A new perspective on environmental controls of marine nitrogen fixation

• Published in: Geophysical Research Letters
• Main Authors: Landolfi, Angela, Koeve, Wolfgang, Dietze, Heiner, Kähler, Paul, Oschlies, Andreas
• Format: Article in Journal/Newspaper
• Language: English
• Published: AGU (American Geophysical Union) 2015
• Subjects: North Atlantic
• Online Access: https://oceanrep.geomar.de/id/eprint/28814/; https://oceanrep.geomar.de/id/eprint/28814/7/Landolfi.pdf; https://doi.org/10.1002/2015GL063756

Growing slowly, marine N2 fixers are generally expected to be competitive only where nitrogen (N) supply is low relative to that of phosphorus (P) with respect to the cellular N:P ratio (R) of non-fixing phytoplankton. This is at odds with observed high N2 fixation rates in the oligotrophic North Atlantic where the ratio of nutrients supplied to the surface is elevated in N relative to the average R (16:1). In this study, we investigate several mechanisms to solve this puzzle: iron limitation, phosphorus enhancement by preferential remineralization or stoichiometric diversity of phytoplankton, and dissolved organic phosphorus (DOP) utilization. Combining resource competition theory and a global coupled ecosystem-circulation model we find that the additional N and energy investments required for exo-enzymatic break-down of DOP gives N2 fixers a competitive advantage in oligotrophic P-starved regions. Accounting for this mechanism expands the ecological niche of N2-fixers also to regions where the nutrient supply is high in N relative to R, yielding, in our model, a pattern consistent with the observed high N2-fixation rates in the oligotrophic North Atlantic.