PROJECT ABSTRACT
NSF Ecology Program Project (0452324)

Multiple Nitrogen Utilization Strategies and Phytoplankton Species Diversity: Nitrate Assimilation in N2-fixing Bloom-Forming Cyanobacteria

Principal Investigators:
Hans W. Paerl, Univ. of North Carolina at Chapel Hill, Institute of Marine Sciences, Morehead City, NC 28557
Co-P.I.’s:
Pia H. Moisander, Univ. of California at Santa Cruz, Dept. of Ocean Sciences, Santa Cruz, CA 95064
Michael F. Piehler, Univ. of North Carolina at Chapel Hill, Institute of Marine Sciences, Morehead City, NC 28557
Rachel T. Noble, Univ. of North Carolina at Chapel Hill, Institute of Marine Sciences, Morehead City, NC 28557

This study addresses the hypothesis that capability to efficiently utilize alternative bioavailable nitrogen sources plays an important role in determining long-term planktonic nitrogen-fixing cyanobacterial species composition in certain freshwater systems. Many nitrogen-fixing cyanobacteria can also utilize nitrate. Whether native nitrogen-fixers will successfully compete with an invasive species in systems experiencing increased nitrate loading, may depend on species-specific ecological trade-offs between nitrate and dinitrogen utilization. To test this hypothesis, the roles of nitrogen (N) uptake and utilization mechanisms potentially promoting dominance of the invasive Cylindrospermopsis raciborskii over the native Anabaena spp. in central Florida lakes, are being addressed. C. raciborskii is a toxin-former and was first reported in the St. Johns River (SJR) watershed lakes in the early 1980’s. Since then it has rapidly expanded and increased in dominance in these systems that have highly variable levels of bioavailable N throughout the year. We hypothesize that efficient use of multiple N sources by C. raciborskii plays a key role in its success in certain freshwater systems. C. raciborskii is present in the community in the SJR in the spring when the nitrate levels are still high in the water column following winter-spring runoff, and it typically increases in abundance in the summer, when N limitation in the system promotes growth of N2-fixing phytoplankton. Although C. raciborskii relies on N2-fixation during N-depleted conditions, its growth during high nitrate concentrations suggests that it is either a good competitor for nitrate, is relying on N2 fixation in the presence of nitrate, or effectively does both and may efficiently shift between the two N sources. Being able to effectively compete for nitrate in the spring months may give C. raciborskii a head-start over other diazotrophs confined to N-deplete summer months. The following Objectives are being pursued: The study addresses competition and ecological tradeoffs of diazotrophic cyanobacteria in situations in which N2 is the only N source and in which the organism is shifting back and forth between nitrate and N2 utilization. The study is evaluating how differences in utilization of different N sources by N2-fixing cyanobacteria may control competitive interactions and ultimately the N2-fixing cyanobacterial species composition. The goal is to develop a mechanistic understanding of shifts in phytoplankton community due to invasion of C. raciborskii. A method iwill be developed to characterize assimilative nitrate reductase (NR) gene narB in N2-fixing cyanobacteria. To study NR expression, an antibody for a diazotrophic NR will be developed based on a translated narB sequence. Kinetic parameters for DIN under different nitrogen conditions will be determined for isolates. Culture (invasive and native strains) and field experiments will address controls of nitrate uptake and NR expression, in relation to expression of nitrogenase (NG), the enzyme complex responsible of N2 fixation.