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Biogeosciences An interactive open-access journal of the European Geosciences Union
https://doi.org/10.5194/bg-2017-63
© Author(s) 2017. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
10 Mar 2017
Review status
This discussion paper is a preprint. It is a manuscript under review for the journal Biogeosciences (BG).
Distribution and drivers of symbiotic and free-living diazotrophic cyanobacteria in the Western Tropical South Pacific
Marcus Stenegren1, Andrea Caputo1, Carlo Berg2, Sophie Bonnet3,4, and Rachel A. Foster1 1Stockholm University, Department of Ecology, Environment and Plant Sciences. Stockholm, Sweden
2Science for Life Laboratory/Department of Biology and Environmental Science, Linnaeus University, Kalmar, Sweden
3Aix Marseille Université, Centre National de la Recherche Scientifique. Marseille/Noumea, New Caledonia, France
4Mediterranean Institute of Oceanography, Institut de Recherche pour le Dévelopement. Marseille, France
Abstract. The abundance and distribution of cyanobacterial diazotrophs were quantified in two regions (Melanesian archipelago, MA and subtropical gyre, SG) of the Western Tropical South Pacific using nifH qPCR assays. UCYN-A1 and A2 host populations were quantified using 18S rRNA qPCR assays including one newly developed assay. All phylotypes were detected in the upper photic zone (0–50 m), with higher abundances in the MA region. Trichodesmium and UCYN-B dominated, composing 81–100 % of nifH copies detected. Het-1 was the next most abundant, and co-occurred with het-2 and het-3. The two UCYN-A lineages were least abundant (<1.0–1.5 % of total nifH copies) and poorly detected (> 47 %). Abundance of the UCYN-A hosts mirrored their respective symbionts; UCYN-A1 and A2 however were detected while their respective hosts were below detection, suggesting a lower partner fidelity or free-living life history. Pairwise comparisons of abundance and environmental parameters supported two groups: deep (45 m) comprised of UCYN-A and surface (0–15m) comprised of Trichodesmium, het-1 and het-2, while UCYN-B overlapped. Temperature, salinity, and PAR, were positively correlated with the latter abundances except UCYN-A. Similar results were identified in a meta-analysis of 11 external datasets. Combined, our results indicate that conditions favoring the UCYN-A symbiosis differ from those of diatom diazotroph associations and free-living diazotrophs.

Citation: Stenegren, M., Caputo, A., Berg, C., Bonnet, S., and Foster, R. A.: Distribution and drivers of symbiotic and free-living diazotrophic cyanobacteria in the Western Tropical South Pacific, Biogeosciences Discuss., https://doi.org/10.5194/bg-2017-63, in review, 2017.
Marcus Stenegren et al.
Marcus Stenegren et al.
Marcus Stenegren et al.

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Short summary
We successully performed quantitiave PCR ''at sea''. We provided the data within three hours, which was used in decisions on further sampling on site. We designed a new primer and probe set for quantifying the UCYN-A1 host. With it we observed discrepancies between host and symbiont, which contradict previous studies. We observed a clear vertical separation between a deep group (UCYN-A with hosts) and a shallow group (remaining targets), mainly separated by the environmental gradient temp./depth.
We successully performed quantitiave PCR ''at sea''. We provided the data within three hours,...
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