High copper and UVR synergistically reduce the photochemical activity in the marine diatom Skeletonema costatum

Many coastal waters are threatened by heavy metal pollution, of which copper (Cu) is one of the most common contaminants. Since surface waters in marine environments and ecosystems contaminated with heavy metals are concurrently exposed to solar ultraviolet radiation (UVR), marine phytoplankton may be stressed by the combination of heavy metals and UVR. In this study, three Cu concentrations (0, 0.5, and 2 μM) and two light conditions: PAR (photosynthetically active radiation, 400–700 nm) and PAB (PAR + UVR, 280–700 nm), were applied to investigate the combined effects on the photosynthetic performance of a marine diatom Skeletonema costatum. The effects on the photosynthetic performance were examined using effective photochemical yields, non-photochemical quenching, the relative UV inhibition on photosystem II (PSII), and the rates of damage and repair to PSII in response to the stressors. Results showed that UVR interacted with Cu more strongly than PAR, and synergistically decreased the photochemical performance of PSII. The contrasting responses to PAR and UVR were primarily ascribed to the repair processes of PSII. Under copper exposure, especially for samples also exposed to UVR, the repair rates were remarkably lower than those under the control (zero Cu addition). This study indicates that the human-induced pollution in coastal water, could have more severe impacts when combined with the stress induced by UVR.