Assessment of UV-B sensitivity of photosynthetic apparatus among microalgae: Short-term laboratory screening versus long-term outdoor exposure

A fifteen-day outdoor cultivation with the UV-B sensitive and the resistant microalgal species was carried out during mid-September to early October 1 995 in order to verify laboratory-identified UV-B sensitivity/ tolerance of the photosynthetic apparatus in these organisms, in the field. Outdoor lig ht conditions during the experiment were basically characterized by a regul ar diurnal change superimposed by unequal daily variation of PAR and ambien t UV-B irradiation. The maximum value of: 370 W m(-2) of PAR and 1.4Wm(-2) of unweighted UV-B irradiation were monitored during midday sunny days. Bot h the UV-B sensitive and the -resistant species suffered from exposure to t he solar irradiation as demonstrated by impairment of photosynthesis (photo synthetic O-2 evolution capacity, Chi rt fluorescence parameters and pigmen ts content) and inhibition of the daily growth rate. Higher UV-B susceptibi lity was consistently observed in the lab-identified UV-B sensitive species , than its resistant counterparts. Outdoor exposure-caused impairment of ph otosynthesis was due to not only the UV-B-induced damage of photosynthetic apparatus but also the visible light-induced photoinhibition. Our results s uggest that the protective mechanism that microalgae employ to counteract e nhanced UV-B stress function both in the laboratory and in the field.