Ocean acidification and the loss of phenolic substances in marine plants@en

Rising atmospheric CO2 often triggers the production of plant phenolics, including many that serve as herbivore deterrents, digestion reducers, antimicrobials, or ultraviolet sunscreens. Such responses are predicted by popular models of plant defense, especially resource availability models which link carbon availability to phenolic biosynthesis. CO2 availability is also increasing in the oceans, where anthropogenic emissions cause ocean acidification, decreasing seawater pH and shifting the carbonate system towards further CO2 enrichment. Such conditions tend to increase seagrass productivity but may also increase rates of grazing on these marine plants. Here we show that high CO2 / low pH conditions of OA decrease, rather than increase, concentrations of phenolic protective substances in seagrasses and eurysaline marine plants. We observed a loss of simple and polymeric phenolics in the seagrass Cymodocea nodosa near a volcanic CO2 vent on the Island of Vulcano, Italy, where pH values decreased from 8.1 to 7.3 and pCO2 concentrations increased ten-fold. We observed similar responses in two estuarine species, Ruppia maritima and Potamogeton perfoliatus, in in situ Free-Ocean-Carbon-Enrichment experiments conducted in tributaries of the Chesapeake Bay, USA. These responses are strikingly different than those exhibited by terrestrial plants. The loss of phenolic substances may explain the higher-than-usual rates of grazing observed near undersea CO2 vents and suggests that ocean acidification may alter coastal carbon fluxes by affecting rates of decomposition, grazing, and disease. Our observations temper recent predictions that seagrasses would necessarily be \"winners\" in a high CO2 world.大气中不断升高的二氧化碳（CO₂）常会诱导植物合成酚类物质（phenolics），这类物质包含诸多可作为植食动物威慑剂、消化抑制剂、抗菌剂或紫外线防护剂的成分。主流植物防御模型，尤其是将碳有效性与酚类生物合成相挂钩的资源可用性模型，早已对这类响应作出了预测。 海洋中的二氧化碳有效性同样在提升，人为排放引发了海洋酸化（Ocean Acidification, OA），降低了海水pH值，并使碳酸盐系统向二氧化碳进一步富集的方向偏移。这类环境条件往往会提升海草的生产力，但也可能加快这些海洋植物被啃食的速率。 本研究表明，海洋酸化的高CO₂/低pH条件，反而会降低而非提升海草与广盐性海洋植物体内的酚类防护物质浓度。我们在意大利武尔卡诺岛附近的火山CO₂喷口区域开展观测，发现该区域pH值从8.1降至7.3，pCO₂浓度提升10倍，海草波喜荡草（Cymodocea nodosa）体内的简单酚类与聚合酚类均出现流失。我们还在美国切萨皮克湾支流开展的原位自由海洋碳富集（Free-Ocean-Carbon-Enrichment, FOCE）实验中，在两种河口物种——丝叶川蔓藻（Ruppia maritima）和穿叶眼子菜（Potamogeton perfoliatus）中观察到了类似的响应。 这类响应与陆生植物所表现出的特征截然不同。酚类物质的流失或可解释海底CO₂喷口附近观测到的高于常规的植食啃食速率，同时表明海洋酸化可能通过改变分解、啃食与病害速率，进而影响近岸碳通量。本研究结果修正了近期的一项预测——即海草在高CO₂世界中必然会成为“赢家”。