Selected Projects


Nitrogen Metabolism in Aureococcus Anophagefferens

Elyse Walker collecting water from Quantuck Bay, NY. Photo by Tom Behling.

Advisor: Chris Gobler, School of Marine and Atmospheric Sciences, Stony Brook University

Summary: In the Northeast United States, one harmful algal bloom-causing organism is Aureococcus anophagefferens, a small alga that forms "brown tides" in bays. Brown tides form when inorganic nitrogen (nitrate, nitrite, and ammonia) concentrations are low, unlike other phytoplankton blooms. A. anophagefferens’ can use organic forms of nitrogen, like urea and amino acids, to grow, which may be their competitive edge. The recent sequencing of the genome of A. anophagefferens by the Department of Energy’s Joint Genome Institute resulted in the discovery of genes for breaking down a variety of organic nitrogen types. For example, A. anophagefferens can use urea, guanine, and amino acids as nitrogen sources. Many more genes for organic nitrogen metabolism have been found, but it is unknown if these genes are expressed, if ever, and under what conditions. My dissertation will examine the expression of nitrogen metabolism genes in culture experiments, field samples, and field experiments. I hope to determine what sources of nitrogen A. anophagefferens uses during brown tides.

Poster: Walker, E.A. & C.J. Gobler (April 2009) Brown tide blooms in New York estuaries, 2007 – 2008. Stony Brook Southampton Coastal and Estuarine Research Program (SCERP) Symposium: Southampton, NY.
Poster: Walker, E. & C. Gobler (April 2010) Using molecular tools to resolve the causes of chronic brown tides on Long Island’s south shore. Stony Brook Southampton Coastal and Estuarine Research Program (SCERP) Symposium: Southampton, NY.
Poster: Walker, E. & C. Gobler (April 2011) Using molecular tools to resolve the causes of chronic brown tides on Long Island’s south shore. Stony Brook Southampton Coastal and Estuarine Research Program (SCERP) Symposium: Southampton, NY.
Paper: Gobler, C.J., D.L. Berry, S.T. Dyhrman, S.W. Wilhelm, A. Salamov, A.V. Lobanov, Y. Zhang, J.L. Collier, A.B. Kustka, L.L. Wurch, A. Kuo, A. Terry, J. Pangilinan, E. Lindquist, S. Lucas, I. Paulsen, T.K. Hattenrath, S.C. Talmage, E.A. Walker, F. Koch, A.M. Burson, M.A. Marcoval, Y.-Z. Tang, G.R. LeCleir, G.M. Berg, K.J. Coyne, E.M. Bertrand, M.A. Saito, V. Gladyshev, I.V. Grigoriev. 2011. Niche of harmful alga Aureococcus anophagefferens revealed through ecogenomics. Proceedings of the National Academy of Sciences 108(11):4352-4357.

Phytoplankton Metal Quotas

Elyse Walker measuring phytoplankton cells on a microscope. Photo by Ben Twining.

Advisor: Benjamin Twining, Senior Research Scientist, Bigelow Laboratory for Ocean Sciences

Summary: Co, Fe, Mn, Ni and Zn are metals that are required for phytoplankton growth. In this study, direct measurements of these metals were made in individual phytoplankton and protozoa cells from the equatorial Pacific Ocean. Metal quotas were determined for individual cells with synchrotron x-ray fluorescence microscopy, and cellular stoichiometries were calculated relative to measured P and S, as well as to C calculated from biovolume. My role in this study was to measure cell dimensions microscopically, calculate their volumes based on shape, and calculate carbon content based on cell volume. These numbers were used to determine the metal "quota" for each cell.

Paper: Twining, B.S., S.B. Baines, B. Bozard, S. Vogt, E. Walker, D.M. Nelson (2010) Metal quotas of plankton in the equatorial Pacific Ocean. Deep Sea Research II.

Pseudo-nitzschia Detection Using the FlowCAMŪ

Elyse Walker collecting water from Sequim Bay, WA. Photo by Jimmie Cotton.

Advisor: Dana Woodruff, Marine Sciences Laboratory, Pacific Northwest National Laboratory

Summary: Toxigenic blooms of Pseudo-nitzschia spp. are increasing in the Puget Sound waters of Washington State. Early detection and rapid screening methodologies are an important component of understanding and mitigating these bloom events. In this preliminary investigation we explored the development of a method using a Fluid Imaging FlowCAMŪ for quickly identifying Pseudo-nitzschia spp. in the waters of Sequim Bay Washington, a coastal embayment that has seen a dramatic increase recently in Pseudo-nitzschia spp. blooms.

Poster: Walker, E.A. and D.L. Woodruff (October 2007) Identification and enumeration of Pseudo-nitzschia in Pacific Northwest coastal waters using the FlowCAMŪ continuous imaging particle analyzer. Fourth Symposium on Harmful Algae in the U.S.: Woods Hole, MA.
Presentation: Walker, E.A. (August 2006) Methodology Development for Monitoring the Diatom Pseudo-nitzschia spp. in Coastal and Estuarine Waters of the Pacific Northwest. Battelle Marine Research Operations Seminar: Sequim, WA.

Phytoplankton and the Algae Online AnalyzerŪ

Elyse Walker operating the AOA. Photo by Mel Ranhofer.

Advisor: Tammi Richardson, Department of Biological Sciences, University of South Carolina

Summary: Phytoplankton are taxonomically diverse and different groups function differently in the environment. All phytoplankton differ in the types of pigments they contain, thus individual pigments and their ratios can be used to quantify different phytoplankton groups. Real-time monitoring allows for a detailed picture of the interactions of biological, chemical, and physical parameters over short time scales and the early detection of events like developing harmful algal blooms. A German company, bbe Moldaenke, has developed the Algae Online AnalyzerŪ (AOA) for continuous monitoring of the composition of the phytoplankton community. The purpose of this study was to compare the results of AOA determinations of phytoplankton community structure to standard analyses by High Performance Liquid Chromatography and CHEMTAX analysis.

Presentation: Walker, E.A. (August 2005) Phytoplankton Community Composition at Clambank Creek, SC, as Measured by HPLC and Spectral Fluorometry. Howard Hughes Undergraduate Award Symposium: Columbia, SC.
Paper: Richardson, T.L., E. Lawrenz, J.L. Pinckney, R.C. Guajardo, E.A. Walker, H.W. Paerl, H.L. MacIntyre (2010) Spectral fluorometric characterization of phytoplankton community composition using the Algae Online AnalyserŪ. Water Research 44(8):2461-2472.

Light, Grazing, and Karenia brevis

Elyse Walker with her K. brevis cultures. Photo by Tammi Richardson.

Advisor: Tammi Richardson, Department of Biological Sciences, University of South Carolina

Summary: Karenia brevis is a mixotrophic phytoplankton that forms annual red tide events off of the Florida Shelf. In this study, we examined the growth of K. brevis under two different light levels and with or without grazers present. Our results indicated that chlorophyll- and gyroxanthin-based growth rates were greater at high light intensity with grazers present. There was significant interaction between light and grazing, indicating that grazing may actually increase bloom persistence. These results were included as part of a broader publication on the method used to approach this question.

Paper: Richardson, T.L., J.L. Pinckney, E.A. Walker, D. Marshalonis (November 2006) Photopigment radiolabelling as a tool for determining in situ growth rates of the toxic dinoflagellate Karenia brevis (Dinophyceae). European Journal of Phycology 41(4):415-423.
Poster: Walker, E.A. (April 2005) Growth Rates of the Toxic Dinoflagellate Karenia brevis in Response to Variations in Light and Grazing. Discovery Day: Columbia, SC.
Award: 1st Place Poster in the Earth Science Category at Discovery Day in Columbia, SC in April of 2005.


For more information, download the pdf version of my CV or contact me.