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• Planète et Univers/Océan, Atmosphère
  

We report the complete mitochondrial genome sequence of the freshwater diatom Asterionella formosa. The large 61.9 kb circular sequence encodes 34 proteins and 25 tRNAs that are universally conserved in other sequenced diatoms. We fully resolved a unique 24 kb region containing highly conserved repeated sequence units, possibly collocating with an origin of replication.

Nitrogen is essential for life but is often a major limiting nutrient for growth in the ocean. Biological dinitrogen fixation is a major source of new nitrogen to surface waters and promotes marine productivity. Yet the fate of diazotroph-derived nitrogen (DDN) in marine ecosystems has been poorly studied, and its transfer to auto- and heterotrophic plankton has not been measured. Here, we use high-resolution nanometer scale secondary ion mass spectrometry (nanoSIMS) coupled with N-15(2) isotopic labelling and flow cytometry cell sorting to examine the DDN transfer to specific groups of natural phytoplankton and bacteria during three diazotroph blooms dominated by the cyanobacterium Trichodesmium spp. in the South West Pacific. During these experiments, 13%+/- 2% to 48%+/- 5% of the fixed N-15(2) was released into the dissolved pool and 6%+/- 1% to 8%+/- 2% of this DDN was transferred to non-diazotrophic plankton after 48 h. The primary beneficiaries of this DDN were diatoms (45%+/- 4% to 61%+/- 38%) and bacteria (22%+/- 27% to 38%+/- 12%), followed by pico-phytoplankton (3%+/- 1% to 21%+/- 14%). The DDN was quickly converted to non-diazotrophic plankton biomass, in particular that of diatoms, which increased in abundance by a factor of 1.4-15 over the course of the three experiments. The single-cell approach we used enabled quantification of the actual transfer of DDN to specific groups of autotrophic and heterotrophic plankton in the surface ocean, revealing a previously unseen level of complexity in the pathways that occur between N-2 fixation and the eventual export of DDN from the photic zone.

The spatial distribution of nanocyanobacteria-like cells and microphytoplankton was investigated along a ~5000 km transect in the North Western Pacific Subtropical Gyre and the Warm Pool. Three different consortia: Richelia intracellularis/ Calothrix rhizosoleniae, Katagnymene spp. and Trichodesmium spp. and the associated diatoms were studied. Although colonies of Trichodesmium spp. were identified in the Warm Pool, we found that up to 94% of diatoms were associated with the individual trichomes identified as Katagnymene spp. In the microphytoplankton assemblages, pennate diatoms lacking cyanobionts were embedded in large quantities of mucilaginous matrix and were characterized by a particular diatom assemblage not seen at other stations. This assemblage was observed in the Warm Pool when numerous dinoflagellates were present as well. As a consequence, we suggest that this high percentage of small pennate diatoms embedded in mucus is the result of an efficient anti-grazing strategy. When large diatoms such as Rhizosolenia spp. and chain-forming Chaetoceros were outside the mucus matrix, they were mainly associated with cyanobiont R. intracellularis/ C. rhizosoleniae in all the areas covered during the cruise. A nanocyanobacteria-like cell group was also identified and seems to have been confined to the Transition Zone and the periphery of the Trichodesmium spp. bloom, suggesting that cyanobacteria species and consortia could be distributed along an oligotrophic gradient.

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