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 p hotosystem II (PSII) was a major target site for photoinhibition by ul traviolet-B radiation (Q(UVB), 280-320 nm) in natural communities. Sec ond, 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), dens e samples of a frazil ice algal community were collected and maintaine d outdoors in the presence or absence of Q(UVB) and/or ultraviolet-A ( Q(UVA), 320-400 nm) radiation. Samples were then collected at interval s over the day to track the time course of UV inhibition of primary pr oduction. The ice algae were assessed for changes in pigment compositi on 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 d ay, The Q(UVB) photoinhibition of PSII quantum yield did not recover b y 6 h after sunset. In contrast, photoinhibition by Q(UVA) and photosy nthetically available radiation (Q(PAR) 400-700 nm) recovered during t he late afternoon. Flourescence-based estimates of carbon fixation rat es were linearly correlated (P = 0.002, r(2) = 0.45) with measured car bon fixation. Fluorescence overestimated the observed Q(UVB) inhibitio n in measured carbon fixation rates by 8% in the morning hours; howeve r, the discrepancy increased during the afternoon. Therefore, research ers should be cautious in using fluorescence measurements to infer ult raviolet inhibition for rates of carbon fixation until there is a grea ter understanding of the coupling of carbon metabolism to PSII activit y for natural populations. Despite these current limitations, fluoresc ence-based technologies represent powerful tools for studying the impa ct of the ozone hole on natural populations on spatial/temporal scales not possible using conventional productivity techniques.