This spring (2016), the Coastal Studies Center hosted the students of EOS2625: Ocean Acidification as they conducted semester-long acidification experiments examining both larval and adult stages of the local green sea urchin. The course was co-taught by Visiting Assistant Professor Meredith White, a researcher who also served on the Maine Ocean Acidification Commission, and marine sciences Laboratory Instructor Elizabeth Halliday Walker.
Human emissions of carbon dioxide are causing acidification of the ocean at a rate unprecedented in the geologic record, and consequently changing ocean chemistry in ways that may present challenges for many marine organisms. In addition to lowering pH, the changes in carbonate chemistry are making it more difficult for organisms to build calcium carbonate shells or skeletal structures. Because the spines, jaws, and internal skeletal structure of sea urchins are all composed of calcium carbonate, there is some concern about how these organisms will fare in the future.
To investigate the effects of ocean acidification on sea urchin growth and chemical composition, adult urchins were kept in two flow-through seawater tanks at the Coastal Studies Center for two months. In one tank, carbon dioxide was bubbled into the water to maintain a pH approximately 0.5pH units lower than the ambient seawater.
Students measured physiological stress over time by seeing how long it took for urchins to right themselves after being flipped over, and measured weight gain over the course of the experiment. To measure calcification during the experiment, Biology Professor Amy Johnson and Research Associate Olaf Ellers shared a unique method they have used in past research on sea urchins. The students injected the urchins with tetracycline at the beginning of the experiment, and because tetracycline binds to calcium, it gets incorporated into any new skeletal structures that are actively being synthesized. Tetracycline has the additional benefit of fluorescing under certain wavelengths of light, so at the end of the experiment the skeletal structures could be photographed under the epifluorescent microscope to visualize a fluorescent band of growth and measure exactly how much the jaws had grown since the beginning of the experiment.
With help from EOS Professor Emily Peterman, students were also able to assess the chemical composition of the carbonate structures using a brand new scanning electron microscope with energy-dispersive X-ray spectroscopy (EDS). In addition to changes in chemical composition, the electron microscope also revealed the beautiful complexity of the sea urchin skeletal structures, which raised many more questions about calcification!
Finally, sea urchins were spawned to conduct a similar experiment on larvae. In many organisms, larvae are more sensitive to changes in the environment. The spaceship-like sea urchin larvae grow skeletal rods as they develop, which are also calcium carbonate, and students found that larvae reared in high-CO2 conditions had shorter skeletal rods.
The culturing techniques made possible by the facilities at the Coastal Studies Center, and the ability to conduct realistic ocean acidification experiments by manipulating pH with carbon dioxide, were essential to the success of the course and helped reveal the complexities of this growing field of research. Most of all, the interdisciplinary collaboration within the course sparked many new lines of inquiry, and revealed how big problems can be attacked in myriad complementary ways.
Story written by Lab Instructor Elizabeth Walker