| 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. |
- Amato, Alberto, et al.
(författare)
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Grazer-induced transcriptomic and metabolomic response of the chain-forming diatom Skeletonema marinoi
- 2018
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Ingår i: ISME Journal. - : Springer Science and Business Media LLC. - 1751-7362 .- 1751-7370. ; 12, s. 1594-1604
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Tidskriftsartikel (refereegranskat)abstract
- Diatoms and copepods are main actors in marine food webs. The prey-predator interactions between them affect bloom dynamics, shape marine ecosystems and impact the energy transfer to higher trophic levels. Recently it has been demonstrated that the presence of grazers may affect the diatom prey beyond the direct effect of grazing. Here, we investigated the response of the chain-forming centric diatom Skeletonema marinoi to grazer cues, including changes in morphology, gene expression and metabolic profile. S. marinoi cells were incubated with Calanus finmarchicus or with Centropages typicus and in both cases responded by reducing the chain length, whereas changes in gene expression indicated an activation of stress response, changes in the lipid and nitrogen metabolism, in cell cycle regulation and in frustule formation. Transcripts linked to G protein-coupled receptors and to nitric oxide synthesis were differentially expressed suggesting involvement of these signalling transduction pathways in the response. Downregulation of a lipoxygenase in the transcriptomic data and of its products in the metabolomic data also indicate an involvement of oxylipins. Our data contribute to a better understanding of the gene function in diatoms, providing information on the nature of genes implicated in the interaction with grazers, a crucial process in marine ecosystems.
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| 3. |
- Bergkvist, Johanna, 1980, et al.
(författare)
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Grazer-induced chain length plasticity reduces grazing risk in a marine diatom
- 2012
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Ingår i: Limnology and Oceanography. - : Wiley. - 0024-3590. ; 57:1, s. 318-324
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Tidskriftsartikel (refereegranskat)abstract
- We show that Skeletonema marinoi suppresses chain formation in response to copepod cues. The presence of three different copepod species (Acartia tonsa, Centropages hamatus, or Temora longicornis) significantly reduced chain length. Furthermore, chain length was significantly reduced when S. marinoi was exposed to chemical cues from caged A. tonsa without physical contact with the responding cells. The reductions in chain length significantly reduced copepod grazing; grazing rates on chains (four cells or more) were several times higher compared to that of single cells. This suggests that chain length plasticity is a means for S. marinoi to reduce copepod grazing. In contrast, chain length was not suppressed in cultures exposed to the microzooplankton grazer Gyrodinium dominans. Size-selective predation may have played a key role in the evolution of chain formation and chain length plasticity in diatoms.
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| 4. |
- Bergkvist, Johanna, 1980
(författare)
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Grazer-induced responses in marine phytoplankton
- 2012
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Phytoplankton are the most important primary producers in the world’s oceans, yet little is known of how they avoid the constant threats from the pelagic grazers. From land it is well known that plants defend themselves by having thorns or producing compounds unpalatable to grazers. This thesis shows how different species of phytoplankton cope with different threats from pelagic grazers. In paper I the focus was on the dinoflagellate Alexandrium minutum, a producer of the highly potent neurotoxins paralytic shellfish toxins (PST). These toxins can cause mass mortalities in marine mammals and can be deadly to humans, who ingest the toxins via filter feeders that accumulate the toxins from algae in their diet. The suggested purpose of these toxins is to act as a defence against zooplankton grazers, like copepods. It has been shown that A. minutum is able to sense water borne cues from the copepods and respond by an increase in PST production. The results of paper I show that not only is A. minutum able to sense copepods, it can also recognize different species and respond by either increasing PST production or not. Cues from one of the copepod species tested, Centropages typicus, resulted in a more than 20 fold increase in PST, whereas another copepod, Pseudocalanus sp., did not have any effect on the PST content. It seems likely that A. minutum can recognize copepods that have the same distribution area as itself, these species would be the most significant grazers on A. minutum and a defence against them would benefit the alga. This strengthens the suggested role of PST as a grazer deterrent. Another group of substances that have been suggested to act as defence against grazers are the polyunsaturated aldehydes (PUA) produced by some phytoplankton, among others, diatoms. The role of PUA has been debated and various effects have been shown for a number of organisms. PUA has been shown to have negative effect on the reproduction of copepods, but results are contradictory. In paper II we investigate another possible defencive effect of PUA, as a structuring agent on the microbial community of bacteria and viruses. Bacteria can infect diatoms or compete for nutrients, and viral infections can terminate phytoplankton blooms. Compounds that affect these organisms can be of great benefit for the producing organism. The results in paper II show that PUA have no effect on either bacteria or viruses, and further questions the role of these compounds. Perhaps the most interesting findings in this thesis are the ones presented in paper III and IV, where diatoms are shown to use chain length plasticity as a defence against copepods. Previously, different factors such as nutrient uptake and flotation has been suggested to be the driving force behind chain formation, but chain formation as a defence has been suggested before. Here I present further support for this. The diatoms responded to cues from copepods by reducing their chain length, and thereby size, with reduced grazing as a result. Reducing chains to single cells would make the diatom Skeletonema marinoi to small for copepods to handle, while larger species like Thalassiosira rotula would still be large enough to be caught. But by reducing chain length the diatoms also reduce the encounter rate with grazers, and thus larger species like T. rotula can escape grazing. Lower grazing rates were also observed on single cells than longer chains. All diatoms did not respond to grazer cues in the same way, Chaetoceros affinis did not reduce chain length when subjected to copepods. C. affinis has long spines that may act a as a defence which could be the reason why it does not reduce its size. I suggest that chain length plasticity may be an evolutionarily adopted trait in chain forming diatoms and that size-selective predation may have played a key role in the evolution of chain formation and chain length plasticity in diatoms.
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| 5. |
- Bergkvist, Johanna, 1980, et al.
(författare)
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Induction of toxin production in dinoflagellates: the grazer makes a difference
- 2008
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Ingår i: Oecologia. - : Springer Science and Business Media LLC. - 0029-8549 .- 1432-1939. ; 156:1, s. 147-154
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Tidskriftsartikel (refereegranskat)abstract
- The dinoflagellate Alexandrium minutum has previously been shown to produce paralytic shellfish toxins (PST) in response to waterborne cues from the copepod Acartia tonsa. In order to investigate if grazer-induced toxin production is a general or grazer-specific response of A. minutum to calanoid copepods, we exposed two strains of A. minutum to waterborne cues from three other species of calanoid copepods, Acartia clausi, Centropages typicus and Pseudocalanus sp. Both A. minutum strains responded to waterborne cues from Centropages and Acartia with significantly increased cell-specific toxicity. Waterborne cues from Centropages caused the strongest response in the A. minutum cells, with 5 to > 20 times higher toxin concentrations compared to controls. In contrast, neither of the A. minutum strains responded with significantly increased toxicity to waterborne cues from Pseudocalanus. The absolute increase in PST content was proportional to the intrinsic toxicity of the different A. minutum strains that were used. The results show that grazer-induced PST production is a grazer-specific response in A. minutum, and its potential ecological importance will thus depend on the composition of the zooplankton community, as well as the intrinsic toxin-producing properties of the A. minutum population.
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| 6. |
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| 7. |
- 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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| 8. |
- Paul, C., et al.
(författare)
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Diatom Derived Polyunsaturated Aldehydes Do Not Structure the Planktonic Microbial Community in a Mesocosm Study
- 2012
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Ingår i: Marine Drugs. - Basel : MDPI AG. - 1660-3397. ; 10:4, s. 775-792
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Tidskriftsartikel (refereegranskat)abstract
- Several marine and freshwater diatoms produce polyunsaturated aldehydes (PUA) in wound-activated processes. These metabolites are also released by intact diatom cells during algal blooms. Due to their activity in laboratory experiments, PUA are considered as potential mediators of diatom-bacteria interactions. Here, we tested the hypothesis that PUA mediate such processes in a close-to-field mesocosm experiment. Natural plankton communities enriched with Skeletonema marinoi strains that differ in their PUA production, a plankton control, and a plankton control supplemented with PUA at natural and elevated concentrations were observed. We monitored bacterial and viral abundance as well as bacterial community composition and did not observe any influence of PUA on these parameters even at elevated concentrations. We rather detected an alternation of the bacterial diversity over time and differences between the two S. marinoi strains, indicating unique dynamic bacterial communities in these algal blooms. These results suggest that factors other than PUA are of significance for interactions between diatoms and bacteria.
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| 9. |
- Ploug, Helle, et al.
(författare)
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Oxygen diffusion limitation and ammonium production within sinking diatom aggregates under hypoxic and anoxic conditions
- 2015
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Ingår i: Marine Chemistry. - : Elsevier BV. - 0304-4203. ; 176, s. 142-149
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Tidskriftsartikel (refereegranskat)abstract
- Sinking diatom aggregates are important components of vertical elemental fluxes and represent 'hotspots' of microbial-driven remineralization in the water column. A combined analytical approach of microsensors and fluorometry was used to measure oxygen (02) and ammonium (NH4+) concentrations in sinking diatom aggregates as a function of the ambient 02 concentrations. Diffusive fluxes of 02 and NH4+ within and around sinking aggregates were subsequently analyzed using a diffusion reaction model. Diffusion limitation of 02 within the diatom aggregates occurred when ambient 02 concentrations decreased beneath 100 mu M. At ambient 02 concentrations of 20 mu M, the measured flux of 02 to aggregates was equivalent to 33% of fluxes when 02 concentrations were in equilibrium with the atmosphere and approximately 50% of the aggregate volume was anoxic. When the diatom aggregates were maintained under hypoxic conditions, was produced in a ratio of 8.9 mol O-2 consumed:1 mol NH4+ produced which is within the expected range during ammonification. The average POC-specific respiration rate and the average PON-specific NH4+ production rate under hypoxic conditions were 0.065 d(-1) and 0.052 d(-1), respectively. Under anoxic conditions, the NH4+ release was 18% of that measured under hypoxic conditions. Our empirical and modeled data revealed diatom aggregates to be microenvironments of elevated NI-It concentrations ranging from 1 to 8 mu M and therefore to be potential sources of NH4+ in the oxygen minimum zones in the ocean. (C) 2015 Elsevier B.V. All rights reserved.
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| 10. |
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