Ocean acidification and the loss of phenolic substances in marine plants

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浓度常会诱导植物产生酚类物质，其中诸多物质可充当植食动物驱避剂、消化抑制剂、抗菌剂或是紫外线防护剂。这类响应可通过主流的植物防御模型进行预测，尤其是将碳有效性与酚类生物合成相关联的资源有效性模型。海洋中的二氧化碳有效性同样在提升：人类活动排放导致海洋酸化（Ocean Acidification, OA），使海水pH值下降，碳酸盐体系向更高二氧化碳富集的方向偏移。此类环境条件通常会提升海草的生产力，但也可能提高这类海洋植物被啃食的速率。本研究表明，海洋酸化（Ocean Acidification, OA）下的高二氧化碳/低pH环境，反而会降低而非提升海草与广盐性海洋植物体内的保护性酚类物质浓度。我们在意大利沃尔坎诺岛一处火山二氧化碳喷口附近的海草喜盐草（Cymodocea nodosa）中观察到了简单酚类与聚合酚类的流失现象：该处pH值从8.1降至7.3，二氧化碳分压（pCO2）浓度提升了10倍。我们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——海菖蒲（Ruppia maritima）与穿叶眼子菜（Potamogeton perfoliatus）中也观察到了类似的响应。这类响应与陆生植物的表现有着显著差异。酚类物质的流失或可解释海底二氧化碳喷口附近观测到的高于常规水平的啃食速率，同时表明海洋酸化可通过改变分解、啃食与病害的速率，进而影响沿岸碳通量。本研究的观测结果修正了近期的相关预测——此前人们认为海草在高二氧化碳世界中必然会成为"赢家"。