Weihao Meng

Ocean warming and acidification (OA) are rapidly altering coastal ecosystems, yet their combined effects on the physiology and functioning of foundation species remain poorly understood. The effects of OA on macroalgae physiology and functioning are species‐specific and depend on their carbon uptake strategy. An increase in available CO ₂ may act as a potential resource, enabling macroalgae to increase passive CO ₂ uptake and down‐regulate energetically costly carbon concentration mechanisms (CCMs). Under combined stressor conditions, OA may, therefore, mitigate the negative effects of ocean warming if energy saved by down‐regulating CCMs is reallocated to maintain physiological functioning. For example, macroalgae may adjust fatty acid composition under increased warming as a compensatory mechanism to maintain cell membrane fluidity. Using experimental mesocosms, we tested for individual and combined effects of ocean warming (ambient and +3°C) and acidification (ambient and 1000 ppm) on the growth, δ ¹³ C, CN ratio, fatty acid content, metabolism and photo‐physiology of the juvenile kelp Laminaria digitata . Consistent with our hypothesis, the growth of L. digitata increased under elevated CO ₂ but only in the absence, compared to the presence, of warming. In contrast, δ ¹³ C values were reduced by warming but remained unaffected by CO ₂ . As predicted, warming increased saturated fatty acid content and reduced polyunsaturated fatty acid content. CO ₂ enrichment, however, had stronger effects on fatty acid content than that of warming and there was no interactive effect of warming and acidification. These results demonstrate that potential energetic savings from elevated CO ₂ cannot mitigate the metabolic costs of ocean warming in L. digitata . These findings link physiological responses at the cellular level to changes in individual performance in a habitat‐forming kelp and highlight the importance of measuring physiological components at multiple levels of biological organisation to prevent underestimating the magnitude of system‐level effects.