| 1. |
- Adam, Birgit, et al.
(författare)
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N2-fixation, ammonium release and N-transfer to the microbial and classical food web within a plankton community
- 2016
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Ingår i: The ISME Journal. - : Springer Science and Business Media LLC. - 1751-7362 .- 1751-7370. ; 10:2, s. 450-459
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Tidskriftsartikel (refereegranskat)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 mum, 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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| 2. |
- Arandia-Gorostidi, Nestor, et al.
(författare)
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Efficient carbon and nitrogen transfer from marine diatom aggregates to colonizing bacterial groups
- 2022
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Ingår i: Scientific Reports. - : Springer Science and Business Media LLC. - 2045-2322. ; 12:1
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Tidskriftsartikel (refereegranskat)abstract
- Bacterial degradation of sinking diatom aggregates is key for the availability of organic matter in the deep-ocean. Yet, little is known about the impact of aggregate colonization by different bacterial taxa on organic carbon and nutrient cycling within aggregates. Here, we tracked the carbon (C) and nitrogen (N) transfer from the diatom Leptocylindrus danicus to different environmental bacterial groups using a combination of C-13 and N-15 isotope incubation (incubated for 72 h), CARD-FISH and nanoSIMS single-cell analysis. Pseudoalteromonas bacterial group was the first colonizing diatom-aggregates, succeeded by the Alteromonas group. Within aggregates, diatom-attached bacteria were considerably more enriched in C-13 and N-15 than non-attached bacteria. Isotopic mass balance budget indicates that both groups showed comparable levels of diatom C in their biomass, accounting for 19 +/- 7% and 15 +/- 11%, respectively. In contrast to C, bacteria of the Alteromonas groups showed significantly higher levels of N derived from diatoms (77 +/- 28%) than Pseudoalteromonas (47 +/- 17%), suggesting a competitive advantage for Alteromonas in the N-limiting environments of the deep-sea. Our results imply that bacterial succession within diatom aggregates may largely impact taxa-specific C and N uptake, which may have important consequences for the quantity and quality of organic matter exported to the deep ocean.
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| 3. |
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| 4. |
- Bergkvist, Johanna, 1980, et al.
(författare)
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Turbulence simultaneously stimulates small- and large-scale CO2 sequestration by chain-forming diatoms in the sea
- 2018
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723. ; 9
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Tidskriftsartikel (refereegranskat)abstract
- Chain-forming diatoms are key CO2-fixing organisms in the ocean. Under turbulent conditions they form fast-sinking aggregates that are exported from the upper sunlit ocean to the ocean interior. A decade-old paradigm states that primary production in chain-forming diatoms is stimulated by turbulence. Yet, direct measurements of cell-specific primary production in individual field populations of chain-forming diatoms are poorly documented. Here we measured cell-specific carbon, nitrate and ammonium assimilation in two field populations of chain-forming diatoms (Skeletonema and Chaetoceros) at low-nutrient concentrations under still conditions and turbulent shear using secondary ion mass spectrometry combined with stable isotopic tracers and compared our data with those predicted by mass transfer theory. Turbulent shear significantly increases cell-specific C assimilation compared to still conditions in the cells/chains that also form fast-sinking, aggregates rich in carbon and ammonium. Thus, turbulence simultaneously stimulates small-scale biological CO2 assimilation and large-scale biogeochemical C and N cycles in the ocean.
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| 5. |
- Eichner, Meri, et al.
(författare)
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Carbonate chemistry in the microenvironment within cyanobacterial aggregates under present-day and future pCO(2) levels
- 2022
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Ingår i: Limnology and Oceanography. - : Wiley. - 0024-3590 .- 1939-5590. ; 67:1, s. 203-218
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Tidskriftsartikel (refereegranskat)abstract
- Photosynthesis and respiration cause distinct chemical microenvironments within cyanobacterial aggregates. Here, we used microsensors and a diffusion-reaction model to characterize gradients in carbonate chemistry and investigate how these are affected by ocean acidification in Baltic vs. Pacific aggregates (Nodularia and Dolichospermum vs. Trichodesmium). Microsensor measurements of O-2 and pH were performed under in situ and expected future pCO(2) levels on Nodularia and Dolichospermum aggregates collected in the Baltic Sea. Under in situ conditions, O-2 and pH levels within the aggregates covered ranges of 80-175% air saturation and 7.7-9.4 in dark and light, respectively. Carbon uptake in the light was predicted to reduce HCO3- by 100-150 mu mol L-1 and CO2 by 3-6 mu mol L-1 in the aggregate center compared to outside, inducing strong CO2 depletion (down to 0.5 mu mol L-1 CO2 remaining in the center) even when assuming that HCO3- covered 80-90% of carbon uptake. Under ocean acidification conditions, enhanced CO2 availability allowed for significantly lower activity of carbon concentrating mechanisms, including a reduction of the contribution of HCO3- to carbon uptake by up to a factor of 10. The magnification of proton gradients under elevated pCO(2) that was predicted based on a lower buffer capacity was observed in measurements despite a concurrent decrease in photosynthetic activity. In summary, we provide a quantitative image of the inorganic carbon environment in cyanobacterial aggregates under present-day and expected future conditions, considering both the individual and combined effects of the chemical and biological processes that shape these environments.
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| 6. |
- Eichner, Meri, et al.
(författare)
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Chemical microenvironments and single-cell carbon and nitrogen uptake in field-collected colonies of Trichodesmium under different pCO2
- 2017
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Ingår i: ISME Journal. - : Springer Science and Business Media LLC. - 1751-7362 .- 1751-7370. ; 11
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Tidskriftsartikel (refereegranskat)abstract
- © 2017 The Author(s)Gradients of oxygen (O2) and pH, as well as small-scale fluxes of carbon (C), nitrogen (N) and O2 were investigated under different partial pressures of carbon dioxide (pCO2) in field-collected colonies of the marine dinitrogen (N2)-fixing cyanobacterium Trichodesmium. Microsensor measurements indicated that cells within colonies experienced large fluctuations in O2, pH and CO2 concentrations over a day–night cycle. O2 concentrations varied with light intensity and time of day, yet colonies exposed to light were supersaturated with O2 (up to ~200%) throughout the light period and anoxia was not detected. Alternating between light and dark conditions caused a variation in pH levels by on average 0.5 units (equivalent to 15 nmol l-1 proton concentration). Single-cell analyses of C and N assimilation using secondary ion mass spectrometry (SIMS; large geometry SIMS and nanoscale SIMS) revealed high variability in metabolic activity of single cells and trichomes of Trichodesmium, and indicated transfer of C and N to colony-associated non-photosynthetic bacteria. Neither O2 fluxes nor C fixation by Trichodesmium were significantly influenced by short-term incubations under different pCO2 levels, whereas N2 fixation increased with increasing pCO2. The large range of metabolic rates observed at the single-cell level may reflect a response by colony-forming microbial populations to highly variable microenvironments.The ISME Journal advance online publication, 11 April 2017; doi:10.1038/ismej.2017.15.
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| 7. |
- Eichner, Meri, et al.
(författare)
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N2 fixation in free-floating filaments of Trichodesmium is higher than in transiently suboxic colony microenvironments
- 2019
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Ingår i: New Phytologist. - : Wiley. - 1469-8137 .- 0028-646X. ; 222:2, s. 852-863
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Tidskriftsartikel (refereegranskat)abstract
- To understand the role of micrometer‐scale oxygen (O2) gradients in facilitating dinitrogen (N2) fixation, we characterized O2 dynamics in the microenvironment around free‐floating trichomes and colonies of Trichodesmium erythraeum IMS101. Diurnal and spatial variability in O2 concentrations in the bulk medium, within colonies, along trichomes and within single cells were determined using O2 optodes, microsensors and model calculations. Carbon (C) and N2 fixation as well as O2 evolution and uptake under different O2 concentrations were analyzed by stable isotope incubations and membrane inlet mass spectrometry. We observed a pronounced diel rhythm in O2 fluxes, with net O2 evolution restricted to short periods in the morning and evening, and net O2 uptake driven by dark respiration and light‐dependent O2 uptake during the major part of the light period. Remarkably, colonies showed lower N2 fixation and C fixation rates than free‐floating trichomes despite the long period of O2 undersaturation in the colony microenvironment. Model calculations demonstrate that low permeability of the cell wall in combination with metabolic heterogeneity between single cells allows for anoxic intracellular conditions in colonies but also free‐floating trichomes of Trichodesmium. Therefore, whereas colony formation must have benefits for Trichodesmium, it does not favor N2 fixation.
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| 8. |
- Farnelid, Hanna, 1983-, et al.
(författare)
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Diverse diazotrophs are present on sinking particles in the North Pacific Subtropical Gyre
- 2019
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Ingår i: The ISME Journal. - : Nature Publishing Group. - 1751-7362 .- 1751-7370. ; 13:1, s. 170-182
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Tidskriftsartikel (refereegranskat)abstract
- Sinking particles transport carbon and nutrients from the surface ocean into the deep sea and are considered hot spots for bacterial diversity and activity. In the oligotrophic oceans, nitrogen (N-2)-fixing organisms (diazotrophs) are an important source of new N but the extent to which these organisms are present and exported on sinking particles is not well known. Sinking particles were collected every 6 h over a 2-day period using net traps deployed at 150 m in the North Pacific Subtropical Gyre. The bacterial community and composition of diazotrophs associated with individual and bulk sinking particles was assessed using 16S rRNA and nifH gene amplicon sequencing. The bacterial community composition in bulk particles remained remarkably consistent throughout time and space while large variations of individually picked particles were observed. This difference suggests that unique biogeochemical conditions within individual particles may offer distinct ecological niches for specialized bacterial taxa. Compared to surrounding seawater, particle samples were enriched in different size classes of globally significant N-2-fixing cyanobacteria including Trichodesmium, symbionts of diatoms, and the unicellular cyanobacteria Crocosphaera and UCYN-A. The particles also contained nifH gene sequences of diverse non-cyanobacterial diazotrophs suggesting that particles could be loci for N-2 fixation by heterotrophic bacteria. The results demonstrate that diverse diazotrophs were present on particles and that new N may thereby be directly exported from surface waters on sinking particles.
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| 9. |
- Hansen, Helle Ploug, et al.
(författare)
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Kobling mellem teori og praksis
- 2013
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Ingår i: Læring i og af klinisk praksis. - 9788717042421 ; , s. 136-146
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Bokkapitel (refereegranskat)abstract
- Abstract in Danish Kapitlet sætter fokus på kobling mellem teori og praksis. Vi argumenterer for, at teoribevidsthed er nødvendig for studerende og sundhedsprofessionelle, her sygeplejersker, for at kunne reflektere, forstå, håndtere og udvikle handlinger, interaktioner og sygepleje i klinisk praksis. Først redegør vi kort for forståelser af teori og praksis. Dernæst præsenterer vi en case fra klinisk praksis. Vi analyserer casen ved hjælp af en argumentationsmodel, her Toulmins argumentationsmodel. Den kan betragtes som et redskab til at styrke sygeplejestuderendes og vejlederes viden om og færdigheder i at kunne reflektere over og begrunde sin sygepleje. Vi argumenterer for, at en manglende teoribevidsthed kan fratage patienten sin stemme i mødet med sundhedsprofessionelle, og at patienten dermed risikerer at blive ”fængslet” i en magtrelation; samtidigt risikerer de sundhedsprofessionelle at ”fængsle sig selv”, de studerende og hinanden i sygeplejerskekollektivets forforståelser.
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| 10. |
- Isabell, Klawonn, et al.
(författare)
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Aerobic and anaerobic nitrogen transformation processes in N-2-fixing cyanobacterial aggregates
- 2015
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Ingår i: Isme Journal. - : Springer Science and Business Media LLC. - 1751-7362 .- 1751-7370. ; 9:6, s. 1456-1466
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Tidskriftsartikel (refereegranskat)abstract
- 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. N-15-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 (>= 1 mm) even when suspended in fully oxygenated waters, whereas anoxia in small aggregates (<1 to >= 0.1 mm) may only arise in lowoxygenated waters (<= 25 mu M). 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 mu M), 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.
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