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Marine Ecology Progress Series

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MEPS 489:1-16 (2013)  -  DOI:

Optimality-based model of phytoplankton growth and diazotrophy

Markus Pahlow*, Heiner Dietze, Andreas Oschlies

GEOMAR Helmholtz Centre for Ocean Research Kiel, Düsternbrooker Weg 20, 24105 Kiel, Germany

ABSTRACT: The notion that excess phosphorus (P) and high irradiance favour pelagic diazotrophy is difficult to reconcile with diazotroph behaviour in laboratory experiments and also with the observed distribution of N2-fixing Trichodesmium, e.g. in the relatively nitrogen (N)-rich North Atlantic Ocean. Nevertheless, this view currently provides the state-of-the-art framework to understand both past dynamics and future evolution of the oceanic fixed N inventory. In an attempt to provide a consistent theoretical underpinning for marine autotrophic N2 fixation we derive controls of diazotrophy from an optimality-based model that accounts for phytoplankton growth and N2 fixation. Our approach differs from existing work in that conditions favourable for diazotrophy are not prescribed but emerge, indirectly, from trade-offs among energy and cellular resource requirements for the acquisition of P, N, and carbon. Our model reproduces laboratory data for a range of ordinary phytoplankton species and Trichodesmium. The model predicts that (1) the optimal strategy for facultative diazotrophy is switching between N2 fixation and using dissolved inorganic nitrogen (DIN) at a threshold DIN concentration; (2) oligotrophy, especially in P and under high light, favours diazotrophy; (3) diazotrophy is compatible with DIN:DIP supply ratios well above Redfield proportions; and (4) communities of diazotrophs competing with ordinary phytoplankton decouple emerging ambient and supply DIN:DIP ratios. Our model predictions appear in line with major observed patterns of diazotrophy in the ocean. The predicted importance of oligotrophy in P extends the present view of N2 fixation beyond a simple control by excess P in the surface ocean.

KEY WORDS: Nitrogen fixation · Phytoplankton · Optimality · Chain model · Resource allocation · Elemental composition

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Cite this article as: Pahlow M, Dietze H, Oschlies A (2013) Optimality-based model of phytoplankton growth and diazotrophy. Mar Ecol Prog Ser 489:1-16.

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