Ocean acidification and the loss of phenolic substances in marine plants

Abstract

Rising atmospheric CO(2) 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. CO(2) availability is also increasing in the oceans, where anthropogenic emissions cause ocean acidification, decreasing seawater pH and shifting the carbonate system towards further CO(2) enrichment. Such conditions tend to increase seagrass productivity but may also increase rates of grazing on these marine plants. Here we show that high CO(2) / 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 CO(2) vent on the Island of Vulcano, Italy, where pH values decreased from 8.1 to 7.3 and pCO(2) 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 CO(2) 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 CO(2) world.

Figure 1. Location of the underwater CO₂ vent and seagrass meadows on the Island of Vulcano, Italy.

(A) Location of near shore transect beneath the volcano, where dense populations of Cymodocea nodosa inhabit shallow grass beds (C,D). (E) The sampling sites spanned a range of high CO₂/low pH conditions at distances of 260, 300 and 380 m from the vent site (*) along the northern shoreline of the bay.

Figure 2. Location of mid-salinity Free Ocean Carbon Enrichment (FOCE) experiments within the St. Mary's River, Maryland (USA) in May-July 2010.

(A) Individual CO₂ diffusers (white) generated halos of high CO₂/low pH conditions (5 cm and 40 cm distances in grey; 100 cm locations in green; 500 cm ambient sites in blue.) (B) Dense meadow of Ruppia maritima from the near shore site. (C) Bubbles emitted from one of the CO₂ diffusers (compare to CO₂ natural vent site, Fig 1.B).

Figure 3. Location of low-salinity Free Ocean Carbon Enrichment (FOCE) experiments in the Severn River, Maryland (USA) in June-July 2011.

(A) Individual CO₂ diffusers (white) generated halos of high CO₂/low pH conditions (5 cm and 40 cm distances in grey; 100 cm locations in green; 500 cm ambient sites in blue.) The FOCE system instrument package was located on shore (B,D) and maintained high CO₂/low pH conditions within mixed-species meadows of widgeon grass (Ruppia maritima) and redhead grass (Potamogeton perfoliatus) (C).