Aerobic and anaerobic nitrogen transformation processes in N2-fixing cyanobacterial aggregates
2015 (English)In: The ISME Journal, ISSN 1751-7362, E-ISSN 1751-7370, Vol. 9, no 1, 1456-1466 p.Article in journal (Refereed) Published
Colonies of N2-fixing cyanobacteria are key players in supplying new nitrogen to the ocean, but the biological fate of this fixed nitrogen remains poorly constrained. Here, we report on aerobic and anaerobic microbial nitrogen transformation processes that co-occur within millimetre-sized cyanobacterial aggregates (Nodularia spumigena) collected in aerated surface waters in the Baltic Sea. Microelectrode profiles showed steep oxygen gradients inside the aggregates and the potential for nitrous oxide production in the aggregates’ anoxic centres. 15N-isotope labelling experiments and nutrient analyses revealed that N2 fixation, ammonification, nitrification, nitrate reduction to ammonium, denitrification and possibly anaerobic ammonium oxidation (anammox) can co-occur within these consortia. Thus, N. spumigena aggregates are potential sites of nitrogen gain, recycling and loss. Rates of nitrate reduction to ammonium and N2 were limited by low internal nitrification rates and low concentrations of nitrate in the ambient water. Presumably, patterns of N-transformation processes similar to those observed in this study arise also in other phytoplankton colonies, marine snow and fecal pellets. Anoxic microniches, as a pre-condition for anaerobic nitrogen transformations, may occur within large aggregates (1 mm) even when suspended in fully oxygenated waters, whereas anoxia in small aggregates (<1 to 0.1 mm) may only arise in low-oxygenated waters (25 μM). We propose that the net effect of aggregates on nitrogen loss is negligible in NO3−-depleted, fully oxygenated (surface) waters. In NO3−-enriched (>1.5 μM), O2-depleted water layers, for example, in the chemocline of the Baltic Sea or the oceanic mesopelagic zone, aggregates may promote N-recycling and -loss processes.
Place, publisher, year, edition, pages
2015. Vol. 9, no 1, 1456-1466 p.
Nitrogen cycle /Cyanobacteria/macroaggregates/O2 and N2O microsensors/isotope pairing technique/Oxygen Minimum Zones
Earth and Related Environmental Sciences Biological Sciences
Research subject Geochemistry; Marine Ecology
IdentifiersURN: urn:nbn:se:su:diva-115034DOI: 10.1038/ismej.2014.232ISI: 000354786700016OAI: oai:DiVA.org:su-115034DiVA: diva2:795477
FunderBaltic Ecosystem Adaptive Management (BEAM)Swedish Research Council Formas