The hidden truth about the symbiotic relationship in lichens, revealed

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Lichens

Look closely at the forest floor, the tree trunks, or even the smother stones- you can see them everywhere. Bright yellow, brownish in color clinging to their substrates. You might mistake them to be moss or something insignificant and undeserving of your attention, but quite often they are the scientifically fascinating lichens.

Growing up as a nature-loving, home-schooled, curious kid in a Montana Trailer park, Toby Spribille was quite fascinated by these organisms he found in the forests of Montana.

In the 1860s, scientists thought these astonishingly beautiful creatures were plants. Only in 1868-69, did the Swedish botanist Simon Schwendener claim in his “dual hypothesis” that lichens were composite organisms made up of algae and fungi in a parasitic relationship. Schwendener’s conclusion that lichens were a modified form of fungi in which the algae was “enslaved” was criticized heavily by his colleagues and later, proved wrong by other scientists.

Other scientists showed that the fungi, usually an ascomycete and algae had a mutually beneficial relationship: the fungus provided shelter to algae in its mycelium; while the algae fed its host with nutrients by producing sugar from carbon dioxide and water through photosynthesis. Two germans, Albert Frank and Anton de Bary later coined a new word for this relationship – “symbiosis” which meant ‘together’ and ‘living’ in Greek.
For over 150 years, lichens have been the model organisms for explaining a symbiotic relationship. Every time scientists tried to artificially unite the fungus and algae to grow lichens in the laboratory, they have failed. Something was amiss and Spribille has finally discovered it! 
Schematic cross section of a foliose lichen: 1. The cortex is the outer layer of tightly woven fungus filaments (hyphae) 2. This photobiont layer has photosynthesizing green algae 3. Loosely packed hyphae in the medulla 4. A tightly woven lower cortex, with anchoring hyphae called rhizines where the fungus attaches to the substrate. Credit: Wikipedia
Schematic cross section of a foliose lichen: 1. The cortex is the outer layer of tightly woven fungus filaments (hyphae) 2. This photobiont layer has photosynthesizing green algae 3. Loosely packed hyphae in the medulla 4. A tightly woven lower cortex, with anchoring hyphae called rhizines where the fungus attaches to the substrate. Credit: Wikipedia
Until now, macrolichens, the largest and most-species rich group of lichens were thought to be an alliance between two organisms. However, Spribille has proved that it is in fact, an alliance between three organisms- including a second type of fungus. It almost seems like the second type of fungus was hiding in plain view, all along!

“There’s been over 140 years of microscopy,” says Spribille. “The idea that there’s something so fundamental that people have been missing is stunning.”

The path to this discovery began when Spribille, armed with a doctorate from Germany, joined the lab of symbiosis specialist John McCutcheon. The duo started studying two lichens that are common in local forests and hang from branches like unruly wigs. One of the lichens, Bryoria tortuosa is yellow and produces a strong poison known as vulpinic acid. Another lichen made up of the same fungus and algae – Bryoria fermontii is dark brown and does not produce the acid.

In an attempt to understand this discrepancy, Spribille studied the genes that the two lichens were activating. The gene expression, however, showed no differences.

Traditionally, lichenologists assumed that lichens consisted of fungi belonging to the ascomycetes family and hence Spribille’s search was limited to ascomycetes genes. He then broadened his search to the entire fungal kingdom and found that a lot of genes that were activated in these lichens belonged to an entirely different family – basidiomycetes.

At first, the duo thought that the basidiomycetes fungus growing on the lichen was a false alarm. Perhaps it was just a contaminant or a pathogen infecting the lichens causing disease. But when Spribille removed all the basidiomycetes gene from his data, everything related to the presence of vulpinic acid disappeared. That was the Eureka moment!

He later found that fungus from the basidiomycetes family was present in both types of lichens but especially abundant in the yellow, toxic one. This abundance enabled the lichen to produce acid that helped defend it against invasion from other microbes.

After ascertaining the presence of basidiomycetes in these two lichens, Spribille began screening about 45,000 samples of lichens he had collected over his entire career – from different lineages, continents. In almost all the macrolichens, he found the genes of basidiomycetes fungi!! After confirming their presence on a molecular level, he analyzed them with his own eyes. 

Under the microscope, the lichens were found to have a spongy loose interior and a stiff, dense crust, much like a loaf of ciabatta bread. The algae were found embedded in the thick crust and the ascomycetes were found branching inwards creating a spongy interior.

The basidiomycetes were found in the outermost part of the crust, surrounding the other two partners. “They’re everywhere in that outer layer,” says Spribille. 

Although the location of the basidiomycetes was seemingly obvious, it took almost 5 years for scientists to figure that out!

That’s because they were embedded in a matrix of sugars on the surface. They had to carefully strip away the matrix with a detergent. Still, they couldn’t identify it easily, as they looked exactly like the ascomycete branches in cross-section. Unless you had a reasonable amount of doubt that there were more than one fungi there, you wouldn’t have persevered to find the third partner. One of the reasons why it lay undiscovered for 150 years.

Only after Spribille labeled the three partners with different fluorescent molecules, did the trinity become absolutely clear and evident.

“Toby took huge risks for many years,” says McCutcheon. “And he changed the field.”

The theme of symbiosis has resonated throughout this research and this discovery was also possible only due to the culmination of a fruitful collaboration between the various scientists, who were experts in natural history, microscopy, lichenologists, and genomics. 

This discovery, which overturned the two organism paradigm may now allow scientists to grow lichens in a laboratory by using the three partners.

Perhaps, it is time to revise the textbook definitions of lichens!

Source: The Atlantic