Influence of light history on the photosynthetic and motility responses of Gymnodinium chlorophorum exposed to UVR and different temperatures

Abstract

In the wake of global climate change, phytoplankton productivity and species composition is expected to change due to altered external conditions such as temperature, nutrient accessibility, pH and exposure to solar visible (PAR) and ultraviolet radiation (UVR). The previous light history is also of importance for the performance of phytoplankton cells. In order to assess the combined impacts of UVR and temperature on the dinoflagellate Gymnodinium chlorophorumwe analyzed the effective photochemical quantum yield (Y), relative electron transport rate vs. irradiance curves (rETR vs. I), percentage of motile cells and swimming velocity. Cells were grown at three different temperatures (15, 20 and 25C) and two PAR intensities: low light (LL, 100lmol photons m2 s1) and high light (HL, 250lmol photons m2 s1). Pre-acclimated cells were then exposed to either PAR only (P), PAR + UV-A (PA) or PAR + UV-A + UV-B (PAB) radiation at two different irradiances, followed by a recovery period in darkness. TheYdecreased during exposure, being least inhibited in P and most in PAB treatments. Inhibition was higher and recov-ery slower in LL-grown cells than in HL-grown cells at 15and 20C, but the opposite occurred at 25C, when exposed to high irradiances. Maximal values of rETR were determined att0as compared to the different (before and after exposure) radiation treatments. The effects of temperature and UVR on rETR were antagonistic in LL-grown cells (i.e., less UVR inhibition at higher temperature), while it was synergistic in HL cells. Swimming velocity and percentage of motile cells were not affected at all tested temperatures and exposure regimes, independent of the light history. Our results indicate that, depending on the previous light history, increased temperature and UVR as predicted under climate change condi-tions, can have different interactions thus conditioning the photosynthetic response ofG. chlorophorum.