Invasive species such as seaweeds often have a broad tolerance, allowing them to colonize novel habitats. During invasion, also new epibacteria can be formed on seaweeds, which have important chemo-ecological effects. Since UV-radiation (UVR) is one of the main factors affecting seaweeds and their epibacteria, we tested its effect on intertidal and subtidal thalli of the invasive seaweed Codium fragile from three sites and monitored photosynthesis, antioxidant activity and epibacteria. Exposure to UV-radiation resulted in photoinhibition with a subsequent low recovery in subtidal thalli from 23°S compared to 27°S and 30°S, which both showed a higher and almost complete recovery. However, a high antioxidant activity was present in all thalli, permitting to explain its relatively high tolerance to new environments. UV-radiation modified the composition of the epibacteria community by reducing its diversity and evenness. Our results showed that C. fragile responds plastic to variable UV-radiation (depending on site and water depth), which contributes to its high invasion potential.
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