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| Titel |
Intriguing diversity among diazotrophic picoplankton along a Mediterranean transect: a dominance of rhizobia |
| VerfasserIn |
M. Moal, H. Collin, I. C. Biegala |
| Medientyp |
Artikel
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| Sprache |
Englisch
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| ISSN |
1726-4170
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| Digitales Dokument |
URL |
| Erschienen |
In: Biogeosciences ; 8, no. 3 ; Nr. 8, no. 3 (2011-03-29), S.827-840 |
| Datensatznummer |
250005581
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| Publikation (Nr.) |
 copernicus.org/bg-8-827-2011.pdf |
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| Zusammenfassung |
| The Mediterranean Sea is one of the most oligotrophic marine areas on earth
where nitrogen fixation has formally believed to play an important role in
carbon and nitrogen fluxes. Although this view is under debate, the
diazotrophs responsible for this activity have still not been investigated
in the open sea. In this study, we characterised the surface distribution
and species richness of unicellular and filamentous diazotrophs across the
Mediterranean Sea by combining microscopic counts with size fractionated in
situ hybridization (TSA-FISH), and 16S rDNA and nifH genes phylogenies. These
genetic analyses were possible owing to the development of a new PCR
protocol adapted to scarce microorganisms that can detect as few as 1 cell ml−1 in cultures.
Low concentrations of diazotrophic cyanobacteria were detected
and this community was dominated at 99.9% by picoplankton hybridized to
the Nitro821 probe, specific for unicellular diazotrophic cyanobacteria
(UCYN). Among filamentous cyanobacteria only 0.02 filament ml−1 of
Richelia were detected in the eastern basin, while small (0.7–1.5 μm) and large
(2.5–3.2 μm) Nitro821-targeted cells were recovered at all stations
with a mean concentration of 3.5 cell ml−1. The affiliation of the
small Nitro821-targeted cells to UCYN-A was confirmed by 16S and nifH
phylogenies in the western Mediterranean Sea. In the central and the eastern
Mediterranean Sea no 16S rDNA and nifH sequence from UCYN was obtained as cells
concentration were close to, or below PCR detection limit. Bradyrhizobium sequences
dominated nifH clone libraries from picoplanktonic size fractions. A few
sequences of γ-proteobacteria were also detected in the central
Mediterranean Sea. While low phosphate and iron concentrations could explain
the absence of Trichodesmium sp., the factors that prevent the development of UCYN-B and
C remain unknown. We also propose that the dominating picoplankters probably
developed specific strategies, such as associations with protists or
particles, and/or photosynthetic activity, to acquire carbon for sustaining
diazotrophy. |
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