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Marine nitrogen fixation: Cyanobacterial nitrogen fixation and the fate of new nitrogen in the Baltic Sea
Stockholm University, Faculty of Science, Department of Ecology, Environment and Plant Sciences. (Marine Ecology)
2015 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Biogeochemical processes in the marine biosphere are important in global element cycling and greatly influence the gas composition of the Earth’s atmosphere. The nitrogen cycle is a key component of marine biogeochemical cycles. Nitrogen is an essential constituent of living organisms, but bioavailable nitrogen is often short in supply thus limiting primary production. The largest input of nitrogen to the marine environment is by N2-fixation, the transformation of inert N2 gas into bioavailable ammonium by a distinct group of microbes. Hence, N2-fixation bypasses nitrogen limitation and stimulates productivity in oligotrophic regions of the marine biosphere.

Extensive blooms of N2-fixing cyanobacteria occur regularly during summer in the Baltic Sea. N2-fixation during these blooms adds several hundred kilotons of new nitrogen into the Baltic Proper, which is similar in magnitude to the annual nitrogen load by riverine discharge and more than twice the atmospheric nitrogen deposition in this area. N2-fixing cyanobacteria are therefore a critical constituent of nitrogen cycling in the Baltic Sea. In this thesis N2 fixation of common cyanobacteria in the Baltic Sea and the direct fate of newly fixed nitrogen in otherwise nitrogen-impoverished waters were investigated. Initially, the commonly used 15N-stable isotope assay for N2-fixation measurements was evaluated and optimized in terms of reliability and practicality (Paper I), and later applied for N2-fixation assessments (Paper II–IV). N2 fixation in surface waters of the Baltic Sea was restricted to large filamentous heterocystous cyanobacteria (Aphanizomenon sp., Nodularia spumigena, Dolichospermum spp.) and absent in smaller filamentous cyanobacteria such as Pseudanabaena sp., and unicellular and colonial picocyanobacteria (Paper II-III). Most of the N2-fixation in the Northern Baltic Proper was contributed by Aphanizomenon sp. due to its high abundance throughout the summer and similar rates of specific N2-fixation as Dolichospermum spp. and N. spumigena. Specific N2 fixation was substantially higher near the coast than in an offshore region (Paper II). Half of the fixed nitrogen was released as ammonium at the site near the coast and taken up by non-N2-fixing organisms including phototrophic and heterotrophic, prokaryotic and eukaryotic planktonic organisms. Newly fixed nitrogen was thereby rapidly turned-over in the nitrogen-depleted waters (Paper III). In colonies of N. spumigena even the potential for a complete nitrogen cycle condensed to a microcosm of a few millimeters could be demonstrated (Paper IV). Cyanobacterial colonies can therefore be hot-spots of nitrogen transformation processes potentially including nitrogen gain, recycling and loss processes. In conclusion, blooms of cyanobacteria are instrumental for productivity and CO2 sequestration in the Baltic Sea. These findings advance our understanding of biogeochemical cycles and ecosystem functioning in relation to cyanobacterial blooms in the Baltic Sea with relevance for both ecosystem-based management in the Baltic Sea, and N2-fixation and nitrogen cycling in the global ocean.

Place, publisher, year, edition, pages
Stockholm: Department of Ecology, Environment and Plant Sciences, Stockholm University , 2015. , 41 p.
Keyword [en]
biogeochemistry, nitrogen cycling, nitrogen fixation, cyanobacteria, Baltic Sea
National Category
Research subject
Marine Ecology
URN: urn:nbn:se:su:diva-122080ISBN: 978-91-7649-278-9 (print)OAI: diva2:862764
Public defence
2015-11-27, sal P216, NPQ-huset, Svante Arrhenius väg 20 A, Stockholm, 13:00 (English)

At the time of the doctoral defense, the following paper was unpublished and had a status as follows: Paper 2: Manuscript.

Available from: 2015-11-05 Created: 2015-10-23 Last updated: 2015-12-15Bibliographically approved
List of papers
1. Simple approach for the preparation of N-15-15(2)-enriched water for nitrogen fixation assessments: evaluation, application and recommendations
Open this publication in new window or tab >>Simple approach for the preparation of N-15-15(2)-enriched water for nitrogen fixation assessments: evaluation, application and recommendations
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2015 (English)In: Frontiers in Microbiology, ISSN 1664-302X, E-ISSN 1664-302X, Vol. 6, 769Article in journal (Refereed) Published
Abstract [en]

Recent findings revealed that the commonly used N-15(2) tracer assay for the determination of dinitrogen (N-2) fixation can underestimate the activity of aquatic N-2-fixing organisms. Therefore, a modification to the method using pre-prepared N-15-15(2)-enriched water was proposed. Here, we present a rigorous assessment and outline a simple procedure for the preparation of N-15-15(2)-enriched water. We recommend to fill sterile-filtered water into serum bottles and to add N-15-15(2) gas to the water in amounts exceeding the standard N-2 solubility, followed by vigorous agitation (vortex mixing >= 5 min). Optionally, water can be degassed at low-pressure (>= 950 mbar) for 10 mm prior to the N-15-15(2) gas addition to indirectly enhance the N-15-15(2) concentration. This preparation of N-15-15(2)-enriched water can be done within 1 h using standard laboratory equipment. The final N-15-atom% excess was 5% after replacing 2-5% of the incubation volume with N-15-15(2)-enriched water. Notably, the addition of N-15-15(2)-enriched water can alter levels of trace elements in the incubation water due to the contact of N-15-15(2)-enriched water with glass, plastic and rubber ware. In our tests, levels of trace elements (Fe, P, Mn, Mo, Cu, Zn) increased by up to 0.1 nmol L-1 in the final incubation volume, which may bias rate measurements in regions where N-2 fixation is limited by trace elements. For these regions, we tested an alternative way to enrich water with N-15-15(2). The N-15-15(2) was injected as a bubble directly to the incubation water, followed by gentle shaking. Immediately thereafter, the bubble was replaced with water to stop the N-15-15(2) equilibration. This approach achieved a N-15-atom% excess of 6.6 +/- 1.7% when adding 2 mL N-15-15(2) per liter of incubation water. The herein presented methodological tests offer guidelines for the N-15(2) tracer assay and thus, are crucial to circumvent methodological draw-backs for future N-2 fixation assessments.

N-2 fixation, cyanobacteria, gas-liquid solution, N-15(2) gas, gas solubility, iron, phosphorus, Nodularia spumigena
National Category
Biological Sciences
Research subject
Marine Ecology
urn:nbn:se:su:diva-120192 (URN)10.3389/fmicb.2015.00769 (DOI)000359484100002 ()
Available from: 2015-09-04 Created: 2015-09-02 Last updated: 2017-12-04Bibliographically approved
2. Cell-specific N2- and carbon fixation of cyanobacteria in a temperate marine system (Baltic Sea)
Open this publication in new window or tab >>Cell-specific N2- and carbon fixation of cyanobacteria in a temperate marine system (Baltic Sea)
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(English)Manuscript (preprint) (Other academic)
National Category
Biological Sciences
Research subject
Marine Ecology
urn:nbn:se:su:diva-122081 (URN)
Available from: 2015-10-23 Created: 2015-10-23 Last updated: 2015-12-15Bibliographically approved
3. N2-fixation, ammonium release and N-transfer to the microbial and classical food web within a plankton community
Open this publication in new window or tab >>N2-fixation, ammonium release and N-transfer to the microbial and classical food web within a plankton community
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2016 (English)In: The ISME Journal, ISSN 1751-7362, E-ISSN 1751-7370, Vol. 10, no 2, 450-459 p.Article in journal (Refereed) Published
Abstract [en]

We investigated the role of N2-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. When incubated with 15N2Aphanizomenon spp. showed a strong 15N-enrichment implying substantial 15N2-fixation. Intriguingly, Aphanizomenon did not assimilate tracers of 15NH4+ from the surrounding water. These findings are in line with model calculations that confirmed a negligible N-source by diffusion-limited NH4+ fluxes to Aphanizomenon colonies at low bulk concentrations (<250 nm) as compared with N2-fixation within colonies. No N2-fixation was detected in autotrophic microorganisms <5 μm, which relied on NH4+uptake from the surrounding water. Aphanizomenon released about 50% of its newly fixed N2 as NH4+. However, NH4+ 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-formingAphanizomenon releases about half of its recently fixed N2 as NH4+, 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, N2-fixing colony-forming cyanobacteria can have profound impact on ecosystem productivity and biogeochemical processes at shorter time scales (hours to days) than previously thought.

National Category
Biological Sciences
Research subject
Marine Ecology
urn:nbn:se:su:diva-122075 (URN)10.1038/ismej.2015.126 (DOI)000368561100015 ()
Available from: 2015-10-23 Created: 2015-10-23 Last updated: 2017-12-01Bibliographically approved
4. Aerobic and anaerobic nitrogen transformation processes in N2-fixing cyanobacterial aggregates
Open this publication in new window or tab >>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
Abstract [en]

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.

Nitrogen cycle /Cyanobacteria/macroaggregates/O2 and N2O microsensors/isotope pairing technique/Oxygen Minimum Zones
National Category
Earth and Related Environmental Sciences Biological Sciences
Research subject
Geochemistry; Marine Ecology
urn:nbn:se:su:diva-115034 (URN)10.1038/ismej.2014.232 (DOI)000354786700016 ()
Baltic Ecosystem Adaptive Management (BEAM)Swedish Research Council Formas
Available from: 2015-03-16 Created: 2015-03-16 Last updated: 2017-12-04Bibliographically approved

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