Award: OCE-0961229

Human dependence on the combustion of fossil fuels for energy production has dramatically altered the EarthÆs atmospheric and oceanic chemistry by releasing CO2 into the atmosphere at geologically unprecedented rates. A portion (roughly one third) of this rising atmospheric CO2 dissolves into the ocean. This dissolution of CO2 into the ocean raises the acidity of the ocean (lowering oceanic pH), and as such, is called ocean acidification. While considerable research has explored how ocean acidification affects marine organisms, most specifically organisms that produce calcium carbonate shells which erode in acidic environments, at the time of this award no research had been done linking the effects of ocean acidification from one group of organisms (a trophic level) to a group of organisms dependent on them (a higher trophic level) for their energy source. The underlying goal of this project was to explore how the base of the ocean food web, and specifically the efficiency by which organic energy moves through trophic levels, will be affected by ocean acidification. We hypothesized that as the ocean acidifies due to rising dissolved CO2, the phytoplankton, who through photosynthesis depend on CO2 to produce their organic respiratory sugars, will alter their cellular physiology and biochemistry. We further hypothesized that these cellular changes in phytoplankton will in turn affect how fast the grazers of phytoplankton, the zooplankton, eat and grow on the altered phytoplankton prey. This is an important question because any change in how fast and efficiently phytoplankton-derived energy moves through succeeding planktonic trophic levels will affect succeeding higher trophic levels (i.e. fish and marine mammals), and many of the EarthÆs biogeochemical cycles. In order to test our hypotheses, we constructed an experimental system that allowed enrichment of seawater with CO2 (acidify) through air-sea gas exchange, which compared to other acidifying techniques, most realistically mimics nature. Our system allowed us to grow plankton under several different ocean acidification scenarios, including current conditions, and two acidified scenarios projected for the end of this century. Several control systems allowed us to monitor CO2 concentration during our experiments, and sophisticated analytical equipment purchased through this award allowed measurement of several water chemistry parameters related to ocean acidification. With the experimental system in place our task turned to testing our hypotheses, which we accomplished by 1) characterizing the physiological, biochemical, and morphological responses of phytoplankton to ocean acidification, and 2) measuring the feeding and growth rates of zooplankton grazers when feeding on these phytoplankton that were acclimated to acidified conditions. We chose two ecologically important phytoplankton to use as our model organisms, Emiliania huxleyi and Rhodomonas sp. Emiliania huxleyi is arguably the EarthÆs dominant marine calcifier. It produces small discs called coccoliths, which are made from calcium carbonate. These discs continually detach and fall from the cell, where some sink to ocean sediments, taking with them particulate carbon produced in the surface waters. The second model phytoplankton, Rhodomonas sp. is a nutritionally-rich phytoplankton, and is an important food resource for zooplankton. The model zooplankton we used were four different species of microzooplankton. This trophic level of zooplankton are small, being between 20-200 µm in length. They are, however, the oceanÆs dominant grazer of phytoplankton. We found that several characteristics of the phytoplankton changed when grown under acidified conditions. For both phytoplankton species tested, cells grown in higher CO2/more acidified conditions were significantly larger that cells grown under current day conditions. Other aspects of the two model phytoplankton that were affected by ocean...