Thousands of species of tiny plantlike organisms may evolve too slowly to keep up with Earth’s changes as the climate crisis continues, according to new research.
These tiny organisms are algae, something you likely picture floating as a green film on top of a pond or lake.
But when algae team up with fungus, they form colonies of lichen — those light-colored, curly patches growing on rocks and trees in your backyard. And with lichen as their vehicle, algae have the versatility to inhabit a vast range of ecosystems around the globe, living anywhere from the Arctic tundra to the most arid desert. Lichen is the dominant vegetation, covering 7% of the planet’s surface.
Lichen grow on the order of millimeters per year and can be thousands of years old. Even if the middle of the lichen begins to break down and decompose, the outer edge will keep spreading out like a ring.
But these slow-growing lichens also move at a snail’s pace when it comes to evolutionary change: They don’t adapt quickly enough to match the pace with which our planet is shifting, especially as it heats up.
Tracking evolutionary change
In order to understand how lichen may adapt, the researchers looked at the genetic relationships of different algae species for comparison, as well as their varied environments.
By creating family trees of algae, researchers could track its evolutionary changes. The scientists realized how long it takes for algae to get used to an environment and its temperatures, precipitation amount and seasonal changes. The algae within lichen can take hundreds of thousands, if not millions, of years to adapt to their preferred climates, according to the new study.
“I was shocked,” said lead study author Matthew Nelsen, a research scientist at Chicago’s Field Museum. “I should have known better from the other papers I’ve read, but I was disturbed to see it. It’s so close to home, on a group of organisms near and dear to my heart.”
A symbiotic relationship
When it comes to lichen, the algae supply food in the form of sugars while the fungus serves as a habitat, creating a symbiotic relationship. And over time, lichen have evolved to appear in the most extreme environments, including around volcanoes.
Their purpose varies depending on where they live, but lichen create oxygen, prevent erosion, provide nesting material, retain moisture and contribute to water cycling in ecosystems. They also serve as a food source for many species including some mammals, like reindeer.
“When you see a lichen, you’re basically looking at all fungal tissue, with some algal cells hidden away and protected inside,” Nelsen said. “Loosely speaking, it’s like a greenhouse — the fungus creates a more hospitable environment for the algae.”
The level of diversity is just part of what makes lichen so charismatic to Nelsen. There are about 20,000 species of fungi that help form lichens — more than all of the species of mammals and birds combined.
“They may be a bit more subtle and not as cute, but it’s still a substantial amount of diversity out there,” Nelsen said. However, he noted that lichen can appear stunning and almost otherworldly in nature.
A changing world
If the planet continues to warm at its current rate, that will outpace what many Trebouxia can adapt to, which could cause ripple effects.
“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,” Nelsen said.
“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.”
Algae and lichen have been able to survive previous shifts in Earth’s global temperatures, but climate change is causing those to occur much more quickly.
“Closely related algal species tend to have similar climatic preferences, as predicted by their evolutionary relationships,” Nelsen said. “The most closely related ones might live in really similar climates, whereas distantly related species might differ more in their climatic tolerance.”
When a climate changes, it’s not uncommon to see plants or animals appear in new environments, where they compete with existing species. While it doesn’t mean that the 7,000 Trebouxia algae will simply disappear, it indicates that changes are on the horizon. If the algae partner of lichen begins to die out, the fungus might be taken out as well, or the algae may have to slowly move to another area.
“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,” Nelsen said.
“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.”
Going forward, Nelsen wants to determine how these lichen survive and even thrive in extreme and diverse environments, test the temperature range limits of algae they can withstand, and understand more about the fungus component of lichen and how it reacts to change.
Nelsen believes lichen research could be applied to other aspects of understanding how climate change will unfold.
“They have interesting stories behind them, and it’s just kind of up to us to try to figure out what exactly they’re doing out there in nature,” Nelsen said.