Understanding cell wall adaptations in desiccation-tolerant microalgae: insights from observations and ultrastructural investigations

Desiccation-tolerant (DT) plants exhibit high cell wall (CW) flexibility and are able to remodel the CW, a property also thought to be present in DT lichen-forming microalgae. Scanning electron microscopy (SEM) observations of partially desiccated Trebouxia gelatinosa (lichenized) and of T. decolorans (free-living, in axenic cultures) revealed the presence of regular, reticulate shapes in the CW that appear when the cell collapses. These regular shapes, which also occur in slightly dehydrated cells, suggest the presence of a "scaffold" of CW components with different biochemical and mechanical properties. These specialised regions of CW might facilitate shrinkage of the remaining regions of the CW while following the cell membrane to prevent plasmolysis during dehydration. Similar reticulate forms have been observed in phylogenetically distant DT microalgae such as Sanguina nivaloides (at cyst stage), Chlorella vulgaris, and Auxenochlorella pyrenoidosa but have never driven much attention, when they might represent an important ultrastructural adaptation to desiccation. A recent study of the ultrastructure and biochemical composition of the CW of Trebouxia sp. TR9 and Coccomyxa simplex revealed that these algae can actively remodel the CW composition during dehydration-rehydration cycles. For these reasons, further studies at SEM and transmission electron microscopy are needed to describe CW shrinkage of lichen-forming and free-living terrestrial microalgae and to verify whether this reticulate shape is a common feature of DT microalgae. This research could provide new insights into the mechanisms these organisms use to survive desiccation.