Switch of Partners in Lichens is Predictable
Lichens, a symbiosis between a fungi and alga or cyanobacteria, have colonized almost every corner of the earth. One of the secret to these composite organisms’ success is their ability to replace one of their partners. In ‘Proceedings of the Royal Society B,’ a team of Senckenberg researchers currently presents the results of a study on the lichen genus Umbilicaria that will allow scientists to predict when this partner turnover will occur. Temperature plays a decisive role in this regard. The researchers hope to use the results to improve their predictions of how symbioses, including the lichens, deal with global climate change.
Lichens are present around the globe and are to be found in the arctic tundra as well as the Sahara. Many species of this symbiosis between a fungus and an alga or a cyanobacterium are able to thrive under a wide range of climatic conditions. This is possible due to a fascinating mechanism: Depending on the climate, one symbiotic partner (for example the alga) can be replaced. Researchers of the Senckenberg Biodiversity and Climate Research Center now studied exactly when one partner has to go and will be replaced by another.
To this end, a team led by evolutionary biologist Dr. Gregor Rolshausen took a closer look at a widespread genus of green algae, Trebouxia, which serves as a preferred symbiotic partner for many lichens, including the species Umbilicaria pustulata and Umbilicaria phaea. These lichens grow in Europe and North America at elevations between 100 and 2,700 meters and exchange their Trebouxia partner on their way from the valleys to the mountaintops.
“This is a bit like switching from summer tires to snow tires. At the foot of the mountain, the temperature is rather warm; therefore, the lichen’s fungal partner enters into a symbiosis with a warm-loving Trebouxia alga, since it is optimally adapted to the climate there. With increasing elevation, the temperatures get cooler, and to ensure the continued productivity of the symbiosis, it is time to switch to an alga that tolerates colder weather,” explains Rolshausen and continues “Surprisingly, this partner turnover always occurs under the same climatic conditions in the mountains that we have studied. Wherever one Trebouxia species in the Umbilicaria lichens on the mountain is replaced by another, we encounter roughly the same climate that is found at the transition from the Mediterranean to the cool-temperate zone.”
More specifically, this means that the Trebouxia algae in the Umbilicaria lichens are exchanged in areas with a mean annual temperature of 12 degrees Celsius and an average temperature of 5 degrees Celsius during the coldest quarter. The exchange occurs rather abruptly in a zone where the ranges of the heat-loving and the cold-tolerant algae species overlap. This exchange zone usually covers an elevation gain of approximately 150 to 200 meters and is found at slightly different heights on each mountain, depending on the local climate profile.
Trebouxia algae form a part of over half of the world’s lichen species. According to the study’s authors, this suggests that such clearly delineated transition zones with specific climate conditions also exist among other lichens. Thus, it would be possible for the first time to predict the partner turnover in lichens.
In addition, lichens are the classic example of a symbiosis, which often assume ecological key functions in nature. “Lichens are pioneer species and contribute to nutrient cycles and soil formation, particularly at higher latitudes. Whether lichens can cope with the changing climate by switching partners will depend on each partners’ individual response to climate change as this will determine if the turnover zone will continue to exist. This is true for symbiosis in general. Our research helps to improve predictions on whether symbiosis will continue to perform their important functions,” adds Rolshausen in closing.
Rolshausen, G., Hallmann, U., Dal Grande, F., Otte, J., Knudsen, K. and Schmitt, I. (2020): Expanding the mutualistic niche: parallel symbiont turnover along climatic gradients. Proceedings of the Royal Society B, doi: 10.1098/rspb.2019.2311