Mechanisms of high-temperature tolerance in the kelp Laminaria saccharina (L.) Lamour. were examined by comparing a heat-tolerant ecotype from Long Island Sound (US), New York, and a population from the Atlantic (ATL) coast of Maine. Greater heat tolerance was not attributable to greater thermal stability of the photosynthetic apparatus: LIS and ATL plants exhibited similar shortterm effects of high temperature on photosynthetic capacity (P,,J and quantum yield (estimated as the ratio of variable to maximum chlorophyll jluorescence, FJF,,,). A s LIS plants had consistently higher N and protein content than A T L plants, the interaction between nitrogen nutrition and high-temperature tolerance was examined. When grown under high N supply and optimal temperature (12" C), LIS plants had a higher density of photosystem 11 reaction centers (RCII), higher activity of two Calvin cyck enzymes (ribulose bisphosphate carboxylase oxjgenase [RUBISCO] and NADP-dependent glyceraldehyde-3=Phos-phate dehydrogenase [G3PDH]), and higher Pmx and F,J F, than A T L plants. Individual A T L plants, furthermore, exhibited close correlations of RCII density and enzyme activity with N and/or protein content. Variation in RCII density and enzyme activity, in turn, largely accounted for plant-to@ant d@erences in P,,,, and Fv/F,. Relationships among these parameters were generally weak or lacking among individual US plants grown under optimal conditions, apparently because luxury N consumption resulted in excess reserves of photosynthetic apparatus components. Exposure of N-repbte LIS and ATL plants to a superoptimal temperature (22" C) for 4 days caused a n increase in the minimum turnover time of the photosynthetic a p paratus (tau) and a decrease in P,,, but had no consistent effect on FJF,,,, RCII density, PSU size (chlorophyll a/RCII), or enzyme activities. When plants were su6jected to concurrent N limitation and heat stress, however, LIS and A T L populations exhibited quite dyferent responses. All photosynthetic parameters of N-limited ATL plants declined shaqly in response to high temperature, resulting in a negative rate of daily net Cfixation. In contrast, LIS plants showed a reduction in PSU size, but maintained other parameters, including daily C fixation, at levels similar to those of N-limited plants at optimal temperature. Overall, the ability of LIS plants to accumulate and maintain high N reserues appears to be critical for heat tolerance and, therefore, for survival during summer pm'ods of simultaneous low N supply and superoptimal temperature. A T L plants, which also experience low summer N supply but not superoptimal temperatures, do not accumulate large reserues of nitrogenous components and are unable I to tolerate the combined stress. Because low N supply often co-occurs with high temperatures in temperate marine systems, largescab declines in algal productivity, such as during El Niiio events, are probably due to the interactive effect of N limitation and heat stress. chlorophyll jluorescence; enzymes; heat stress;...
