Photosynthesis Research 58: 259–268, 1998. © 1998 Kluwer Academic Publishers. Printed in the Netherlands. 259 Regular paper Compensatory changes in Photosystem II electron turnover rates protect photosynthesis from photoinhibition Michael J. Behrenfeld 1,∗ , Ondrej Prasil 2 , Zbigniew S. Kolber 1 , Marcel Babin 3 & Paul G. Falkowski 1 1 Institute of Marine and Coastal Sciences, Rutgers University, 71 Dudley Road, New Brunswick, NJ 08903-0231, USA; 2 Institute of Microbiology, MBU, AV ˇ CR, 379 81 Tˇ reboˇ n, Czech Republic; 3 Laboratoire de Physique et Chimie Marines, Universit´ e Peirre et Marie Curie and CNRS, Villefranche-Sur-Mer F 06230, France; ∗ Author for correspondence (e-mail: behren@ahab.rutgers.edu; fax: +1-732-932-3036) Received 14 July 1998; accepted in revised form 25 September 1998 Key words: carbon fixation, phytoplankton Abstract Exposure of algae or higher plants to bright light can result in a photoinhibitory reduction in the number of func- tional PS II reaction centers (n) and a consequential decrease in the maximum quantum yield of photosynthesis. However, we found that light-saturated photosynthetic rates (P max ) in natural phytoplankton assemblages sampled from the south Pacific ocean were not reduced despite photoinhibitory decreases in n of up to 52%. This striking insensitivity of P max to photoinhibition resulted from reciprocal increases in electron turnover ( 1 /τ PSII ) through the remaining functional PS II centers. Similar insensitivity of P max was also observed in low light adapted cultures of Thalassiosira weissflogii (a marine diatom), but not in high light adapted cells where P max decreased in proportion to n. This differential sensitivity to decreases in n occurred because 1 /τ PSII was close to the maximum achievable rate in the high light adapted cells, whereas 1 /τ PSII was initially low in the low light adapted cells and could thus increase in response to decreases in n. Our results indicate that decreases in plant productivity are not necessarily commensurate with photoinhibition, but rather will only occur if decreases in n are sufficient to maximize 1 /τ PSII or incident irradiance becomes subsaturating. Abbreviations: PS II – Photosystem II; DCMU – 3-(3,4-dichlorophenyl)-1,1-dimethylurea Introduction Although the detrimental effects of excessive light ex- posure on plant photosynthesis have been recognized for more than a century (Ewart 1895–1897), only dur- ing the past few decades has damage to Photosystem II (PS II) reaction centers been recognized as the fun- damental mechanism responsible for photoinhibition (Kok 1956; Kyle 1987; Barber 1991, 1992a; Prasil et al. 1992). This light-dependent inactivation of PS II may either be rapidly reversible (Osmond 1994) or entail irreversible damage to core PS II reaction cen- ter proteins (D1), requiring de novo protein synthesis for repair (Prasil et al. 1992). For both phytoplankton and terrestrial plants, photodamage to PS II reaction centers can be detected with high sensitivity from changes in variable chlorophyll fluorescence (Björk- man 1987a,b; Neale 1987; Baker et al. 1994; Long et al. 1994). 1 The ease with which variable fluorescence measurements can be made has led to their common usage as a diagnostic for photoinhibition, although the consequence of PS II inactivation on photosynthetic electron flow remains controversial. In the field, diurnal patterns of variable fluores- cence frequently exhibit midday depressions that are roughly symmetric relative to local noon. Full recov- ery from the midday minimum is often observed by late afternoon (Falkowski et al. 1994), but a mild