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（marine heatwaves）是一类可对海洋生物造成深远且持久影响的极端事件。野外观测表明，大型海藻对海洋热浪具有高度易感性，但其背后的生理驱动机制仍不甚明晰。此外，海洋热浪与海洋增温、酸化协同作用的效应尚未得到研究。

为填补这一认知空白，我们开展了一项实验室培养实验，测试了分布范围最南端（南纬43°至31°）的厚膜藻（Phyllospora comosa）幼体对海洋热浪、海洋增温和酸化的生长及生理响应。我们采用"折叠因子设计"，将海洋热浪叠加于"当前pH与温度"和"未来（2100年预测的pH与温度）"两种海洋环境条件中。实验分别在热浪期间以及为期7天的恢复期后检测了生物响应。

热浪在当前和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。恢复期过后，几乎没有证据表明热浪会对生长、光合或呼吸产生持久的负面影响。

暴露于海洋热浪、未来海洋环境或二者共同作用下，会导致脂肪酸饱和度升高。这一调整可能通过降低膜流动性抵消了高温升高的负面影响——膜流动性会随温度升高而增强。此外，在未来环境条件下，厚膜藻似乎下调了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值有所降低。

由该下调过程节省的能量并未被用于生长，而是大概率被用于调整脂肪酸组成。这一调整机制或可使厚膜藻及其他大型海藻通过海洋酸化带来的益处，耐受海洋增温和海洋热浪带来的负面影响。