IMPACT OF ULTRAVIOLET-B RADIATION ON PHOTOSYSTEM-II ACTIVITY AND ITS RELATIONSHIP TO THE INHIBITION OF CARBON FIXATION RATES FOR ANTARCTIC ICE ALGAE COMMUNITIES

One goal of the Icecolors 1993 study was to determine whether or not photosystem II (PSII) was a major target site for photoinhibition by ultraviolet-B radiation (Q(UVB), 280-320 nm) in natural communities. Second, the degree to which Q(UVB) inhibition of PSII could account for Q(UVB) effects on whole cell rates of carbon fixation in phytoplankton was assessed. On 1 October, 1993, at Palmer Station (Antarctica), dense samples of a frazil ice algal community were collected and maintained outdoors in the presence or absence of Q(UVB) and/or ultraviolet-A (Q(UVA), 320-400 nm) radiation. Samples were then collected at intervals over the day to track the time course of UV inhibition of primary production. The ice algae were assessed for changes in pigment composition and rates of carbon fixation. The quantum yield of PSII (phi(Ile)(0)) was measured by Pulse Amplitude Modulated fluorometry. Over the day, phi(Ile)(0) declined due to increasing time-integrated dose exposure of Q(UVB). The Q(UVB)-driven inhibition of phi(Ile)(0) increased from 4% in the early morning hours to a maximum of 23% at the end of the day, The Q(UVB) photoinhibition of PSII quantum yield did not recover by 6 h after sunset. In contrast, photoinhibition by Q(UVA) and photosynthetically available radiation (Q(PAR) 400-700 nm) recovered during the late afternoon. Flourescence-based estimates of carbon fixation rates were linearly correlated (P = 0.002, r(2) = 0.45) with measured carbon fixation. Fluorescence overestimated the observed Q(UVB) inhibition in measured carbon fixation rates by 8% in the morning hours; however, the discrepancy increased during the afternoon. Therefore, researchers should be cautious in using fluorescence measurements to infer ultraviolet inhibition for rates of carbon fixation until there is a greater understanding of the coupling of carbon metabolism to PSII activity for natural populations. Despite these current limitations, fluorescence-based technologies represent powerful tools for studying the impact of the ozone hole on natural populations on spatial/temporal scales not possible using conventional productivity techniques.