Resistance mechanisms to UVR stress of two Nostoc commune strains from warm and cold deserts unveiled by differential gene expression

Cyanobacteria can survive in extreme environments, such as drylands or polar regions, and are found in aquatic or terrestrial habitats around the world. Their different resistance traits to cope with extreme and contrasted environments, such as the production of pigments to absorb UV ra diation (UVR), have been well studied. However, a better understanding of their ecophysiological responses is necessary to clarify how these processes work. In the present study, we analyzed the ecophysiological performance of two Nostoc commune strains under UVR. Strain ULC002 was isolated from a microbial mat in an Antarctic lake (Larsemann Hills, East Antarctica) and CANT2 from a biological soil crust in a semiarid region (limestone quarry in Gádor, SE Spain). To analyze their resistance to UVR, both strains were incubated for 3 days under a UV-B and –A radi ation of 2 W/m2 and 10 W/m2, respectively, followed by a recovery period without UVR. The chlo rophyll a, scytonemin and carotenoid contents were extracted from the cultures and measured with a spectrophotometer at different time points, as well as the photosynthetic efficiency (Fv/Fm, measured by Pulse-Amplified Modulation spectrophotometry). Both strains showed a decrease in Fv/Fm values under UVR but the chlorophyll a contents showed a different pattern, being more stable in UL002 than in CANT2. In view of these results, the RNA of each strain was extracted at three different time points and sequenced by Illumina Novaseq. We analysed the differential
gene expression of both strains to understand the genetic basis of their ecophysiological respons es to UVR.