Seawater carbonate chemistry and metabolism and growth of epilithic and endolithic coral-reef turf algal

Turf algal assemblages are ubiquitous primary producers on coral reefs, but little is known about the response of this diverse group to ocean acidification (OA) across different temperatures. We tested the hypothesis that CO2 influences the functional response of epilithic and endolithic turf assemblages to increasing temperature. Replicate carbonate plugs covered by turf were collected from the reef and exposed to ambient and high pCO2 (1000 µatm) conditions for 3 weeks. Each pCO2 treatment was replicated across six temperatures (24.0-31.5 °C) that spanned the full seasonal temperature range on a fringing reef in Moorea, French Polynesia, and included one warming treatment (3 °C above daily average temperatures). Temperature and CO2 enrichment had complex, and sometimes interactive, effects on turf metabolism and growth. Photosynthetic and respiration rates were enhanced by increasing temperature, with an interactive effect of CO2 enrichment. Photosynthetic rates were amplified by high CO2 in the warmest temperatures, while the increase in respiration rates with temperature were enhanced under ambient CO2. Epilithic turf growth rates were not affected by temperature, but increased in response to CO2 enrichment. We found that CO2 and temperature interactively affected the endolithic assemblage, with the highest growth rates under CO2 enrichment, but only at the warmest temperatures. These results demonstrate how OA may influence algal physiology and growth across a range of ecologically relevant temperatures, and indicate that the effects of CO2 enrichment on coral-reef turf assemblages can be temperature dependent. The complex effects of CO2 enrichment and temperature across a suite of algal responses illustrates the importance of incorporating multiple stressors into global change experiments.藻丛群落（turf algal assemblages）是珊瑚礁上广泛分布的初级生产者，但目前对这一多样类群在不同温度条件下对海洋酸化（Ocean Acidification, OA）的响应尚缺乏系统认知。本研究验证了如下假说：二氧化碳（CO₂）可调控石表与石内藻丛群落对温度升高的功能响应。研究人员从珊瑚礁采集多份被藻丛覆盖的碳酸盐岩芯复制品，将其分别置于环境与高浓度二氧化碳分压（pCO₂, 1000 µatm）条件下培养3周。每组二氧化碳分压处理均设置6个温度梯度（24.0~31.5 ℃），覆盖法属波利尼西亚茉莉亚岛岸礁的完整季节温度区间，其中包含1个升温处理（较日均温高3 ℃）。温度与二氧化碳富集处理对藻丛的代谢与生长存在复杂且常具交互作用的影响。温度升高可提升光合与呼吸速率，且该效应受到二氧化碳富集的交互调控。在最高温条件下，高二氧化碳分压可进一步放大光合速率；而在环境二氧化碳分压条件下，呼吸速率随温度升高的增幅更为显著。石表藻丛的生长速率不受温度影响，但会随二氧化碳富集而显著提升。本研究发现，二氧化碳与温度对石内藻丛群落存在交互影响：仅在最高温条件下，二氧化碳富集可使石内藻丛生长速率达到峰值。本研究结果阐明了海洋酸化如何在一系列生态相关的温度范围内调控藻类生理与生长，并表明二氧化碳富集对珊瑚礁藻丛群落的影响具有温度依赖性。二氧化碳富集与温度对藻类多项响应指标的复杂影响，凸显了在全球变化实验中纳入多重胁迫因子的必要性。