Fungal Symbiosis: Mycorrhizae and Lichens

Introduction

Fungal symbiosis represents one of nature’s most successful evolutionary strategies, where fungi form mutually beneficial relationships with other organisms. Two predominant forms of these relationships—mycorrhizae and lichens—have profound impacts on ecosystem functioning and biodiversity. Mycorrhizal associations between fungi and plant roots facilitate nutrient exchange that benefits both partners, while lichens represent composite organisms formed by fungi and photosynthetic partners that can colonize harsh environments.

Mycorrhizal Symbiosis

Definition and Types

Mycorrhizae (from Greek mykes “fungus” and rhiza “root”) are symbiotic associations between soil fungi and plant roots. In these relationships, fungi colonize the host plant’s root tissues to create an interface for metabolic exchanges.

Functional Mechanisms

The mycorrhizal relationship works through bidirectional nutrient exchange:

1. Plant to Fungus: Photosynthetically derived carbon compounds (primarily hexoses) flow from plant to fungus, providing energy.
2. Fungus to Plant: The fungus supplies:
   • Phosphorus (often limiting in soils)
   • Nitrogen
   • Water
   • Micronutrients (zinc, copper, etc.)

The extensive fungal mycelium effectively increases the absorptive surface area of the root system by 100-1000 times, dramatically improving nutrient acquisition.

Ecological Significance

Mycorrhizal networks serve numerous ecological functions:

• Common Mycorrhizal Networks (CMNs): Also called “Wood Wide Web,” these underground networks connect multiple plants, allowing resource sharing and communication.
• Seedling Establishment: Mycorrhizal fungi improve seedling survival rates.
• Soil Structure: Fungal hyphae produce glomalin, a glycoprotein that improves soil aggregation and carbon sequestration.
• Plant Diversity: Mycorrhizal fungi can mediate plant competition and contribute to plant community structure.
• Stress Protection: Enhanced tolerance to drought, salinity, heavy metals, and pathogens.

Lichen Symbiosis

Definition and Structure

Lichens are composite organisms resulting from symbiosis between fungi (mycobiont) and photosynthetic partners (photobiont)—usually green algae or cyanobacteria. Rather than a simple partnership, lichens represent a complex integration where partners form a unique organism with properties distinct from either component.

Lichen Morphology

Lichens exhibit three primary growth forms:

Lichen Reproduction

Lichens reproduce through several mechanisms:

1. Sexual Reproduction: Involves only the fungal partner, producing spores that must find compatible photobionts.
2. Asexual Reproduction: Through fragmentation or specialized structures:
   • Soredia: Small clusters of algal cells wrapped in fungal hyphae
   • Isidia: Finger-like outgrowths containing both partners
   • Lobules: Small leaf-like structures that detach

Ecological Roles

Lichens perform crucial ecological functions:

• Pioneer Species: First colonizers of bare surfaces, initiating soil formation.
• Bioindicators: Sensitivity to air pollution makes them valuable environmental monitors.
• Nutrient Cycling: Contribute to weathering of rocks and nutrient release.
• Food Web: Provide food for invertebrates, birds, and mammals.
• Habitat: Create microhabitats for small invertebrates.

Evolutionary Perspectives

Origins and Antiquity

• Mycorrhizal associations are ancient, dating back approximately 400 million years, coinciding with plant colonization of land.
• Fossil evidence from the Rhynie chert (Early Devonian, 410 million years ago) shows arbuscular mycorrhizal structures nearly identical to modern forms.
• Lichen-like symbioses may be even older, with some estimates suggesting origins over 600 million years ago.

Evolutionary Significance

• The evolution of mycorrhizal associations likely facilitated the terrestrialization of plants.
• Lichens demonstrate how symbiosis can lead to novel ecological innovations, allowing colonization of extreme environments.
• Both relationships highlight how symbiosis can drive evolutionary innovation through combining complementary metabolic capabilities.

Applications and Human Relevance

Agricultural Applications

Mycorrhizal fungi offer significant benefits for sustainable agriculture:

Lichen Applications

Lichens have diverse uses:

• Biomonitoring: Environmental pollution assessment
• Natural Dyes: Traditional textile colorants
• Traditional Medicine: Anti-microbial and anti-inflammatory compounds
• Perfume Industry: Fixatives for scents
• Food Source: Emergency food in some cultures; ingredient in traditional dishes

Conservation Implications

Both mycorrhizal networks and lichen communities face threats from:

• Climate change
• Air pollution
• Land-use changes
• Agricultural intensification
• Nitrogen deposition

Conservation strategies include:

• Maintaining forest continuity to preserve mycorrhizal networks
• Reducing air pollutants to protect sensitive lichen species
• Incorporating fungal symbionts in ecosystem restoration efforts
• Preserving ancient woodlands as reservoirs of fungal diversity

Future Research Directions

Current frontiers in fungal symbiosis research include:

• Harnessing mycorrhizal networks in climate change mitigation
• Understanding cross-kingdom communication via fungal networks
• Developing lichen-inspired biomaterials
• Exploring pharmaceutical potential of lichen compounds
• Using metagenomic approaches to understand symbiont diversity

FAQs

Q: Can plants survive without mycorrhizal fungi? A: While some plants can survive without mycorrhizae, approximately 80-90% of terrestrial plants form these associations. Plants without mycorrhizae often struggle in natural environments, particularly in nutrient-limited soils.

Q: Are lichens plants? A: No, lichens are not plants. They are composite organisms consisting of fungi and photosynthetic partners (algae or cyanobacteria). Despite their plant-like appearance, they lack true roots, stems, and leaves.

Q: How do fungal networks help trees communicate? A: Mycorrhizal fungi connect multiple trees, forming a “Wood Wide Web.” Through this network, trees can exchange nutrients, defense signals, and carbon. When one tree is attacked by insects, it can send chemical warning signals through the fungal network to neighboring trees.

Q: Are mycorrhizal fungi the same as the mushrooms we eat? A: Some edible mushrooms are the fruiting bodies of ectomycorrhizal fungi (like porcini and truffles), but many culinary mushrooms are saprotrophic decomposers rather than mycorrhizal symbionts.

Q: Can lichens grow on any surface? A: Lichens are incredibly adaptable and can grow on diverse substrates including rock, bark, soil, leaves, and even man-made materials like concrete, metal, and glass. Some species are highly specialized to particular substrates.

Q: How quickly do lichens grow? A: Lichens grow extremely slowly, typically 0.1-10 mm per year depending on the species and environmental conditions. Some crustose lichens on rocks may grow less than 1 mm per year.

References

Allen, M.F. (1991). The Ecology of Mycorrhizae. Cambridge University Press. https://doi.org/10.1017/CBO9780511525025

Brodo, I.M., Sharnoff, S.D., & Sharnoff, S. (2001). Lichens of North America. Yale University Press. https://yalebooks.yale.edu/book/9780300082494/lichens-of-north-america

Bungartz, F., Nash, T.H., & Sanders, W.B. (2023). Lichen Biology and the Environment. Cambridge University Press. https://doi.org/10.1017/9781108769341

Gorzelak, M.A., Asay, A.K., Pickles, B.J., & Simard, S.W. (2015). Inter-plant communication through mycorrhizal networks mediates complex adaptive behaviour in plant communities. AoB Plants, 7, plv050. https://doi.org/10.1093/aobpla/plv050

Nash, T.H. (2008). Lichen Biology (2nd ed.). Cambridge University Press. https://doi.org/10.1017/CBO9780511790478

Smith, S.E., & Read, D.J. (2008). Mycorrhizal Symbiosis (3rd ed.). Academic Press. https://doi.org/10.1016/B978-0-12-370526-6.X5001-6

van der Heijden, M.G.A., Martin, F.M., Selosse, M.A., & Sanders, I.R. (2015). Mycorrhizal ecology and evolution: the past, the present, and the future. New Phytologist, 205(4), 1406-1423. https://doi.org/10.1111/nph.13288