N2-fixation, ammonium release and N-transfer to the microbial and classical food web within a plankton community.
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 (15)N2, Aphanizomenon spp. showed a strong (15)N-enrichment implying substantial (15)N2-fixation. Intriguingly, Aphanizomenon did not assimilate tracers of (15)NH4(+) 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.
Max Planck Institute for Marine Microbiology, Bremen, Germany.
Birgit
B
Adam
Department of Ecology, Environment and Plant Sciences, Stockholm University, Stockholm, Sweden.
Isabell
I
Klawonn
Department of Ecology, Environment and Plant Sciences, Stockholm University, Stockholm, Sweden.
Jennie B
JB
Svedén
Department of Biological and Environmental Sciences, University of Gothenburg, Gothenburg, Sweden.
Johanna
J
Bergkvist
Department of Biological and Environmental Sciences, University of Gothenburg, Gothenburg, Sweden.
Nurun
N
Nahar
Department of Ecology, Environment and Plant Sciences, Stockholm University, Stockholm, Sweden.
Jakob
J
Walve
Max Planck Institute for Marine Microbiology, Bremen, Germany.
Sten
S
Littmann
Swedish Museum of Natural History, Stockholm, Sweden.
Martin J
MJ
Whitehouse
Max Planck Institute for Marine Microbiology, Bremen, Germany.
Gaute
G
Lavik
Max Planck Institute for Marine Microbiology, Bremen, Germany.
Marcel M M
MM
Kuypers
Department of Ecology, Environment and Plant Sciences, Stockholm University, Stockholm, Sweden.|Department of Biological and Environmental Sciences, University of Gothenburg, Gothenburg, Sweden.
Helle
H
Ploug
D064751:Ammonium Compounds / Q000378:metabolism*; D046890:Aphanizomenon / Q000254:growth & development / Q000378:metabolism*; D017377:Diatoms / Q000254:growth & development / Q000378:metabolism*; D017753:Ecosystem; D020387:Food Chain; D052836:Heterotrophic Processes; D009584:Nitrogen / Q000378:metabolism; D009586:Nitrogen Fixation; D010933:Plankton / Q000254:growth & development / Q000378:metabolism*; D012621:Seasons; D012623:Seawater / Q000382:microbiology*
D064751:Ammonium Compounds; D009584:Nitrogen