Colonies of N 2 -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.15 N-isotope labelling experiments and nutrient analyses revealed that N 2 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 N 2 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 (X1 mm) even when suspended in fully oxygenated waters, whereas anoxia in small aggregates (o1 to X0.1 mm) may only arise in lowoxygenated waters (p25 lM). We propose that the net effect of aggregates on nitrogen loss is negligible in NO 3 À -depleted, fully oxygenated (surface) waters. In NO 3 À -enriched (41.5 lM), O 2 -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.

Denitrification is a critical process that can alleviate the effects of excessive nitrogen availability in aquatic ecosystems subject to eutrophication. An important part of denitrification occurs in benthic systems where bioturbation by meiofauna (invertebrates <1 mm) and its effect on element cycling are still not well understood. Here we study the quantitative impact of meiofauna populations of different abundance and diversity, in the presence and absence of macrofauna, on nitrate reduction, carbon mineralization and methane fluxes. In sediments with abundant and diverse meiofauna, denitrification is double that in sediments with low meiofauna, suggesting that meiofauna bioturbation has a stimulating effect on nitrifying and denitrifying bacteria. However, high meiofauna densities in the presence of bivalves do not stimulate denitrification, while dissimilatory nitrate reduction to ammonium rate and methane efflux are significantly enhanced. We demonstrate that the ecological interactions between meio-, macrofauna and bacteria are important in regulating nitrogen cycling in soft-sediment ecosystems.

We investigated the role of N 2 -fixation by the colony-forming cyanobacterium, Aphanizomenon spp., for the plankton community and N-budget of the N-limited Baltic Sea during summer by using stable isotope tracers combined with novel secondary ion mass spectrometry, conventional mass spectrometry and nutrient analysis. + fluxes to Aphanizomenon colonies at low bulk concentrations (o250 nM) as compared with N 2 -fixation within colonies. No N 2 -fixation was detected in autotrophic microorganisms o5 μm, which relied on NH 4 + uptake from the surrounding water. Aphanizomenon released about 50% of its newly fixed N 2 as NH 4 + . However, NH 4 + did not accumulate in the water but was transferred to heterotrophic and autotrophic microorganisms as well as to diatoms (Chaetoceros sp.) and copepods with a turnover time of 5 h. We provide direct quantitative evidence that colony-forming Aphanizomenon releases about half of its recently fixed N 2 as NH 4 + , which is transferred to the prokaryotic and eukaryotic plankton forming the basis of the food web in the plankton community. Transfer of newly fixed nitrogen to diatoms and copepods furthermore implies a fast export to shallow sediments via fast-sinking fecal pellets and aggregates. Hence, N 2 -fixing colony-forming cyanobacteria can have profound impact on ecosystem productivity and biogeochemical processes at shorter time scales (hours to days) than previously thought.

Filamentous, nitrogen-fixing cyanobacteria form extensive summer blooms in the Baltic Sea. Their ability to fix dissolved N2 allows cyanobacteria to circumvent the general summer nitrogen limitation, while also generating a supply of novel bioavailable nitrogen for the food web. However, the fate of the nitrogen fixed by cyanobacteria remains unresolved, as does its importance for secondary production in the Baltic Sea. Here, we synthesize recent experimental and field studies providing strong empirical evidence that cyanobacterial nitrogen is efficiently assimilated and transferred in Baltic food webs via two major pathways: directly by grazing on fresh or decaying cyanobacteria and indirectly through the uptake by other phytoplankton and microbes of bioavailable nitrogen exuded from cyanobacterial cells. This information is an essential step toward guiding nutrient management to minimize noxious blooms without overly reducing secondary production, and ultimately most probably fish production in the Baltic Sea.Electronic supplementary materialThe online version of this article (doi:10.1007/s13280-015-0660-x) contains supplementary material, which is available to authorized users.

SummaryWe analysed N 2 -and carbon (C) fixation in individual cells of Baltic Sea cyanobacteria by combining stable isotope incubations with secondary ion mass spectrometry (SIMS). Specific growth rates based on N 2 -and C-fixation were higher for cells of Dolichospermum spp. than for Aphanizomenon sp. and Nodularia spumigena. The cyanobacterial biomass, however, was dominated by Aphanizomenon sp., which contributed most to total N 2 -fixation in surface waters of the Northern Baltic Proper. N 2 -fixation by Pseudanabaena sp. and colonial picocyanobacteria was not detectable. N 2 -fixation by Aphanizomenon sp., Dolichospermum spp. and N. spumigena populations summed up to total N 2 -fixation, thus these genera appeared as sole diazotrophs within the Baltic Sea's euphotic zone, while their mean contribution to total C-fixation was 21%. Intriguingly, cell-specific N 2 -fixation was eightfold higher at a coastal station compared to an offshore station, revealing coastal zones as habitats with substantial N 2 -fixation. At the coastal station, the cell-specific C-to N 2 -fixation ratio was below the cellular C:N ratio, i.e. N 2 was assimilated in excess to C-fixation, whereas the C-to N 2 -fixation ratio exceeded the C:N ratio in offshore sampled diazotrophs. Our findings highlight SIMS as a powerful tool not only for qualitative but also for quantitative N 2 -fixation assays in aquatic environments.