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This study aimed to elucidate for the first time the combined effects of marine heatwaves (MHWs) and light limitation simulated in mesocosm on critical physiological descriptors of the surfgrass Phyllospadix torreyi , which constitutes highly productive meadows along the intertidal and subtidal rocky shores of the Pacific coast of North America. Our results revealed that short-term exposure (~7 days) to extreme thermal anomalies of + 9 °C had positive effects on the photosynthetic capacities of P. torreyi , as indicated by increments in maximum photosynthetic rates, photosynthetic efficiency (α), maximum electron transport rate, and effective quantum yield. Despite that its photosynthetic performance was enhanced, exposure to warming caused a decrease in its internal carbon reserves (i.e. energy status), likely as a consequence of carbon mobilization/utilization to activate heat-stress responses. Plants exposed to light limitation (i.e. sub-saturating irradiance of 30 μmol photon m−2 s−1) generally exhibited an increase in α and/or a decrease in respiration, which ultimately allowed for a reduction in plant compensation irradiance. The combination of low light and seawater warming resulted in a decrease in non-structural carbohydrates content, daily net-productivity, and leaf growth rates. Gross photosynthetic rates at control saturating irradiance exhibited higher activation energy and, thus, greater responsiveness to seawater warming than plants kept under light limitation. While our results indicated that unusual warming events might favor the photosynthetic performance of P. torreyi, combining this condition with a drastic light reduction can lead to internal carbon depletion and potentially compromise plant survival in the long term. • First study to assess physiological responses of surfgrass to warming and low light. • Unexpectedly, warming can favor photosynthetic performance of P. torreyi. • Phyllospadix torreyi can partially photoacclimate under light limitation. • Productivity and growth decreased under the combination of low light and warming. • Marine heatwaves combined with low irradiance could compromise P. torreyi survival. [ABSTRACT FROM AUTHOR]