Moss is one of the oldest land plants, a non-vascular organism that first colonized terrestrial environments more than 400 million years ago. Despite this ancient simplicity, moss possesses a biological capacity that allows it to survive in conditions that would kill most other plants. The plant’s life cycle requires liquid water for reproduction, yet it can endure the complete absence of moisture for extended periods, appearing brown and lifeless. This adaptation allows moss to thrive globally despite its dependence on water for reproduction.
Poikilohydry: The Biological Mechanism for Survival
The ability of moss to survive long periods without water is governed by a strategy known as poikilohydry, meaning its internal water content fluctuates freely and equilibrates with the surrounding environment. Unlike vascular plants, which employ mechanisms like a waxy cuticle and extensive root systems to maintain a stable internal water balance, mosses do not attempt to resist water loss. Instead, their cells are built to tolerate profound dehydration, a process that can remove up to 95% of their water content.
The cellular machinery prepares for this state of dormancy by synthesizing protective molecules. The sugar trehalose is one such molecule, acting as a stabilizing agent for the cell membrane and internal organelles when water is withdrawn. As the cell dries, trehalose forms a glassy matrix that holds cellular structures in place, preventing them from collapsing or rupturing. This protective sugar layer ensures that the cellular architecture remains intact until moisture returns.
Another adaptation involves the production of Late Embryogenesis Abundant (LEA) proteins. These proteins are hydrophilic, meaning they are attracted to water, and they help stabilize other proteins and nucleic acids, such as DNA, when the cell becomes desiccated. The LEA proteins bind to these components, preventing them from aggregating or denaturing in the absence of surrounding water molecules. This biochemical defense mechanism allows the moss to enter a deep, dormant state where metabolism is virtually shut down without fatal damage.
This physiological shutdown means that the moss is not merely resisting desiccation but is fully tolerating it at a cellular level, a strategy known as desiccation tolerance. The moss Syntrichia ruralis, a desert species, provides an example of this mechanism, using these proteins to shield its photosynthetic apparatus from damage caused by high light exposure. By pausing its biological clock and protecting its cellular infrastructure, the moss transforms into a resilient package ready for reawakening.
Factors Determining Desiccation Survival Duration
The duration a moss can remain in this desiccated, dormant state is variable, depending on internal physiology and external environmental conditions, ranging from weeks to many years. Some hardy species, such as Anoectangium compactum, have demonstrated an ability to revive after being dried and stored for 19 years. The desert moss Syntrichia caninervis has regenerated after 17 years of desiccation.
The specific species of moss is a primary factor, as desert-dwelling varieties have evolved superior tolerance mechanisms compared to those found in moist temperate forests. Survival is maximized in cool, dark environments, which minimizes metabolic expenditure and prevents damaging free radicals. For instance, exposure to high light and heat during the dry period can reduce survival time by increasing oxidative stress, even in the dormant state.
Rapid, complete dryness often aids in long-term survival by forcing the moss into the deepest form of dormancy quickly. Low ambient humidity facilitates this, allowing the plant to shut down systems before cellular damage accumulates. This resilience was demonstrated when spores of Physcomitrium patens survived nine months on the exterior of the International Space Station. Based on this data, researchers estimated these spores could survive in the vacuum of space for up to 15 years.
The Rapid Process of Rehydration
When water returns, the moss’s recovery is swift, reversing the dormant state in a matter of minutes or hours. The non-vascular structure of moss allows it to absorb liquid water over its entire surface through capillary action, acting like an efficient sponge. This rapid absorption quickly hydrates the cells, causing collapsed cellular structures to regain their shape and volume.
Once rehydrated, metabolic functions are reactivated, with respiration often resuming within one minute of contact with water. Photosynthesis, the mechanism for producing food, typically takes slightly longer to recover, with measurable net carbon exchange often achieved within 15 to 20 minutes. This quick return contrasts sharply with the slower recovery rates seen in many other drought-tolerant organisms.
For mosses that have endured long periods of dryness, prehydration (exposure to humid air before liquid water) can significantly improve survival and regeneration. This gradual reintroduction of moisture helps the cells acclimate and repair minor damage before the metabolic shock of liquid water absorption. Studies have shown that a few hours of prehydration can increase the survival rate of moss desiccated for over a decade.