*Article* **Comparative Study of Algal Responses and Adaptation Capability to Ultraviolet Radiation with Different Nutrient Regimes**

**Lingxiao Ren 1,\*, Jing Huang <sup>2</sup> , Keqiang Ding <sup>1</sup> , Yi Wang <sup>1</sup> , Yangyang Yang <sup>3</sup> , Lijuan Zhang <sup>1</sup> and Haoyu Wu <sup>1</sup>**


**Abstract:** Frequent outbreaks of harmful algal blooms (HABs) represent one of the most serious outcomes of eutrophication, and light radiation plays a critical role in the succession of species. Therefore, a better understanding of the impact of light radiation is essential for mitigating HABs. In this study, *Chlorella pyrenoidosa* and non-toxic and toxic *Microcystis aeruginosa* were mono-cultured and co-cultured to explore algal responses under different nutrient regimes. Comparisons were made according to photosynthetically active radiation (PAR), UV-B radiation exerted oxidative stresses, and negative effects on the photosynthesis and growth of three species under normal growth conditions, and algal adaptive responses included extracellular polymeric substance (EPS) production, the regulation of superoxide dismutase (SOD) activity, photosynthetic pigments synthesis, etc. Three species had strain-specific responses to UV-B radiation and toxic *M. aeruginosa* was more tolerant and showed a higher adaptation capability to UV-B in the mono-cultures, including the lower sensitivity and better self-repair efficiency. In addition to stable µmax in PAR ad UV-B treatments, higher EPS production and enhanced production of photosynthetic pigments under UV-B radiation, toxic *M. aeruginosa* showed a better recovery of its photosynthetic efficiency. Nutrient enrichment alleviated the negative effects of UV-B radiation on three species, and the growth of toxic *M. aeruginosa* was comparable between PAR and UV-B treatment. In the co-cultures with nutrient enrichment, *M. aeruginosa* gradually outcompeted *C. pyrenoidosa* in the PAR treatment and UV-B treatment enhanced the growth advantages of *M. aeruginosa*, when toxic *M. aeruginosa* showed a greater competitiveness. Overall, our study indicated the adaptation of typical algal species to ambient UV-B radiation and the stronger competitive ability of toxic *M. aeruginosa* in the UV-radiated waters with severer eutrophication.

**Keywords:** *Microcystis aeruginosa*; *Chlorella pyrenoidosa*; ultraviolet B radiation; photosynthetic efficiency; adaptation capability; nutrient enrichment
