Lichen algae remodel their cell walls in response to drying

Lichens consist of a symbiotic partnership between a fungus and one or more photosynthetic organisms belonging to either the cyanobacteria or the green algae. Lichens occur in a wide variety of environments, including extremely arid ones in which the lichen will undergo repeated cycles of desiccation and rehydration (D/R). Plants that tolerate desiccation are known to do so in part because they have high cell wall (CW) flexibility which allows a ‘controlled collapse’ of the cell without lasting damage, but the mechanisms by which lichens survive similar conditions are poorly understood.

In a recent paper published in Annals of Botany, María González-Hourcade and colleagues investigated the structural and biochemical features of the CW of two lichen microalgae, one adapted to rapid D/R cycles, the other to milder seasonal dry periods. The researchers exposed the two algae to up to four cycles of D/R and then visualized their CW structure using scanning and transmission electron microscopy. They also carried out biochemical analysis of the CW composition of each alga.

The authors found that during the desiccation process, the cells progressively shrunk and deformed, though the plasma membranes stayed in contact with the CWs. Within certain limits, this change could be rapidly reversed with the addition of water. The alga adapted to seasonal dryness was less tolerant of rapid drying and very low moisture levels than the species acclimated to a more extreme environment, with the latter withstanding as low as 10% relative humidity, while the former required at least 60%.

Biochemically, the experiments revealed that the composition of CW polysaccharides changes after desiccation, but that the changes differed in the two species studied. “Biochemical remodelling of the CW appears to play a crucial role in controlling its biomechanical properties, helping lichen microalgae to cope with rapid changes in the cell’s hydric status” the authors write. They also note that the study “supports the notion that CW remodelling is an active and species-specific process, induced by exposure to desiccation conditions similar to those encountered in the natural habitats in which each alga/lichen thrives.”