Data accompanying: Adjustments in fatty acid composition is a mechanism that can explain resilience to marine heatwaves and future ocean conditions in the habitat-forming seaweed Phyllospora comosa

Marine heatwaves are extreme events that can have profound and lasting impacts on marine species. Field observations have shown seaweeds to be highly susceptible to marine heatwaves, but the physiological drivers of this susceptibility are poorly understood. Furthermore, the effects of marine heatwaves in conjunction with ocean warming and acidification are yet to be investigated. To address this knowledge gap, we conducted a laboratory culture experiment in which we tested the growth and physiological responses of Phyllospora comosa juveniles from the southern extent of its range (43 - 31° S) to marine heatwaves, ocean warming and acidification. We used a "collapsed factorial design" in which marine heatwaves were superimposed on current (today's pH and temperature) and future (pH and temperature projected by 2100) ocean conditions. Responses were tested both during the heatwaves, and after a seven-day recovery period. Heatwaves reduced net photosynthetic rates in both current and future conditions, while respiration rates were elevated under heatwaves in the current conditions only. Following the recovery period, there was little evidence of heatwaves having lasting negative effects on growth, photosynthesis or respiration. Exposure to heatwaves, future ocean conditions or both caused an increase in the degree of saturation of fatty acids. This adjustment may have counteracted negative effects of elevated temperatures by decreasing membrane fluidity, which increases at higher temperatures. Furthermore, P. comosa appeared to down-regulate the energetically expensive carbon-concentrating mechanism (CCM) in the future conditions with a reduction in δ13 C values detected in these treatments. Any saved energy arising from this down-regulation was not invested in growth and was likely invested in the adjustment of fatty acid composition. This adjustment is a mechanism by which P. comosa and other seaweeds may tolerate the negative effects of ocean warming and marine heatwaves through benefits arising from ocean acidification.

海洋热浪是一类极端海洋事件，可对海洋生物造成深远且持久的影响。野外观测表明，海藻对海洋热浪具有高度易感性，但此类易感性背后的生理驱动机制仍鲜为人知。此外，海洋热浪与海洋变暖、酸化共同作用的效应尚未得到研究。为填补这一认知空白，本研究开展了室内培养实验，探究采自其分布范围南部区域（南纬31°—43°）的囊叶藻（*Phyllospora comosa*）幼体对海洋热浪、海洋变暖及酸化的生长与生理响应。本实验采用简化析因设计（collapsed factorial design），将海洋热浪叠加于当前（现今海水pH与温度）及未来（预计至2100年的海水pH与温度）两种海洋环境条件中。实验分别在热浪发生期间及为期7天的恢复周期结束后，对各项响应指标进行了测定。结果显示，海洋热浪在当前与未来环境条件下均降低了净光合速率；而呼吸速率仅在当前环境条件下的热浪处理组中出现升高。恢复周期结束后，几乎未观测到海洋热浪对生长、光合或呼吸速率产生持久负面影响的证据。单独暴露于海洋热浪、未来海洋环境，或同时暴露于二者时，脂肪酸的饱和程度均有所升高。此类适应性调节可通过降低膜流动性抵消高温带来的负面影响——膜流动性会随温度升高而增强。此外，在未来海洋环境条件下，囊叶藻似乎会下调耗能较高的碳浓缩机制（carbon-concentrating mechanism, CCM），此类处理组中检测到的δ¹³C值亦出现下降。由该下调过程节省的能量并未用于生长，而是大概率投入到了脂肪酸组成的适应性调节中。这种调节机制是囊叶藻与其他海藻通过利用海洋酸化带来的益处，来耐受海洋变暖和海洋热浪负面影响的可行途径。