Grantee Research Project Results

Comparison of Nitrogen Uptake Patterns in Phytoplankton and Bacteria From Multiple Marine Ecosystems

EPA Grant Number: FP916328
Title: Comparison of Nitrogen Uptake Patterns in Phytoplankton and Bacteria From Multiple Marine Ecosystems
Investigators: Bradley, Paul B.
Institution: College of William and Mary-VA
EPA Project Officer: Lee, Sonja
Project Period: January 1, 2004 through December 31, 2006
Project Amount: $105,116
RFA: STAR Graduate Fellowships (2004) RFA Text |  Recipients Lists
Research Category: Fellowship - Oceanography , Fellowship - Aquatic Ecology and Ecosystems , Academic Fellowships

Objective:

This research will focus on the variations in microbial nitrogen utilization between three different types of marine ecosystems: estuarine, coastal, and oceanic. The objective of this research project is to quantify the fate of dissolved inorganic and organic nitrogen into either autotrophic or heterotrophic biomass. The main hypothesis is that the traditional dogma of bacteria dominating organic nitrogen consumption and phytoplankton using mainly inorganic nitrogen does not accurately reflect microbial nitrogen use in marine environments.

Approach:

Previous approaches used to examine phytoplankton and bacterial nitrogen use, such as size fractionation, have a limited ability to distinctly separate these two groups for analytical purposes. This research employs a new approach, namely cell sorting on a flow cytometer, to isolate autotrophs from heterotrophs on the basis of the presence of autofluorescent pigments (e.g., chlorophyll). The sorting capabilities of flow cytometry are combined with traditional ¹⁵N isotopic tracer techniques to resolve the variations in autotrophic and heterotrophic nitrogen utilization in three different marine environments: the Chesapeake Bay (estuarine), the Mid-Atlantic Bight (coastal), and the Gulf of Mexico (oceanic). At each of these locations, water samples from various depths are incubated with ¹⁵N-labelled ammonium, nitrate, urea, and an amino acid mixture before being concentrated and preserved for subsequent sorting and analysis to determine uptake rates for each nitrogen substrate. The uptake rates then are used to quantify nitrogen fluxes into autotrophic and heterotrophic biomass.

Bacteria are known to utilize ammonium in many marine environments, and they are capable of out-competing phytoplankton for this inorganic nitrogen source. Some phytoplankton, on the other hand, prefer organic nitrogen compounds, such as urea. Thus, it is expected that enhanced use of inorganic nitrogen by bacteria will lead to an increased dependence by phytoplankton on organic nitrogen compounds. Such a scenario contradicts the current dogma and could have significant implications for nutrient management strategies in estuarine and coastal waters.

Supplemental Keywords:

fellowship, phytoplankton, heterotrophic bacteria, flow cytometry, dissolved organic nitrogen, dissolved inorganic nitrogen, marine ecosystems, cell sorting,, RFA, Ecosystem Protection/Environmental Exposure & Risk, Scientific Discipline, ECOSYSTEMS, Aquatic Ecosystem, Aquatic Ecosystems & Estuarine Research, Oceanography, Aquatic Ecosystems, Ecology and Ecosystems, Environmental Monitoring, marine phytoplankton, nutrient dynamics, oceanic phytoplankton, phytoplankton, aquaculture, microbial nitrogen uptake, heterotrophic bacteria, ecosystem monitoring, ecosystem response

Relevant Websites:

2004 STAR Graduate Fellowship Conference Poster (PDF, 1p., 147KB, about PDF)

Progress and Final Reports:

2004

2005

Final