A Tale of Two Microbes

by Katie Kelly

Growing up amongst vast monocultures of corn and soybean made me appreciate the details. I could easily spend hours outside sitting in the backyard staring at the grass and rocks. The largest rock in my backyard always caught my interest.

Blue-gray circles cover most of the rock (Figure 1). The patches didn’t look to be part of the rock; in fact, they resembled moss, but these patches were not spongy or green. I would pick at these enigmatic patches and cut them up, but they did not possess any apparent roots. They did not produce any leaves or flowers. No stems appeared throughout the year. Over the years, I noticed similar perplexing patches on live trees and decaying wood. My mom told me that this patch was lichen, a living organism. In truth, she was half-right.

Figure 1. The rock of interest. A) Small boulder/large rock from New Lenox, IL. B) Blue-gray patches of interest. Credit: Carrie Kelly.

Lichen is two organisms living together, or “symbiotically.” A lichen species is shorthand for a pair of two microorganisms: an algal species and a fungal species1. The unicellular algal species is referred to as the “photobiont” because it photosynthesizes and provides carbon nutrients to the fungal partner^1,2. Trebouxia species are the most common photobiont in lichens, making up 40% of all observed lichens³.The fungal species is referred to as the “mycobiont” and is responsible for the majority of visible growth of the lichen. Lichen may also contain a third symbiont which is bacterial³.

Lichen diversity is vast due to the huge number of lichenizing fungal species and the ability to have diverse symbiotic partners. Approximately 25% of all fungi are lichenized, with a majority of fungal species members of the family Ascomyceta (members are “ascomycetes”)³. It’s important to note that the estimated number of fungal species on Earth is between 712,000 and 1.5 million^4,5. Even with the most conservative estimate, that’s a lot of lichenizing fungi. Its dominance is evident: lichen is found on every continent, even the barren Antarctica⁴. So no matter where I go, I will find lichen—and photograph them, too (Figure 1, 2, concluding pictures).

Figure 2. Lichen found in Hamilton, MT. Credit: Katie Kelly

This diversity in symbiotic partners leads to diverse morphologies. The visible morphologies of lichens are categorized by their shapes: flat, crust-like lichen are crustose; bead-like clusters are squamulose; leaf-like lichen are foliose; and branched tube lichens are fruticose (Figure 2)⁶. The color of lichen depends on the metabolism of the mycobiont⁷. Whether a lichen is blue or yellow can depend on whether the fungal species is producing sugar chains or amino acids⁷.

Figure 3. Lichen morphology. These panels represent the major morphologies of lichen, respectively: crustose, squamulose, foliose, and fruticose⁶.

Although the visible morphologies of lichens vary between species, the microbiological organization of the photobiont and the mycobiont is largely conserved. Differences in color and morphology aside, the major structure of lichens is fairly consistent across different partners. A cross section of a lichen ‘body’ or thallus shows specific layers: a thin, tightly-packed layer of fungal hyphae; a layer of algal cells able to access light; a large gap with some hyphae, used for gas exchange; and often a portion of fungal tissue which anchors the lichen to its base (Figure 4).

Figure 4. Structure of lichen. The general architecture of lichen is conserved in all of the different morphologies (Figure 3). This consists of the fungal upper cortex, which is what we can see; the algal layer; a more open cavity consisting of mainly fungal hyphae (seen as strings in this figure); and the lower fungal cortex, which binds or anchors the lichen⁸.

The beauty and amazing diversity of lichens masks one of the more sinister traits. Although many believe the photobiont and mycobiont live mutually benefitting each other, the jury is still out. Why? Some researchers claim the fungal counterpart is actually parasitic. The fungal counterpart will wrap itself around the algal cell, penetrating its hyphae into the algal cell wall to take nutrients⁹. Figure 5 demonstrates this invasive ability, the same method plant pathogens use to infect plants⁹. An algal cell can even be crushed by the force of the fungus and die (Figure 5B).

However, the major difference between Figure 5A and 5B is the species of algal cell. Both alga are in the same genus (Trebouxia), but 5A is Trebouxia erici and 5B is Trebouxia gelatinosa. This suggests specificity to relationships between certain species of photobionts and mycobionts.

Although you may be thinking “That poor Trebouxia algal cell!” right now, don’t worry. Some Trebouxia species have been shown to benefit from this association. Research has shown that lichen partners are more able to withstand extreme drying out and oxidative stress than they would on their own². This highly adaptive trait allows them to withstand long periods without water and high amounts of chemical stress. This research suggests how this resilient partnership has flourished on every continent.

My favorite cartoon science teacher always says, “Let’s take a closer look!” and I live by that motto. Lichens are more than meets the eye: a small symbiosis, a mini-battleground for nutrients, a resilient partnership that weathers the storm. It’s amazing what we can see when we look a little closer.

References

• 1. Lisci, M., Monte, M. & Pacini, E. Lichens and higher plants on stone: a review. Int. Biodeterior. Biodegrad. 51, 1–17 (2003).
• 2. Kranner, I. et al. Antioxidants and photoprotection in a lichen as compared with its isolated symbiotic partners. Proc. Natl. Acad. Sci. U. S. A. 102, 3141–3146 (2005).
• 3. Honegger, R. in Fungal Associations (ed. Hock, P. D. B.) 165–188 (Springer Berlin Heidelberg, 2001).
• 4. Hawksworth, D. L. The magnitude of fungal diversity: the 1·5 million species estimate revisited. Mycological Research (2001). Available at: /core/journals/mycological-research/article/the-magnitude-of-fungal-diversity-the-15-million-species-estimate-revisited/EC6F0D9391AA820DE7AEBB043172CB0B. (Accessed: 11th November 2016)
• 5. Schmit, J. P. & Mueller, G. M. An estimate of the lower limit of global fungal diversity. Biodivers. Conserv. 16, 99–111 (2007).
• 6. Morphology of Lichens. Available at: http://www.ucmp.berkeley.edu/fungi/lichens/lichenmm.html. (Accessed: 11th November 2016)
• 7. Nash, T. H. Lichen flora of the greater Sonoran Desert region. (Lichens Unlimited, Arizona State University, 2002).
• 8. Lichens - Basic Morphology. Available at: http://www.arctic.uoguelph.ca/cpl/organisms/plants/terrestrial/lichens/basicmorph.htm. (Accessed: 11th November 2016)
• 9. Ahmadjian, V. & Jacobs, J. B. Relationship between fungus and alga in the lichen Cladonia cristatella Tuck. Nature 289, 169–172 (1981).