Some cyanobacteria can protect themselves from ultraviolet radiation by producing sunscreen pigments. In particular, the sheath pigment scytonemin protects cells against long-wavelength UVA radiation and is only found in cyanobacteria which are capable of extracellular polysaccharide (EPS) production. The presence of a putative glycosyltransferase encoded within the scytonemin gene cluster, along with the localization of scytonemin and EPS to the extracellular sheath, prompted us to investigate the relationship between scytonemin and EPS production under UVA stress. In this study, it was hypothesized that there would be a relationship between the biosynthesis of scytonemin and EPS under both UVA and oxidative stress, since the latter is a by-product of UVA radiation. EPS production was measured following exposure of wild-type Nostoc punctiforme and the non-scytonemin-producing strain SCY59 to UVA and oxidative stress. Under UVA, SCY59 produced significantly more EPS than the unstressed controls and the wild type, while both strains produced more EPS under oxidative stress compared to the controls. The results suggest that EPS secretion occurs in response to the oxidative stress by-product of UVA rather than as a direct response to UVA radiation.

Research on the UVA, UVB and oxidative (as reactive oxygen species, ROS) stress response in cyanobacteria has typically focused on each individual stress condition, with limited studies addressing the intersection. Here, we evaluated the transcriptomic responses of the model cyanobacterium Nostoc punctiforme after exposure to each of these conditions. Overall, response to UVA was characterized by more gene down‐regulation than the UVB or ROS response, although UVB affected over fourfold more genes than UVA or ROS. Regarding expression patterns, responses to UVA and ROS were more similar and differentiated from those to UVB. For example, genes involved in ROS metabolism were up‐regulated under both UVA and ROS. However, when it came to RNA and protein metabolism, there were more up‐regulated genes under UVB and ROS compared to UVA. This suggests that the response to UVB and ROS is more active than the response to UVA, which stimulated more genes in secondary metabolism. Histidine kinases and response regulators were often differentially expressed, demonstrating that regulatory systems were at the base of the patterns. This study provides background for future studies targeting different genes, proteins and systems sensitive to these conditions. It also highlights the significance of considering multiple stress conditions.