Graduate Thesis Or Dissertation
 

Phosphorus control of nitrogen fixation

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  • Under conditions of fixed N-limitation, as with most oligotrophic systems, the process of biological N₂ fixation (diazotrophy) is favored, provided the necessary trace elements and vitamins are sufficient. Despite the well understood contributions of N₂ fixation in oligotrophic regions, the nutritional and ecological controls of marine diazotrophs have not been well characterized. The aim of this thesis is to explore the role of phosphorus (P) in regulating productivity and nitrogen fixation rates of diazotrophs and to examine the method of phosphorus fractionation as a proxy for Pstatus. Phosphorus is an essential element required by all marine organisms for many components of the cell. Herein, we demonstrate that P plays an important role in regulating productivity and nitrogen fixation rates of diazotrophs. Chapter two characterizes the spatial variation of nitrogen and carbon fixation rates and presents a series of experiments designed to determine the role of inorganic and organic phosphorus limitation in diazotrophic productivity. We present a spatially extensive record of dinitrogen (N₂) fixation rates and distributions of N2 fixing microorganisms along with the results of exogenous P addition experiments conducted during a series of cruises in the North Pacific Subtropical Gyre (NPSG). These experiments produced three major findings: (1) methylphosphonate (MPn) and dissolved inorganic phosphorus (DIP) were utilized with equal metabolic efficiency over a single photoperiod, (2) the bulk of the enhanced N2 fixation rates were within the range reported for the Hawaii Ocean Time-series (HOT) suggesting that P levels in this region can be saturating but were not at the time of sampling and (3) MPn and DIP additions stimulated C fixation rates beyond estimated contributions by diazotrophs, and hence both DIP and bioavailable dissolved organic phosphorus (DOP) additions could lead to enhancement of net primary productivity on short time-scales. Chapter three presents an initial investigation into compartmentalization of phosphorus in natural assemblages of Trichodesmium in relation to the bulk plankton community and explores the use of a phosphorus fractionation method as a potential proxy for Pstatus. We evaluated intracellular P compartmentalization by both the bulk natural assemblage of plankton and isolated colonies of Trichodesmium from five cruises within the North and South Pacific subtropical Gyres (NPSG and SPSG, respectively). Using a trichloroacetic acid extraction method, samples were analyzed for total and acid soluble fractions of particulate P. Field data suggest that natural populations of Trichodesmium demonstrate a unique internal composition of primarily inorganic phosphorus while the bulk plankton assemblage are composed of primarily organic P pools.
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