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One of life’s most important symbiotic partnerships may be threatened by a warming climate.

Lichens — a composite organism made from cyanobacteria or algae entangled within the body of a fungus — may be evolutionarily outpaced by changing climatic conditions thanks to a slow rate of evolution in the algal component of the ancient collaboration.

at the Field Museum in Chicago and his colleagues have been investigating how the climatic preferences of the lichen algae — which Nelsen describes as the “understudied” partner in lichen — have changed over evolutionary time, and how this relates to what the algae are facing with ongoing climate change.

The team focused on a single genus of algae, Trebouxia, which is found in about 7000 species of lichen. For comparison, there are only about 6400 described mammal species on Earth today.

“It really puts it in perspective that this is quite a lot of diversity these algae are responsible for maintaining,” says Nelsen.

The team collected data on where Trebouxia occurs across the world, and noted the climatic conditions at each location. The group also used a database of Trebouxia genes to create a global family tree for the algae, which revealed which forms of the algae were ancestral to the others. Using all of this information, Nelsen’s team could estimate how rapidly Trebouxia – and the lichens it lives within – have adapted to changing climatic conditions in the past.

The team found that the algae were slow to adapt to new climates, shifting temperature preferences by less than 1°C every million years. This rate is “substantially lower” than the 1°C to 4°C global temperature rise predicted over the next 80 years or so, says Nelsen.

“It is surprising that it’s so low,” says Nelsen, noting that the rate is at the low end of what has been calculated for a wide range of plant and animal groups in previous studies. “It’s just really disheartening to see actual numbers and data showing how discordant these two rates are: the predicted rate of global climate change, versus the rate at which [preferences] changed in the past.”

at the New York Botanical Garden says that having a longer generation time could make rapid evolution less likely, and that lichen biology may partially explain the slow pace of evolution.

“What is the generation time of an organism that can essentially live forever, in many ways?” says Lendemer.

Nelsen predicts that lichens that depend on Trebouxia will disappear from many of the places they are found in today, although there may also be some migration of the lichens to locations with tolerable temperatures and humidity.

“They’re going to have to shift their distribution,” he says. “They’re going to have to disperse to roll with the punches.”

Then there is the matter of whether the habitat at the migratory destination is even suitable for the lichens, says Lendemer. Environmental degradation caused by humans means that there are limits on the available area for lichens to spread into, regardless of how welcoming the climate might be. Other lichens may need to form new algal-fungal partnerships to survive.

All of this could have far-reaching ecological impacts. Lichens play crucial roles in soil stabilisation and moisture retention and can serve as food or shelter to animals.

“The overarching picture of this representing a potentially dire situation is certainly real, because we know that lichens are greatly impacted by rapid changes,” says Lendemer – for instance, the changes due to pollution or physical disturbance of the environment by humans.

For Nelsen, potential next steps include using this study’s approach to gain insight into the fungal partners’ rate of climate adaptation, and lab studies to verify the thermal limits of the algal symbionts.