Red Sea Synechococcus exhibits greater resilience to UV-B radiation than pico-eukaryotic phytoplankton

1.UV-B radiation can be a significant stressor to primary producers in tropical seas, particularly to organisms residing close to the surface. We investigated the UV-B sensitivity of natural pico-phytoplankton communities in the Red Sea, one of the warmest seas on Earth. The communities were incubated either in full sunlight or under UV-B-reduction conditions. The mean lethal UV-B radiation dose (LRD50) of natural Synechococcus was 57 kJ to 73% non-detectable compared to 8 KJ to 45 % non-detectable for pico-eukaryotes. UV-B-induced Fv/Fm change ({Delta}Fv/Fm) in natural pico-phytoplankton communities ranged from -0.22 to 0.06 (mean: -0.08). Changes in Synechococcus growth under UV-B-reduction were more pronounced in cooler waters. The dominant pico-phytoplankton in the Red Sea, Synechococcus, exhibited a strong resistance to UV-B, highlighting their exceptional adaptation to the local conditions. Our results underscore the importance of considering the UV-B sensitivity of natural phytoplankton communities in the context of global change, which may differ from those of cultured strains.