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Abstract: Previous studies showed that exposure of Arthrospira spp. spirals to natural levels of solar radiation in the presence of UV radiation (UVR, 280–400nm) led to the breakage of its spiral structure. However, the underlying mechanisms have not yet been explored. Here, we showed that associated accumulation of reactive oxygen species (ROS) resulted in the spiral breakage by oxidizing the lipids of sheath or cell membrane in Arthospira platensis, and presence of UVR brought about higher accumulation level of the ROS. Activities of superoxide dismutase (SOD) and catalase (CAT) were inhibited by high levels of solar PAR, addition of UVR led to further inhibition of CAT activity. High levels of ROS also decreased the content of photosynthetic pigments, damaged photosystem II (PSII) and inhibited the photosynthesis and growth. It is concluded that both UV and high PAR levels could generate higher amounts of ROS, which decreased the photosynthetic performances and led to spiral breakage of A. platensis. [Copyright &y& Elsevier]

• Karenia mikimoto i inhibited development and photosynthesis of S. fusiforme embryos. • Inhibitory effects of K. mikimotoi on the embryos follows a cell density-dependent manner. • The inhibitory effects was mainly caused by allelochemicals released by K. mikimotoi. Lots of research has demonstrated that macroalgae can strongly inhibit the growth of harmful algal bloom (HAB) species in general. However, the effects of HABs or HAB-forming species on macrophytes are still largely uncharacterized until now. In the present study, the effects of the dinoflagellate Karenia mikimotoi cell density gradient, live cell suspension (LC), ruptured cell suspension (RC) as well as the cell-free supernatant (FC) of K. mikimotoi at 1000 μg Chl a l−1 (~1.0 × 105 cells ml−1) on the development and photosynthesis of Sargassum fusiforme embryos were investigated in a series of laboratory experiments. The results showed that co-cultivation with K. mikimotoi at 500 μg Chl a l−1(~5.0 × 104 cells ml−1) and higher cell densities significantly (P <0.05) inhibited the development, pigment content and photosynthetic activities of the embryos. In addition, the inhibitory effects increased with increased cell densities and prolonged exposure time. Compared to the embryos cultured with the F/2 medium (Control), exposure to LC, RC and FC of K. mikimotoi at 1000 μg Chl a l−1for 2 weeks all led to decreased relative growth rate (RGR), chlorophyll (Chl) a content, carotenoids (Car) content and photosynthetic activities of the embryos, with LC and RC exhibiting the maximal and the minimal suppression. The dominant inhibitory effects of FC on the embryos indicated that the suppression was mainly caused by the allelochemicals, while the slightest inhibitory effects of RC on the embryos suggested that some intracellular growth-promoting substances were synchronously released when K. mikimotoi cells lyzed. In addition, the most severe growth suppression of embryos by LC indicated that intact cell contact by K. mikimotoi probably also contributed to the inhibitory effects. These results indicated that a dense HAB formed by K. mikimotoi could seriously suppress the development and photosynthesis of S. fusiforme embryos and eventually reduce the seedlings stock. Image, graphical abstract [ABSTRACT FROM AUTHOR]