Lichens are the most famous and successful examples of symbiosis on Earth, but an unexpected discovery of a third player in this composite organism has given their study a much needed jolt.
Lichens are special. They can be grey, green, yellow, black, flaky, crusty, leafy, bush-like, powdery – the list goes on. Moreover, they are everywhere: you’ve almost certainly seen them, even if you did not identify them – from the Arctic Tundra and deserts to rainforests. In fact, 6% of Earth’s land is estimated to be covered by more than 20,000 known species of lichen, and more than 2,000 of which are abundant in India.
So what are lichens and how are they able to take on such diverse forms?
The first part of that question has been a lot easier to answer than the second. Lichens are ‘composite’ organisms, meaning they are made up of two major components – a fungus and a photosynthesising partner, making them the most famous example of the ecological interaction we know as symbiosis. The fungus usually belongs to a group called ascomycete and the photosynthesiser is either an algae or cyanobacteria. Swiss botanist Simon Schwendener discovered this interaction about 140 years ago and this piece of knowledge has been one of the backbones of lichenology since then.
The second part of the question has been more difficult to answer. Catherine Aime, a mycologist from Purdue University, Indiana, along with a team of international scientists, attempted to get a clearer picture. “We were trying to determine why two different ‘species’ of lichens had the exact same mycobiont (fungal part) and phytobiont (algal part), but looked completely different,” she wrote in an email.
In pursuit of an explanation, Aime and team inadvertently stumbled upon evidence proving that the century-old wisdom of lichen’s fungi-algae symbiosis is only two-thirds of the complete story. They realised that a third living component to these species was causing the change in appearance – a type of yeast called basidiomycete. Their results were published in the journal Science on July 21.
To confirm their findings, the scientists collected samples belonging to 52 different groups of lichens from six continents and tested them. They could detect basidiomycete yeast in all of them. “These yeasts had not previously been found anywhere else in nature, they were completely unknown to science,” said Aime.
To get a better picture of its nature and its evolutionary history, they subjected it to a slew of genetic analyses. The results suggested that the yeast appeared to be of the same evolutionary age as the other fungal portion of the lichens. “This was some indirect evidence that this partnership is as old as these lichens,” said Aime. Further studies will shed light on the exact nature of yeast’s role in this symbiosis, but Aime suspects that they may be providing chemical protection to the lichen.
Big implications
This discovery impacts our understanding of lichens in several ways, according to Sanjeeva Nayak from the National Botanical Research Institute, Lucknow. Nayak was not involved in the new study. “It starts from basic questions such as what type of association these organisms have? What is the role of each partner in the association? Why or how did this association necessitate? How long back did this take place?” Nayak asks, also wondering what will happen to lichen taxonomy after this. “Should we now consider the third partner for nomenclature now?”
Smitha Hegde, a biologist at Aloysius College, Mangaluru, welcomes this shake-up in the field. “Lichens, although primitive, are least understood. [The discovery] will now open up possibilities of several new biochemical pathways which these forms use to adapt, thrive and survive.”
Hegde, who specialises in ferns – another primitive species – rues that lower forms of life are generally paid less attention to unless they have commercial importance. “In this trend of product-based research, knowledge-based research has taken a back seat,” she said.
This could be one reason this discovery took so long. Aime said that the availability of better tools today that allow us to ask much more refined questions than in the past has also played a role. Another reason the yeast escaped our notice for more than a century, said Nayak, could be that it has no visible or distinct structure, shape or colour in any lichen thallus. “Lichenologists are dependent on mostly visible characters (even under microscopes).”
Moreover, whenever the yeast did form noticeable structures, scientists dismissed it as contamination, infection or superficial growth of other fungus, Nayak said. “Such contaminations are common on lichens.” The team involved in the current study were able to eliminate the contamination possibility using DNA and fluorescent analysis to confirm their exact location within the thallus.
Now that a missing piece of the lichen puzzle has been found, scientists expect that many other mysteries will be solved. The abundance of these yeasts and their location could correlate with the previously unexplained variations in forms of lichen. This new knowledge will also help scientists synthesise lichen in the lab, something they have been unsuccessful at so far. “Earlier attempts to generate lichens in the lab were done without knowledge of this third partner, including it in the mix now could theoretically lead the way to successful synthesis,” said Aime.
The possibilities are numerous and this could very well herald a new era in lichenology. “I am excited,” says Nayak. “The new finding will change our view towards lichens. Now on there will be more chaos, more confusion in lichenology, more material for research.”