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- Bertilsson, Stefan, et al.
(författare)
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Photochemically induced changes in bioavailable carbon and nitrogen pools in a boreal watershed
- 1999
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Ingår i: Aquatic Microbial Ecology. - Oldendorf : Inter-Research. - 0948-3055 .- 1616-1564. ; 19:1, s. 47-56
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Tidskriftsartikel (refereegranskat)abstract
- In several recent studies, a net stimulation of bacterial growth has been demonstrated after exposing humic surface waters to solar radiation or artificial ultraviolet radiation. This stimulation has been attributed to a photochemical release of bioavailable carbon or nitrogen compounds (ammonium). In a synoptic experiment, we exposed 0.2 mu m filtered water from 12 different habitats in a river system, dominated by allochthonous carbon input, to mild artificial UV radiation. A significant photochemical release of carboxylic acids of low molecular weight occurred. Furthermore, the exposure increased carbon-limited bacterial yield on average by a factor of 1.7. No photochemical production of free ammonium could be detected, which was in accordance with the lack of effects of radiation on bacterial growth yield under nitrogen-limited conditions. We conclude that, in boreal systems dominated by allochthonous carbon input, photochemical production of bioavailable carbon rather than nitrogen compounds is likely to positively influence the total substrate pool available for bacterial utilization.
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| 4. |
- Legrand, Catherine, et al.
(författare)
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Induced phagotrophy in the photosynthetic dinoflagellate Heterocapsa triquetra
- 1998
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Ingår i: Aquatic Microbial Ecology. - : Inter-Research Science Center. - 0948-3055 .- 1616-1564. ; 15:1, s. 65-75
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Tidskriftsartikel (refereegranskat)abstract
- Mixotrophy by the photosynthetic dinoflagellate Heterocapsa triquetra was investigated using fluorescently labelled algae (FLA) (size 1, 3, 6 mu m). Experiments were conducted in nitrogen-and phosphorus-replete/depleted medium under light and dark conditions. Incubations ranged from several hours to several days. The dinoflagellate was capable of phagotrophy when exposed to light and dark periods in nutrient-depleted medium. H. triquetra showed similar ingestion rates in the light (range: 0.1 to 0.4 FLA dino(-1) d(-1)) and in the dark (range: 0.06 to 0.4 FLA dino(-1) d(-1)). The dinoflagellate was able to selectively ingest the different FLA. The cyanobacterium Synechococcus sp, was not ingested, whereas a small round flagellate and the diatom Thalassiosira pseudonana were observed inside the cells. The flagellate was ingested at higher rates than the diatom in both the light and the dark. About 40% of the labelled flagellate was removed from the suspension by H. triquetra in the light and 22% in the dark. The diatom was removed from the suspension at the same rate (27 to 30%) in both light and dark treatments. After 5 d incubation in nutrient-depleted medium and no addition of FLA, the proportion of small H. triquetra cells (<1000 mu m(3)) increased from 50 to over 75%. In the presence of FLA, the distribution of H. triquetra cell volumes showed that the proportion of larger cells (>2000 mu m(3)) increased from 6 to >20% during long incubations in the light and the dark. Since the frequency of observed cells with ingested FLA varied from 3 to 20% for the same period, the proportion of larger cells may be the phagotrophic proportion of the population. However, only 8 (dark) to 12% (light) of the observed H. triquetra cell volume increase can be explained in terms of carbon from the ingestion of fluorescently labelled phytoplankton. We conclude that phagotrophy in H. triquetra may be important in maintaining the population in environments of low nutrient concentration and low light intensity.
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| 5. |
- Legrand, Catherine, et al.
(författare)
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Uptake of high molecular weight dextran by the dinoflagellate Alexandrium catenella
- 1998
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Ingår i: Aquatic Microbial Ecology. - : Inter-Research Science Center. - 0948-3055 .- 1616-1564. ; 16:1, s. 81-86
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Tidskriftsartikel (refereegranskat)abstract
- Carbohydrate macromolecules (dextrans) labeled with fluorescein isothiocyanate (FITC) were taken up by the dinoflagellate Alexandrium catenella at a substrate concentration of 5 mg C l(-1) The FITC-labeled dextrans appeared in what resembled food vacuoles inside the dinoflagellate cells. Between 5 and 50% of the cells contained fluorescent green vacuoles. A. catenella took up dextrans of high molecular weight (2000 kDa) but did not show significant uptake of lower molecular weight (20 kDa) dextrans. The uptake of the 2000 kDa dextran was higher with addition of humic substances to the growth medium and did not seem related to the presence of bacteria. Phagocytosis of fluorescent microspheres (0.36 mu m) by A. catenella was also investigated. Although aggregation of fluorescent microspheres was observed in the sulcal region of the cells, no evidence was found of phagocytosis of bacterial-size prey by A. catenella. These observations show that A. catenella has the capacity to take up high molecular weight organic molecules. perhaps by pinocytosis.
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| 9. |
- Wikner, Johan, 1961-, et al.
(författare)
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Bacterioplankton intra-annual variability: importance of hydrography and competition
- 1999
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Ingår i: Aquatic Microbial Ecology. - Oldendorf : Inter-Research. - 0948-3055 .- 1616-1564. ; 20:3, s. 245-260
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Tidskriftsartikel (refereegranskat)abstract
- Field data from a 1.5 yr intensive study of 1 coastal (0 to 20 m) and 2 offshore stations (0 to 100 m) in the northern Baltic were analysed. Specific interest was paid to the difference in the spatiotemporal variation of bacterioplankton and its controlling factors. Less than 31% of the annual bacterial biomass production (P-b) occurred in the photic zone during the productive season at the offshore stations. This suggested an uncoupling between P-b and phytoplankton carbon fixation, which was further supported by the lack of a significant correlation between these variables in the photic zone. The basin with high allochthonous loading and long residence time showed high P-b relative to autochthonous carbon fixation and low variance of P-b and bacterial abundance (N-b), suggesting an important contribution of terrestrial dissolved organic carbon to the carbon and energy supply. Bacterial per capita growth rate (r(c)) was highest during spring, while P-b was highest during summer at all stations. The seasonal variation in P-b was mainly explained by variation in the r(c), rather than in N-b. A positive correlation of N-b with temperature. and a negative correlation with salinity, suggested that >61% of the seasonal variation in N-b was a consequence of the formation of a stratified photic zone with a higher carrying capacity. Temperature limitation of r(c) only occurred in the stratified photic zone, suggesting that other growth factors were sufficient during this period. A density limitation of the maximum r(c) was observed at all stations during autumn and winter in both depth layers, suggesting competition to be of periodic importance. Bacterioplankton with a low r (intrinsic growth rate) and high K (carrying capacity) strategy dominated when sedimenting particles were a major resource in the aphotic zone, while the opposite strategy dominated during winter at low cell densities, when dissolved substrates were the major resource.
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| 10. |
- Wikner, Johan, 1961-, et al.
(författare)
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Differences in consumption of allochthonous DOC under limnic and estuarine conditions in a watershed
- 1999
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Ingår i: Aquatic Microbial Ecology. - Oldendorf : Inter-Research. - 0948-3055 .- 1616-1564. ; 17:3, s. 289-299
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Tidskriftsartikel (refereegranskat)abstract
- A temporal study of the bacterial utilisation of allochthonous dissolved organic carbon (DOC) was performed in a lake and estuary situated in the same boreal watershed. A greater utilisation of allochthonous DOC was observed under estuarine as compared to Limnic conditions. However, the bacterial biomass yield was not significantly different between the environments, indicating a lower growth efficiency on allochthonous DOC by the estuarine bacterioplankton. The largest fraction of the allochthonous DOC utilised (20 %) was observed under estuarine conditions and coincided with the spring flood. The utilisation of allochthonous DOC and bacterial biomass yield in the experimental bottles was Limited by the availability of nitrogen or phosphorus in both environments. Our results showed that the major bacterial transformation of DOC occurred in the estuarine rather than the Limnic environment, due to the shift in physicochemical milieu and bacterial assemblage. However, the estuarine bacteria also respired a greater fraction of the DOC utilised than their limnic counterparts.
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