869 / 2024-09-19 17:37:31

The Contribution of Diazotroph to the Temporal Decoupling of Nitrogen Fixation and Export Production: Insights from the Hawaii Ocean Time-series

Nitrogen Fixation,Export Production,Diazotroph,Grazing Pressure

Session 3 - The nitrogen cycle towards a sustainable ocean: from microbes to global biogeochemistry

Biological nitrogen fixation, carried out by a specialized group of prokaryotes known as diazotrophs, is the process by which inert dinitrogen (N₂) gas is converted into bioavailable forms of nitrogen. Since most marine organisms cannot directly utilize N₂, this transformation plays a crucial role in sustaining oceanic productivity. Notably, nitrogen fixation supplies approximately 50% of the bioavailable nitrogen in marine ecosystems, underpinning primary production and driving carbon sequestration through its export to the deep ocean. Although there is an approximate balance on larger temporal and spatial scales, evidence suggests a potential decoupling between diazotroph-derived nitrogen (DDN) and its subsequent export from the euphotic zone, particularly over shorter timescales. To investigate this, we leverage data from the Hawaii Ocean Time-series (HOT), focusing on nitrogen fixation rates, nifH gene abundances, and associated export production to track seasonal differences in diazotroph community composition and dominant species. Building on this, the study delves into the variability in grazing pressures and DDN export efficiencies across different diazotroph groups. These factors help explain the temporal decoupling between DDN and its export, shedding light on its seasonal fate—whether retained in circulation or exported to deeper ocean layers through sedimentation. By addressing these mechanisms, the research provides a theoretical basis for quantifying and predicting the spatiotemporal variability in the coupling and decoupling of nitrogen fixation and export production, advancing our understanding of ocean biogeochemistry.