Plasticity and Adaptation of Marine Organisms to Ocean Acidification
Brian Hopkinson, University of Georgia
Anthropogenic CO2 emissions are increasing the concentration of CO2 in the surface ocean leading to a drop in seawater pH. Both the rise in CO2 and drop in pH will affect marine organisms from the tiny microalgae, phytoplankton, that perform most of the photosynthesis in the ocean to more recognizable creatures such as corals and fish. Ocean acidification will make it more difficult for many organisms to make hard-parts, such as shells, skeletons, and spines, out of calcium carbonate as the concentration of carbonate in the ocean declines. On the other hand, photosynthetic rates in organisms such as algae, seagrasses, and corals generally increase as CO2 becomes more available.
Organisms respond to the changes in seawater chemistry caused by ocean acidification. In some cases taking advantage of the changes like increased CO2 to grow more rapidly and in other cases attempting to mitigate the effects of harmful changes, such as reduced carbonate. Two significant modes of response are distinguished: phenotypic plasticity and genetic adaptation. Phenotypic plasticity refers to physiological, behavioral, etc adjustments an individual takes to deal with environmental change. These responses are usually fast: minutes to weeks. Adaptation is evolutionary change (i.e. a change at the genetic level) that helps a species survive in its modified environment. These responses take many generations, which depending on the life span of the organism could be as short as several weeks to thousands of years.
Plasticity of marine organisms in response to ocean acidification is somewhat well-understood and my talk will focus on the important examples of photosynthetic responses in marine phytoplankton and the response of calcification in corals. Phytoplankton have a system called the CO2 concentrating mechanism that elevates and regulates the concentration of CO2 around RubisCO, insulating them from the changing environmental conditions. As seawater CO2 rises, this CO2 concentrating mechanism can be down-regulated (turned down), which saves energy. Reallocation of this energy to other processes is believed to account for the increased growth rates of phytoplankton at high CO2. Corals calcify by raising the pH of the calcifying fluid in an extracellular space adjacent to the existing skeleton. This fluid becomes more acidic as seawater becomes more acidic, but corals are able to partially compensate.
Adaptation in response to ocean acidification is not very well understood. Recent experiments in sea urchins have shown that these organisms do show signatures of adaptation to ocean acidification, but the ecological significance of this is still unclear. Some species of phytoplankton lose components of the CO2 concentrating mechanism when adapted to high CO2. Plasticity and adaptation allow marine organisms to cope with or even take advantage of some degree of ocean acidification. However, continued acidification will be harmful to many marine species.