摘要：Climate change-related events, such as marine heatwaves, are increasing seawater temperatures, thereby putting pressure on marine biota. The cosmopolitan distribution and significant contribution to marine primary production by the genus <i>Ruppia</i> makes them interesting organisms to study thermal tolerance and local adaptation. In this study, we investigated the photosynthetic responses in <i>Ruppia</i> to the predicted future warming in two contrasting bioregions, temperate Sweden and tropical Thailand. Through DNA barcoding, specimens were determined to <i>Ruppia cirrhosa</i> for Sweden and <i>Ruppia maritima</i> for Thailand. Photosynthetic responses were assessed using pulse amplitude-modulated fluorometry, firstly in short time incubations at 18, 23, 28, and 33 °C in the Swedish set-up and 28, 33, 38, and 43 °C in the Thai set-up. Subsequent experiments were conducted to compare the short time effects to longer, five-day incubations in 28 °C for Swedish plants and 40 °C for Thai plants. Swedish <i>R. cirrhosa</i> displayed minor response, while Thai <i>R. maritima</i> was more sensitive to both direct and prolonged temperature stress with a drastic decrease in the photosynthetic parameters leading to mortality. The results indicate that in predicted warming scenarios, Swedish <i>R. cirrhosa</i> may sustain an efficient photosynthesis and potentially outcompete more heat-sensitive species. However, populations of the similar <i>R. maritima</i> in tropical environments may suffer a decline as their productivity will be highly reduced.
