Effect of UV-B Radiation and Humic Substances on Growth and Motility of Gyrodinium aureolum

We investigated the effect of ultraviolet-B radiation (280-320 nm) on the growth and motility of the marine dinoflagellate Gyrodinium aureolum in the presence of humic substances. The specific growth rate after 7 d of the $UV-B_BE$ (biologically effective radiation) exposure $(2.15 kJ m^-2 d^-1)$ decreased by 92% compared to the control. The growth rate for UV-B treated cells increased by 18, 26, and 10% when cells were cultured in medium enriched with humic substances (1.7, 4.2, and 8.3 mg $liter^-1$). However, the growth rate was lower at the highest humic substance than at the two lower concentrations. When humic substances were used as a filter to shade the organisms from UV-B radiation without substantial changes in the amount of white light, instead of being addedt to the medium, the growht rate increased with the concentration of humic substances. UV-B radiation had littler or no effect on the motility of G. aureolum, whether humic substances were added or not. Phototactic orientation of the UV-B-treated cells was significantly weaker than the control to actinic white light (400-700 nm, 47 $m^-2$). The presence of humic substances during treatment with UV-B radiation reduced the inhibition of the positive phototaxis in G. aureolum.