A poor climate for liking lichens

They’re an unassuming organism that most of us pay little mind to, but lichens – a composite creature formed from the marriage of algae and fungi – are actually incredibly important cornerstone species.

They’re critical for establishing new ecosystems, forming a crust over rock which they slowly break down, creating a carpet of soil for new organisms to grow on and thrive. They provide food for animals in the local ecosystem, and prevent soil from eroding away. They even help condition the air that we breathe, because they allow algae to live all over the world, in all sorts of climates, breathing in carbon dioxide and breathing out oxygen.

In fact, we probably owe lichens a huge debt of gratitude; without them, life on Earth may never have evolved to be so complex and rich.

But a new study, out today in Frontiers in Microbiology, suggests time might be running out for these ever-useful, ever-versatile organisms, because the algae that make up part of the lichen symbiosis cannot adapt quickly to a changing climate.

“In this study, we set out to learn how rapidly the climate preferences of these algae have evolved over time, and relate them to predictions about future rates of climate change,” says Matthew Nelsen, a research scientist at the Field Museum, Chicago, US, and the lead author of the new paper.

What did the researchers find? 

“Terrible, awful things,” says Nelsen. “We found that the predicted rate of modern climate change vastly exceeds the rate at which these algae have evolved in the past. This means that certain parts of their range are likely to become inhospitable to them.”

The group of lichen-forming algae that Nelsen and his colleagues examined is called Trebouxia. When these algae take up residence in a lichen, the fungus provides the physical structure while the miniscule algae provide food via photosynethsis.

“When you see a lichen, you’re basically looking at all fungal tissue, with some algal cells hidden away and protected inside,” says Nelsen. “Loosely speaking, it’s like a greenhouse – the fungus creates a more hospitable environment for the algae.”

To figure out whether Trebouxia could withstand a rapidly changing climate, Nelsen and colleagues compared family trees of related algal species, looking at the climate preferences of more and less closely related algae. 

By cross-referencing these phylogenetic trees with historic changes in climate and other weather variables, and undertaking statistical analysis, they found that the upper thermal limits of Trebouxia evolve at a slower rate than the lower thermal limits. In simple terms, that means it’s harder for Trebouxia to adapt to warming than to cooling. They also found that Trebouxia algae have evolved at a slower pace in general than many other species.

Ultimately, they caution that it could take hundreds of thousands, if not millions of years, for Trebouxia to adapt to the kinds of temperature changes we’re on track to see within the next century.

“I was shocked,” says Nelsen about the team’s findings. “I should have known better from the other papers I’ve read, but I was disturbed to see it.” 

More than 7,000 species of lichen are powered by Trebouxia, making it the most common algal partner in the lichen world. But if the planet continues to warm at the rates predicted, many Trebouxia species will struggle to survive, likely having downstream impacts on all sorts of species within their finely adapted ecosystems.

“Lichens are the dominant vegetation on 7% of the Earth’s surface,” explains Nelsen. “They play roles in ecosystem hydrology by retaining moisture. They also play roles in carbon and nitrogen cycling, and some of them are used by animals for food or nesting materials.”

That’s not to say that all 7,000 Trebouxia lichens are destined for extinction.

“I think we’re going to see the ranges of these things shift, and that could lead to some shuffling of the relationships with fungi– we might get partnerships that weren’t there previously,” says Nelsen. 

“Since algae are the food source for the fungus, they’re the ones photosynthesizing and making sugars to give to the fungus. If they’re forced to move, then the fungal partner would either have to move too, or develop a new partnership.”