Photosynthesis Research 42: 51-64. 1994. @ 1994 Kluwer Academic Publishers. Printed in theNetherlands. Regularpaper Inhibition of PS II photochemistry by PAR and UV radiation in natural phytoplankton communities* llya R. Vassilievl,2, Ondrej Prasil1,3, Kevin D. Wymanl, Zbigniew Kolberl, Alfred K. Hanson, Jr.4,Jennifer E. Prentice4& Paul G. Fa1kowski1,* 10ceanographic and Atmospheric SciencesDivision, Department oj Applied Science, Brookhaven National Laboratory, Upton, N~ 11973-5000, USA; 2Presentaddress: Department oj Biochemistry, University oj Nebraska, Lincoln, NE 68583-0718, USA; 3 Permanent Address: 1nstitute oj Microbiology, ClechAcademy oj Sciences, Trebon, 37981, Clech Republic; 4Graduate School oJOceanography, University oj Rhode ls1and, Narragansett, RI02882, USA (*Author Jor correspondence) Received 6 Apri11994; accepted in revised farm 5 Ju1y 1994 Key words: absorptioncross section 9f PS II, chlorophyll fluorescence, photoinhibition, phytoplankton, QA, quantum efficiency of PS II, UV radiation ~ Abstract The effects ofPAR and UV radiation on PS II photochemistry were examined in natural phytoplankton communities from coa'Stalwaters off Rhode Island (USA) and the subtropical Pacific. The photochernical energy conversion efficiency, the functional absorption cross section and the kinetics of electron transfer on the acceptor side of PS II were derived from variable fluorescence parameters using both pump and probe and fast repetition rate techniques. In both environments, the natural phytoplankton communities displayed marked decreases in PS II photochernical energy conversion efficiency that were correlated with increased PAR. In the coastal waters, the changes in photochernical energy conversion efficiency were not statistically different for samples treated with supplementary UV-B radiation or screened to exclude ambient UV-B. Moreover, no significant light-dependent changes in the functional absorption cross section of PS II were observed. The rate of electron transfer between QA- and QB was, however, slightly reduced in photodamaged cells, indicative of damage on the acceptor side. In the subtropical Pacific, the decrease in photochemical energy conversion efficiency was significantly greater for samples exposed to naturallevels of VY-A and/Dr UV-B compared with those exposed to PAR alone. The cells displayed large diurnal changes in the functional absorption cross section of PS II, indicative of non-photochemical quenching in the antenna. The changes in the functional absorption cross section were highly correlated with PAR but independent of UV radiation. The time course of changes in photochemical efficiency reveals that the photoinhibited reaction centers rapidly recover (within an hOUf or two) to their preillumination values. Thus, while we found definitive evidence for photoinhibition of PS II photochernistry in both coastal and open ocean phytoplankton communities, we did not find any effect of UV-B on the former, but a clear effect on the latter. The results ofthis study indicate that the effects ofUV-B radiation on phytoplankton photosynthesis are as dependent on the radiative transfer properties of the water body and the mixing rate, as on the wavelength and energy distribution of the radiation and the absorption cross sections of the biophysical targets. Introduction * The subrnittedmanuscript has been authored under Contract No. DE-ACO2-76CHOOO16 with the U.S. Department of Energy. Accordingly.the U.S. Govemment retainsa nonexclusive. roya1ty- free license to publishor reproduce the published form of this con- tribution. or a11ow others to do so. for U.S. Govemment purposes. The discovery ofthe rapid turnoverofDl (Kyle et al. 1984) andits apparent correlation with photoinhibition stimulated considerable research on the molecular and