Marine Nitrogen Cycling and Food Webs

Dear Colleagues,

Nitrogen is a key element in marine food webs. First, because it limits primary producers in most of the ocean. Second, because it is essential for the building of major biomolecules, as proteins and nucleic acids, which are required for maintaining marine life. The nitrogen cycle in the ocean is closely linked with those of other biologically important elements (e.g., carbon and phosphorus cycles) by stoichiometric requirements. Recent recalculations of the global nitrogen cycle have noted a remarkable balance between sources and sinks of nitrogen in the ocean that critically depends on a spatial separation of biologically-mediated assimilation and loss processes. With a longer lifetime of reactive nitrogen in the ocean than in terrestrial ecosystems, about it is still uncertain how the nitrogen cycle will be affected by global changes such as the input of antropogenic nitrogen and the warming of surface waters.

New discoveries have indicated the major importance of the biological control of atmospheric nitrogen fixation and denitrification in much larger areas of the ocean than previously considered (e.g., temperate upwelling regions). The diversity of nitrogen fixers has just started to be revealed using advanced molecular and genomic methods. The impact of fixed nitrogen extends over the entire food web and is transported to traditionally considered nitrogen-deficient regions by long-distance travelling upper consumers (such as tuna). Zooplankton grazing may be one of the main controls of the cycling of nitrogen, affecting, for instance, the spatial decoupling between nitrogen fixation and denitrification, and the remineralization of organic nitrogen by bacteria and archaea, as suggested by recent studies. Zooplankton and mesopelagic fish actively contribute to the transfer of nitrogen, from productive surface waters to deep ocean layers. Methodological developments have also challenged the existing database of experimental nitrogen rate measurements as well as the reconstruction of long-term changes in food webs from alterations in climate and local oceanography affecting the nitrogen cycle.

Major challenges for current research are to determine the consequences of climate change for ocean nitrogen cycling and their effects on other biogeochemical processes and ecosystem functions and services.

The aim of this Special Issue is to provide an overview of recent approaches to the study of the cycling of nitrogen in marine food webs through contributions dealing with the following:

• The molecular diversity of marine nitrogen fixers;
• Rates and magnitude of nitrogen cycling (diazotrophy, the uptake and recycling of dissolved inorganic and organic nitrogen);
• Spatial and temporal patterns of nitrogen-influenced changes in food web structure;
• Nitrogen control on ocean carbon sequestration;
• The environmental and climatic influence of ocean nitrogen cycling.

as well as other relevant topics using specific or generic methodologies applied to the nitrogen cycle (e.g., genomics, stable isotopes, microscopic analysis, etc.).

Dr. Antonio Bode
Dr. Marta M. Varela
Guest Editors

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