Algal blooms are increasing worldwide, driven by elevated nutrient inputs. However, it is still unknown how tropical benthic algae will respond to heatwaves, which are expected to be more frequent under global warming. In the present study, a multifactorial experiment was carried out to investigate the potential synergistic effects of increased ammonium inputs (25 μM, control at 2.5 μM) and a heatwave (31°C, control at 25°C) on the growth and physiology (e.g., ammonium uptake, nutrient assimilation, photosynthetic performance, and pigment concentrations) of two bloom-forming algal species, Cladophoropsis sp. and Laurencia sp. Both algae positively responded to elevated ammonium concentrations with higher growth and chlorophyll a and lutein concentrations. Increased temperature was generally a less important driver, interacting with elevated ammonium by decreasing the algaes' %N content and N:P ratios. Interestingly, this stress response was not captured by the photosynthetic yield (Fv/Fm) nor by the carbon assimilation (%C), which increased for both algae at higher temperatures. The negative effects of higher temperature were, however, buffered by nutrient inputs, showing an antagonistic response in the combined treatment for the concentration of VAZ (violaxanthin, antheraxanthin, zeaxanthin) and thalli growth. Ammonium uptake was initially higher for Cladophoropsis sp. and increased for Laurencia sp. over experimental time, showing an acclimation capacity even in a short time interval. This experiment shows that both algae benefited from increased ammonium pulses and were able to overcome the otherwise detrimental stress of increasingly emerging temperature anomalies, which provide them a strong competitive advantage and might support their further expansions in tropical marine systems.

中文翻译：

---

热带水华形成中藻类 Cladophoropsis sp.和 Laurencia sp.——对铵富集和模拟热浪的响应

在养分投入增加的推动下，全球范围内的藻华正在增加。然而，目前尚不清楚热带底栖藻类将如何应对热浪，预计在全球变暖的情况下热浪将更加频繁。在本研究中，进行了一项多因素实验，以研究增加铵输入（25 μM，控制在 2.5 μM）和热浪（31°C，控制在 25°C）对生长和生理的潜在协同效应（例如，两种形成水华的藻类Cladophoropsis sp.的铵吸收、养分同化、光合作用性能和色素浓度） 。和劳伦西亚sp。两种藻类都对铵浓度升高做出积极反应，生长速度加快，叶绿素a和叶黄素浓度也提高。温度升高通常是一个不太重要的驱动因素，它通过降低藻类的%N 含量和N:P 比率与升高的铵相互作用。有趣的是，这种胁迫反应既没有被光合产量（F v/ F m）捕获，也没有被碳同化（%C）捕获，两种藻类的碳同化在较高温度下都会增加。然而，较高温度的负面影响被营养物质输入所缓冲，在联合处理中对 VAZ（紫罗兰黄质、蒽黄质、玉米黄质）浓度和菌体生长表现出拮抗反应。Cladophoropsis sp.的铵吸收最初较高。劳伦西亚sp有所增加。在实验时间内，即使在很短的时间间隔内也显示出适应能力。该实验表明，这两种藻类都受益于增加的铵脉冲，并且能够克服日益出现的温度异常带来的有害压力，这为它们提供了强大的竞争优势，并可能支持它们在热带海洋系统中的进一步扩张。