Acoustic emissions and water dynamics in desiccating lichens: insights into hyphae cell wall collapsing and cavitation events

Lichens, as poikilohydric symbiotic organisms, have to cope with fluctuations in water content, which significantly impact their metabolic activities and, in particular, photosynthesis. Recent studies on the mechanisms of water movement within the lichen thallus suggest that during desiccation the photobiont layer of foliose lichens retains a higher water content than the surrounding fungal pseudo-tissues. The starting hypothesis was that cavitation events in the fungal hyphae could cause water release to the algal cells. However, measurements of ultrasonic acoustic emissions (UAE) conducted on desiccating lobes of Flavoparmelia caperata and Lobaria pulmonaria showed that these occur at such low water contents that they are not compatible with the starting hypothesis. The aim of this study was to understand what phenomena caused the recorded UAE. Thalli of the two species were rehydrated to full turgor, then two sets of hydrated thalli were equilibrated to a saturated NaCl solution and to silica gel in order to obtain relative water contents (RWC) of 16.6 ± 1.1% and 4.8 ± 0.5% respectively, i.e. RWC above and below the value corresponding to an increase in UAE rate (RWCUAE). Thalli were observed at the cryo-SEM to identify possible ultra-structural changes that could explain the energy emission associated with UAE. As expected, at full turgor in both species almost no hyphae were collapsed (collapsed hyphae, CH < 13%), while above RWCUAE they started collapsing (CH c. 75 %), and below RWCUAE they were all heavily collapsed (CH c. 100%), suggesting that the energy associated with UAE was released by their collapse. These findings stimulate further research to explore alternative mechanisms responsible for water dynamics in lichens and the potential implications for lichen biology and their adaptation to water stress.