1–5. https://doi.org/10.4161/psb.1.1.2277

Huey, C. J., Gopinath, S. C. B., Uda, M. N. A., Zulhaimi, H. I., Jaafar, M. N., Kasim, F. H., & Yaakub, A. R. W. (2020). Mycorrhiza: a natural resource assists plant growth under varied soil conditions. 3 Biotech, 10(5), 204. https://doi.org/10.1007/s13205-020-02188-3

Islam, M. R., Tudryn, G., Bucinell, R., Schadler, L., & Picu, R. C. (2017). Morphology and mechanics of fungal mycelium. Scientific reports, 7(1), 13070. https://doi.org/10.1038/s41598-017-13295-2

Jo Feeney, M., Miller, A. M., & Roupas, P. (2014). Mushrooms-Biologically Distinct and Nutritionally Unique: Exploring a "Third Food Kingdom". Nutrition today, 49(6), 301–307. https://doi.org/10.1097/NT.0000000000000063

McGinnis MR, Tyring SK. Introduction to Mycology. In: Baron S, editor. Medical Microbiology. 4th edition. Galveston (TX): University of Texas Medical Branch at Galveston; 1996. Available from: https://www.ncbi.nlm.nih.gov/books/NBK8125/

Rhodes C. J. (2017). The whispering world of plants: 'The Wood Wide Web'. Science progress, 100(3), 331–337. https://doi.org/10.3184/003685017X14968299580423

Schüßler, A., Martin, H., Cohen, D., Fitz, M., & Wipf, D. (2007). Arbuscular mycorrhiza: studies on the geosiphon symbiosis lead to the characterization of the first glomeromycotan sugar transporter. Plant signaling & behavior, 2(5), 431–434. https://doi.org/10.4161/psb.2.5.4465

Virágh, M., Merényi, Z., Csernetics, Á., Földi, C., Sahu, N., Liu, X. B., Hibbett, D. S., & Nagy, L. G. (2022). Evolutionary Morphogenesis of Sexual Fruiting Bodies in Basidiomycota: Toward a New Evo-Devo Synthesis. Microbiology and molecular biology reviews : MMBR, 86(1), e0001921. https://doi.org/10.1128/MMBR.00019-21