Abstract
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 15N2, Aphanizomenon 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-forming Aphanizomenon 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.
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Acknowledgements
We thank the staff at Stockholm University’s Baltic Sea Centre for their hospitality and support during this study at the Askö field station. We also thank Marcel Günter for his assistance in the field and during experiments, Gabriele Klockgether and Hannah Marchant for assistance with the MS analysis and Ragnar Elmgren for constructive comments to the manuscript. The Nordsim ion microprobe facility is operated under an agreement between the research funding agencies of Denmark, Iceland, Norway and Sweden, the Geological Survey of Finland and the Swedish Museum of Natural History. This work was supported by The Swedish Research Council for Environment, Agricultural Sciences and Spatial Planning, FORMAS (Grant no. 215-2009-813 to Ragnar Elmgren, JW and HP, and Grant no. 215-2010-779 to HP), the Max Planck Society (MPG), by the Baltic Ecosystem Adaptive Management programme (http://www.su.se/beam) to HP, the University of Stockholm, the University of Gothenburg and the Swedish Research Council (VR, Dnr: 621-2011-4406 to HP).
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Adam, B., Klawonn, I., Svedén, J. et al. N2-fixation, ammonium release and N-transfer to the microbial and classical food web within a plankton community. ISME J 10, 450–459 (2016). https://doi.org/10.1038/ismej.2015.126
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DOI: https://doi.org/10.1038/ismej.2015.126
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