Cold and Desiccation Tolerance of Mosses and Lichens

After a wonderful vacation in Costa Rica (with ocean temperatures of ~85^oF) it was a shock to return to the Midwest and experience the Polar Vortex! Temperatures in West Lafayette, Indiana plummeted to a high of -2^oF and a low of -17^oF on January 30, 2019. Thankfully, temperatures rebounded to 56^oF yesterday (February 3, 2019) and it almost felt like spring!

Inspecting our magnificent 60-year old maple and ash trees yesterday, I was in awe at the rapid recovery of the epiphytes (mosses and lichens) on their bark. Our ash tree is one of the few remaining in the area because of the Emerald Ash Borer; we have been treating the tree for the last several years to protect the tree from this invasive species.

Epiphytic moss and lichens on our maple tree (February 3, 2019)

Epiphytic moss and lichens on our maple tree (February 3, 2019)

Epiphytic moss at the base of our ash tree (February 3, 2019)

Mosses and lichens are notoriously cold hardy and desiccation tolerant (Takezawa (2018), Tuba et al. (1996)). Their cold hardiness (freezing tolerance) is a function of their desiccation tolerance: by losing water this prevents intracellular ice formation (Lenné et al. (2010)). Upon rehydration they rapidly repair damaged membranes and their photosynthetic apparatus achieving full recovery in a few hours (Proctor et al. (2007)). This is facilitated by the production of proteins called rehydrins during the recovery from desiccation (Oliver et al. (2005)).

Lichens are composite organisms that arise from a symbiotic association of algae and/or cyanobacteria with multiple fungal species. The algae and/or cyanobacteria provide photosynthate for the symbiotic fungus. The fungus helps gather moisture and nutrients from the environment, and produce protective antioxidants in copious quantities. They are estimated to occupy 6% of Earth's land surface and may be among the oldest living things (for further details, please see the Wikipedia article on Lichen).

References:

Lenné, T., Bryant, G., Hocart, C.H., Huang, C.X., Ball, M.C. Freeze avoidance: a dehydrating moss gathers no ice. Plant Cell Environ. 33: 1731-1741 (2010)

Oliver, M.J., Velten, J., Mishler, B.D. Desiccation tolerance in bryophytes: a reflection of the primitive strategy for plant survival in dehydrating habitats? Integr. Comp. Biol. 45: 788-799 (2005)

Proctor, M.C., Ligrone, R., Duckett, J.G. Desiccation tolerance in the moss Polytrichum formosum: physiological and fine-structural changes during desiccation and recovery. Ann. Bot. 99: 75-93 (2007)

Takezawa, D. Mechanisms underlying freezing and desiccation tolerance in bryophytes. Adv. Exp. Med. Biol. 1081: 167-187 (2018)

Tuba, Z., Csintalan, Z., Proctor. M.C.F. Photosynthetic responses of a moss, Tortula ruralis, ssp. ruralis, and the lichens Cladonia convoluta and C. furcata to water deficit and short periods of desiccation, and their ecophysiological significance: a baseline study at present-day CO₂ concentration. New Phytol. 133: 353-361 (1996)