At least one of the two cooperation partners in question may not be able to adapt quickly enough.

You have undoubtedly come across them, perhaps even in your own backyard: lichens. You can find them on rock, but also, for example, on the branches of trees. They really grow everywhere: in Dutch forests, but also on the Arctic tundra. And the lichens perform a variety of functions: in some places they serve as food for reindeer, but they are also overzealous producers of oxygen.

Collaboration

What few people realize when they see lichens is that we are dealing here with a special partnership. A lichen consists of an algae and a fungus. “When you see a lichen, you’re actually looking at fungal tissue with some algae cells in it,” explains researcher Matthew Nelsen. “Loosely speaking, it is actually a kind of greenhouse: the fungus creates a more hospitable environment for the algae.” And in return, the algae provide the fungus with food – through photosynthesis. It’s a million-year evolutionary success story. But climate change now threatens to throw a spanner in the works, Nelsen and colleagues discovered.

The research

For their study, the scientists focused on algae belonging to the genus Trebouxia† The algae – also in the Netherlands – regularly collaborate with fungi. “In our study, we wanted to find out how fast the climate preferences of these algae have evolved over time and compare that rate to the predicted rate at which the climate will change in the future,” explains Nelsen. The findings are, in Nelsen’s words, “terrible.” “We found that the predicted rate of modern climate change is much higher than the rate at which these algae evolved in the past. It means that certain parts of their habitat are likely to become uninhabitable for them.”

The researchers came to this conclusion after examining how the Trebouxia-algae in the past reacted to natural (and much more slowly progressing) changes in climate. To do this, they first made a family tree to map out how different Trebouxiaalgae were related. They then also determined for each of those species in what time and climate they lived. “By then looking at the difference between the climate preferences of a modern species and its ancestor and the time elapsed between the periods in which these two species lived, we were able to determine the rate of adaptation.” And so it turns out that in the past it took the algae quite some time – hundreds of thousands, if not millions of years – to adapt to temperature changes expected in the next century.

Changes

It is therefore expected that the algae will find it difficult to keep up with current climate change and will lose out in some (rapidly warming) areas. Exactly what that means for the lichens of which these algae are part, is highly dependent on what the fungi will do in a changing climate, and this was not examined in this study. “Since algae are the food source for the fungi, they are the ones that photosynthesize and make sugars to give to the fungi,” Nelsen said. “If the algae are forced to move, the fungus has to move with it or find a new partner.” And so the million-year-old success story of these common collaborations can suddenly become quite messy in the coming decades. “I think habitats are going to change and that could mean that relationships with the fungi are going to change too — we might even see collaborations that weren’t there before,” Nelsen predicts.

Importance of lichens

And that is still a rosy scenario. Because at the moment it cannot be ruled out that lichens in some areas may be due to the loss of those single-celled, but crucial partner in crime, disappear anyway. “I think the lichens are able to survive in parts of their current habitat,” Nelsen said. “But that will vary from species to species. It will also depend on the extent to which such a population already lives at the climatic limit.”

It is clear that the disappearance of lichens can have far-reaching consequences. “In certain areas, they stabilize the soil (and prevent erosion) or help the soil to hold more water. In addition, they serve as a food source and shelter for invertebrates (some organisms nest under larger lichens, others eat them). And some mammals also depend on lichens for their food.” In addition, lichens can also play a role in the carbon cycle, especially in ecosystems where they are the dominant ‘vegetation’ (and such ecosystems cover as much as 7 percent of the earth’s surface. “We are certainly not saying that all lichens in these areas will disappear, Nelsen emphasizes again, “We’re just stating that they play a role in ecosystems, and if some of those lichens are lost, then new lichens have to replace them that can take on this role.”

Trebouxia algae are a food source for about 7,000 different fungal species. It results in a variety of lichen species, including the one in this image: curb finger moss. Image: MeegsC (via Wikimedia Commons†

Whether there are already lichens in which a climate change struggling Trebouxia-algae is trapped, is unclear. “It’s hard to say,” Nelsen said. “When it comes to lichens, we identify and recognize them by the fungus: the algae are hidden deep in the thallus formed by the fungus. And these fungi can also switch to a new algae partner without changing their appearance. So when we know that a particular lichen is gone, we know the fungus is gone. But we don’t really know whether the algal partner is now living on its own, has started working with another fungus, or has disappeared as well. In a similar way, an algae species can disappear, while the fungus persists and mates with another algae species (without changing the appearance of the fungus). So it’s a challenge to determine exactly who is struggling without zooming in on the algae cells and determining whether they look healthy. That said, I do suspect that some of the algae that are already living at the limit are struggling. Think, for example, of algae that have adapted to cold conditions and are now unable to adapt quickly enough to higher temperatures.”

In any case, more research into the state of the lichens and the individual cooperation partners within them is desperately needed. In addition, the study underlines once again how important it is that we vigorously combat the rapid, self-induced global warming. Because in addition to famous climate icons such as the polar bear, there are also plenty of species in our proverbial and literal backyard that could be threatened by global warming. A lichen may not be the most appealing example of this, but it is a crucial part of some ecosystems into which the effects of warming could seep through considerably via such a wilting algal partner, among other things.