SGER: Reassessing the Role of Planktonic Amoeboid Protists in Estuarine Microbial Food Webs

Columbia University, New York NY

Investigators

Abstract

Recent methodological advances have revealed that naked amoeboid protists (amebas) are much more abundant in the plankton than previously thought. Using a newly-established set of biovolume relationships, the carbon content of planktonic amebas in the Hudson River estuary (HRE) was estimated to comprise 3-36% of total planktonic heterotrophic protist carbon, comparable to the ciliates, a group that has received much more research attention. Amebas are therefore likely to play an important role in carbon transfer through planktonic food webs in estuaries, challenging current views of microbial C fluxes through estuarine plankton. Nevertheless, C-flux through the ameba pool has not been adequately quantified in any marine system and sampling for amebas in the HRE has heretofore been restricted to a single, shore-based location in the mesohaline portion of the estuary. This project will quantify the significance of naked planktonic amebas to total heterotrophic protist carbon and C flux through the microbial food web of the HRE. Biomasses of attached and free-living bacteria and the major groups of heterotrophic protists will be quantified using microscopy-based techniques. Growth rates of planktonic amebas will be measured under natural conditions for the first time. Data will be gathered from 5 locations along the HRE salinity gradient over 3 months, providing observations from a wide range of environmental settings. The outcome will be the first comprehensive, quantitative understanding of planktonic heterotrophic protist standing stocks and C fluxes within this important estuary, and the first time amebas and other major protist groups in the plankton have been adequately quantified together in any marine system. This project meets SGER guidelines as a venture into an emerging area using recently-developed expertise to potentially transform our established view of estuarine planktonic microbial communities. SGER funding would also be highly leveraged through a companion project funded by the Wallace Foundation and by the in-kind contribution of time and expertise to the project by Co-PI, O. Roger Anderson. Planktonic protists are important links in the food web from bacteria to larger organisms, including invertebrates and fish. However, because of methodological impediments, previous efforts to assess the importance of planktonic protists have overlooked the role of planktonic amebas. Using a combination of well established and newly-available techniques, this project will quantify the contribution of amebas to microbial standing stocks and carbon fluxes in the HRE. This effort has the potential to transform current understanding of planktonic microbial C fluxes in estuaries. This will be the first study of planktonic protists in the lower HRE, and the first study to compare microbial food webs in different regions of the estuary. By providing information on elemental fluxes through lower trophic levels, the results of this study will be relevant to environmental management of one of the most populated estuaries in the US. The HRE provides a model for other estuaries, especially systems with high particle load. Determining the environmental correlates of ameba standing stocks, growth rates, and C flux will facilitate transferring the results of this study to other systems. This project is closely related to, and highly leveraged by, a high-profile study of water-quality in the HRE. The water-quality study will have a large, dedicated web presence and is slated for extensive press coverage in the coming weeks. The project also presents an opportunity for significant and unique knowledge transfer related to the identification and enumeration of planktonic amebas from a senior scientist (Anderson) to a junior colleague (Juhl). In addition, an undergraduate will participate in the research and use a portion of the data collected for her Senior research thesis.