The calcifying Haptophyte Emiliania huxleyi appears to be acutely sensitive to the rising concentration of ocean pCO2. Documented responses by E. huxleyi to elevated pCO2 include modifications to their calcification rate and cell size, malformation of coccoliths, elevated growth rates, increased organic carbon production, lowering of PIC:POC ratios, and elevated production of the active climate gas DMS. Changes in these parameters are mechanisms known to elicit alterations in grazing behavior by microzooplankton, the oceans dominant grazer functional group. The investigators hypothesize that modifications to the physiology and biochemistry of calcifying and non-calcifying Haptophyte Emiliania huxleyi in response to elevated pCO2 will precipitate alterations in microzooplankton grazing dynamics. To test this hypothesis, they will conduct controlled laboratory experiments where several strains of E. huxleyi are grown at several CO2 concentrations. After careful characterization of the biochemical and physiological responses of the E. huxleyi strains to elevated pCO2, they will provide these strains as food to several ecologically-important microzooplankton and document grazing dynamics. E. huxleyi is an ideal organism for the study of phytoplankton and microzooplankton responses to rising anthropogenic CO2, the effects of which in the marine environment are called ocean acidification; E. huxleyi is biogeochemically important, is well studied, numerous strains are in culture that exhibit variation in the parameters described above, and they are readily fed upon by ecologically important microzooplankton.
Intellectual Merit
The implications of changes in microzooplankton grazing for carbon cycling, specifically CaCO3 export, DMS production, nutrient regeneration in surface waters, and carbon transfer between trophic levels are profound, as this grazing, to a large degree, regulates all these processes. E. huxleyi is a model prey organism because it is one of the most biogeochemically influential global phytoplankton. It forms massive seasonal blooms, contributes significantly to marine inorganic and organic carbon cycles, is a large producer of the climatically active gas DMS, and is a source of organic matter for trophic levels both above and below itself. The planned controlled study will increase our knowledge of the mechanisms that drive patterns of change between trophic levels, thus providing a wider array of tools necessary to understand the complex nature of ocean acidification field studies, where competing variables can confound precise interpretation.
Broader Impacts
This research project will provide support for two early-career scientists, a graduate student, and numerous undergraduate students. Undergraduate student involvement will come largely from SPMCs long-standing two resident programs: the award-winning Multicultural Initiative in the Marine Sciences: Undergraduate Participation (MIMSUP) and Research Experience for Undergraduates (REU). The mission statement of the MIMSUP program is to increase diversity within the next generation of marine scientists through coursework and research. Students participating in MIMSUP conduct independent research projects, present at national and international meetings, and conduct community outreach. The REU program provides the portal for our research to reach a nation-wide audience;just last year SPMC received 118 REU applications representing 90 different universities. Undergraduates from WWUs Biology and Environmental Science departments will also participate in this research through courses taught by the project PIs. Finally, SPMC has received funding to hire a K-12 outreach coordinator who will develop a series of workshops, public lectures and presentations, and contributions to K-12 curriculum. Findings from this research will be incorporated into all of these outreach efforts.
Human dependence on the combustion of fossil fuels for energy production has dramatically altered the Earth’s 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 Earth’s 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 Earth’s 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 ocean’s 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 acidification were the total lipid and total organic carbon carbon per cell were higher in cells grown under acidified conditions. As we hypothesized, the changes in phytoplankton state had a significant effect on the grazing rate and growth rate of the four different microzooplankton grazers. All species of grazers tested grazed at higher rates on phytoplankton grown under acidified conditions. Additionally, the growth rate of the grazers and the efficiency by which the grazers converted their ingested carbon into their own new biomass was, on average, higher when feeding of phytoplankton grown under acidified conditions. The confirmation of our hypotheses, that ocean acidification will have a cascading effect across planktonic food webs, begs the question: what does this mean for a future ocean? So many important biogeochemical processes are, in part, driven by the interactions between phytoplankton and zooplankton. Some examples are nutrient remineralization in surface waters, production of dissolved organic matter, carbon export to depth, and secondary production. The magnitude of these important processes depends on the rate by which zooplankton consume and grow on their phytoplankton prey. Our results showing that these rates are affected when zooplankton consume phytoplankton grown under future ocean acidification scenarios suggest that biogeochemical cycling in Earth’s surface waters will be affected in an acidifying ocean.
