There is big news in the world of lichens. These slow growing organisms have long been known to be a collaboration between a fungus and a photosynthetic algae or cyanobacteria. A recent publication in Science may have changed all that.
Researchers have discovered another fungus living in the tissues of lichens. Unlike the dominant fungal type, also known as Ascomycetes, the new fungus is a Basidiomycete that exists as single cells, more closely related to yeast. A survey has found these new fungal cells in 52 genera of lichens, raising the prospect of a previously undetected third partner in the ancient symbiosis.
Interestingly, despite many attempts, it has never been possible to synthesise lichen in the laboratory by combining the two known partners, and now we might know why. Lichenologists have always recognised a mycobiont (fungal partner) and a photobiont (the photosynthetic organism that makes food) and now we may have to find a word for the new fungal component.
Toby Spribille of the University of Graz in Austria and his colleagues were trying to understand why two species of lichen that were made up of the same species of mycobiont and photobiont were differently coloured and contained varying levels of a toxin known as vulpinic acid.
Using an approach that examined the messenger RNAs produced by the organism, they tried to find the genes that produced the toxin, but neither the mycobiont or the photobiont had genes that matched the transcript. By broadening their search to include other types of fungi, they found genes belonging to a rare fungus called a Cystobasidiomycete.
Unable to see the cells responsible for this unusual finding, they used fluorescent in situ hybridisation (FISH) to light up cells containing genes for the algae, the ascomycete and the cystobasidiomycete. By linking different colours to each organism, they produced videos showing the distribution of each cell type. The new fungus existed as single cells inside the cortex, where it may play a structural role as well as providing chemical defence.
It is hard to overstate the importance of this discovery. Spribille was quoted in the New York Times as saying that lichens are as diverse as vertebrates. And yet we did not know until now that the symbiosis that allows lichens to exist has more than two partners.
The authors have described a new order of fungi called the Cyphobasidiales. It is not everyday that scientists are able to add new taxa at such a high level. It is like discovering the Primates. By creating a phylogenomic tree and applying a molecular clock, they found that this group has been around for 200 million years, probably since the beginning of lichens.
The 52 genera that have been examined thus far are widespread (on six continents) but are still a small portion of lichens, so there may be more to discover. Interestingly, the continent that is not included is Australia. Perhaps we do not have enough lichenologists to provide samples to the international community. It is possible that some lichens do not contain this new order of fungi. What is not in doubt is that now scientists will be looking at lichens more closely.
Lichens grow very slowly. Individuals can be hundreds or even thousands of years old. Now it seems that our knowledge of this ancient symbiosis has also grown slowly, as it has taken 150 years to find the third partner.
Given the sophisticated techniques required to untangle this conundrum, I suppose it was not possible to know about the silent partner, the yeast in the mix, until now. But it certainly gives rise to some exciting science.