Responses of Biofilm-Forming Halophilic Calothrix and Coelastrella Strains to Environmental Stressors Associated with Climate Change

Research into the effects of environmental stressors associated with global climate change (GCC) on cyanobacteria and microalgae is scarce, with bloom-forming planktonic cyanobacteria being the exception. This study aimed to address the issue by assessing morphological and biochemical changes in cyanobacterial and microalgal cells exposed to an increased temperature (T), ultraviolet radiation (UVR) and carbon dioxide (CO₂) concentration. The strains selected were <i>Calothrix</i> sp. SLM0211 and <i>Coelastrella</i> sp. SLM0503, which were isolated from a coastal environment in the central Mediterranean island of Malta. Elevated UVR had a pronounced effect on <i>Calothrix</i> sp. filaments, which produced screening compounds and resorted to trichome coiling to enhance self-shading. Enhanced growth was observed in cultures of <i>Calothrix</i> sp. grown at an increased CO₂ concentration, which produced significantly high amounts of biomass, chlorophylls and carotenoids. An increased T resulted in stunted growth and low biomass accumulation in both strains. Each strain exhibited a unique response to T and UVR stressors, which stimulated the production of exopolymeric substances (EPS) and mycosporine-like amino acids (MAAs) in cultures of <i>Calothrix</i> sp. and lipid production in <i>Coelastrella</i> sp. cells. Our findings indicate that the effects of stressors related to GCC on cyanobacterial and microalgal cells are strain-specific, making changes at community and ecosystem levels difficult to predict.