Two Antarctic Nostoc sp. strains from freshwater and terrestrial habitats show contrasting resistance mechanisms to desiccation and re-wetting

Filamentous heterocystous cyanobacteria belonging to Nostoc genus are worldwide ecosystem engineers in dry and oligotrophic environments. In Antarctica, members of this genus are of ten dominant in both edaphic and lacustrine habitats. Yet little is known about their adaptation mechanisms enabling them to thrive such contrasting environments conditions, and during their dispersal between these habitats. Here we studied the response to short-term desiccation ex posure and rehydration of two Antarctic terrestrial (ULC180) and freshwater (ULC008) Nostoc strains sharing 97.7% of ANI similarity. We compared the concentration of different pigments and osmolytes (i.e. trehalose and sucrose), the photosynthetic efficiency as well as the differential
gene expression after RNA-seq (Illumina NovaSeq 2x150 bp) between the controls (T0) and the different treatments (D = after 3h of desiccation; RW1 = after 10 min of rehydration; RW2 = 24h of rehydration; RW3 = 72h of rehydration). Both strains reacted to dehydration by accumulating su crose, whereas trehalose was present in lower concentrations. Only the freshwater strain showed a recovery in chlorophyll a content after 72h of rehydration. Transcriptomic profiles showed that both strains protected their cells during dehydration by inducing stress-related genes, such as those for the production of carotenoids, trehalose and nitrogen fixation, but these were significant ly up-regulated only in the terrestrial strain. The latter one also responded with a higher number of up-regulated genes compared to the freshwater strain, including those necessary to protect the
photosystem II from degradation (e.g. psbA2 gene), enabling a stronger resistance to dehydration of ULC180 compared to ULC008.