INHIBITION OF PS-II PHOTOCHEMISTRY BY PAR AND UV-RADIATION IN NATURALPHYTOPLANKTON COMMUNITIES

The effects of PAR and UV radiation on PS II photochemistry were exami ned in natural phytoplankton communities from coastal waters off Rhode Island (USA) and the subtropical Pacific. The photochemical energy co nversion efficiency, the functional absorption cross section and the k inetics of electron transfer on the acceptor side of PS II were derive d from variable fluorescence parameters using both pump and probe and fast repetition rate techniques. In both environments, the natural phy toplankton communities displayed marked decreases in PS II photochemic al energy conversion efficiency that were correlated with increased PA R. In the coastal waters, the changes in photochemical energy conversi on efficiency were not statistically different for samples treated wit h supplementary UV-B radiation or screened to exclude ambient UV-B. Mo reover, no significant light-dependent changes in the functional absor ption cross section of PS II were observed. The rate of electron trans fer between Q(A)(-) and Q(B) was, however, slightly reduced in photoda maged cells, indicative of damage on the acceptor side. In the subtrop ical Pacific, the decrease in photochemical energy conversion efficien cy was significantly greater for samples exposed to natural levels of UV-A and/or UV-B compared with those exposed to PAR alone. The cells d isplayed large diurnal changes in the functional absorption cross sect ion of PS II, indicative of non-photochemical quenching in the antenna . The changes in the functional absorption cross section were highly c orrelated with PAR but independent of UV radiation. The time course of changes in photochemical efficiency reveals that the photoinhibited r eaction centers rapidly recover (within an hour or two) to their preil lumination values. Thus, while we found definitive evidence for photoi nhibition of PS II photochemistry in both coastal and open ocean phyto plankton communities, we did not find any effect of UV-B on the former , but a clear effect on the latter. The results of this study indicate that the effects of W-B radiation on phytoplankton photosynthesis are as dependent on the radiative transfer properties of the water body a nd the mixing rate, as on the wavelength and energy distribution of th e radiation and the absorption cross sections of the biophysical targe ts.