Ocean acidification weakens seagrass defenses
Chesapeake Bay, USA / Island of Vulcano, Italy. The world’s oceans absorb carbon dioxide (CO2) and slow the pace of climate change. At the same time the absorbed CO2 lowers the pH of ocean waters, changing seawater chemistry in the process called ocean acidification. This can have devastating impacts on corals and shellfish, disrupting the process of calcification essential for the construction of coral reefs and the cultivation of oysters and clams in aquaculture. However, the added CO2 could boost the photosynthesis and growth of coastal seagrasses, according to recent studies suggesting that seagrasses will be “winners” in future acidified seas (1). This is a critical question for those hoping to manage or restore coastal waterways because seagrass meadows reduce coastal erosion and serve as nursery grounds for fish and shellfish, but have been in steep decline world-wide.
The results of a recent study published in the journal PLoS ONE challenge the assumption that seagrasses would necessarily be “winners” in a future high CO2/ low pH world (2). The team, led by Dr. Tom Arnold from Dickinson College, examined the effects of ocean acidification on seagrasses, particularly their ability to produce a range of protective chemicals called phenolics.
“Like most plants,” explained Arnold, “seagrasses produce phenolic substances that can act as structural and chemical defenses,inhibiting the growth of disease organisms and deterring fish and other grazers from consuming the leaves.” Indeed ecologists have long understood that plant phenolics can have numerous roles in plants. Slight changes to their chemical structure give them useful properties as antimicrobials, antioxidants, sunscreens for harmful UV radiation, bitter-tasting deterrents, digestion reducers, and flower pigments, as well as making them the building blocks for forming wood. “On land, more CO2 often means more phenolic substances in plants,” Arnold explained “and this too could be beneficial if it helps protect them from insects or disease”.
To test this a team of researchers, including Dr. Whitman Miller from the Smithsonian Environmental Research Center and three undergraduate students from Dickinson College1, simulated ocean acidification using an instrument called a F.O.C.E., which stands for Free Ocean Carbon Enrichment. The instrument generates high CO2 / low pH seawater and releases it into experimental areas of seagrass meadows. By dialing in the correct settings on the F.O.C.E. they mimicked conditions predicted to occur within the next 100 years in several meadows of aquatic plants, including widgeon grass Ruppia maritima and redhead grass Potamogeton perfoliatus, in the Chesapeake Bay. In each of their experiments they found that high CO2 conditions led to a dramatic loss of the phenolic protective substances in these plants. “We were quite surprised.” said Arnold “This was different than what has been observed on land.”
According to the authors, the surprising observation may be caused by something else that plagues many coastal waterways – nutrient pollution. On land, plants often struggle to acquire nutrients such as nitrogen and phosphorous for growth and, thus, excess CO2 is diverted to the production of phenolics. This might protect plants from grazers at a time when they would find it most difficult to regrow lost tissues (3). However, coastal plants may respond differently because they are often bathed in nutrient-rich waters, the result of nutrient pollution (called “eutrophication”). In this scenario excess CO2 can be combined with nutrients to fuel rapid plant growth instead of phenolic synthesis (4).
To confirm the discovery, Arnold traveled to the Island of Vulcano in the Mediterranean Sea, where CO2 is emitted not only from volcanic craters on land but also from underwater volcanic seeps, creating a natural laboratory for the study of ocean acidification. Here a short swim towards the underwater seep provides a glimpse of the future. As CO2 levels increase closer to the seeps the ecosystem changes visibly – seagrasses and some seaweeds thrive, while creatures such as sea urchins and molluscs disappear (5). On Vulcano Arnold joined forces with Dr. Jason Hall-Spencer from the University of Plymouth and Dr. Marco Milazzo from the Dipartimento di Scienze della Terra e del Mare at the University of Palermo. Together they compared populations of the seagrass Cymodocea nodosa growing near the seeps. They analyzed populations growing at control sites, where the average pH was 8.1 and concentrations of CO2 were 422 ppm – well within the “normal” range for ocean waters. Closer to the seeps, however, the average pH was as low as 7.3 and CO2 levels were nearly ten times higher. They once again found the surprising decrease in concentrations of the phenolic protective substances near the vents, confirming the work done thousands of miles away in the Chesapeake Bay.
“What this means for seagrasses and the creatures that depend upon them isn’t clear yet”, says Arnold. “It is something we are working to understand.”
Recently, he traveled to the world’s second largest sand island, North Stradebroke Island in Australia, to study unusual underground springs flow through mangrove forests to acidify coastal areas. There the team is working to determine if high CO2 causes seagrasses growing there to become more vulnerable to grazing by local rabbitfishes2. Other groups are studying CO2 impacts on seagrass pathogens, such as the slime-mold like microbe that triggers outbreaks of the infamous wasting disease, which is believed to have contributed to a world-wide die-off of eelgrass in the 1930s. “We wonder, will seagrasses really be ‘winners’ in future acidified seas? If ocean acidification stimulates the growth of seagrasses but at the same time reduces their natural defense mechanisms, what does this mean for grazers such as fishes, turtles and dugongs and microbes that cause disease?” he explains. “We just don’t know. We really need this information before we can predict how seagrasses, and therefore coastal communities, will respond.”