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A study of microphytoplankton and cyanobacteria consortia in four oligotrophic regimes in the western part of the North Pacific subtropical gyre and in the warm pool

  • Girault, Mathias
  • GREGORI, Gerald
  • BARANI, Aude
  • Arakawa, Hisayuki
Publication Date
Jan 01, 2016
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The spatial distribution of nanocyanobacteria-like cells and microphytoplankton was investigated along a ~5000 kmtransect 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. Althoughcolonies of Trichodesmium spp. were identified in the Warm Pool, we found that up to 94% of diatoms were associatedwith the individual trichomes identified as Katagnymene spp. In the microphytoplankton assemblages, pennate diatomslacking cyanobionts were embedded in large quantities of mucilaginous matrix and were characterized by a particulardiatom assemblage not seen at other stations. This assemblage was observed in the Warm Pool when numerous dinoflagellateswere present as well. As a consequence, we suggest that this high percentage of small pennate diatomsembedded in mucus is the result of an efficient anti-grazing strategy. When large diatoms such as Rhizosolenia spp. andchain-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 andseems to have been confined to the Transition Zone and the periphery of the Trichodesmium spp. bloom, suggestingthat cyanobacteria species and consortia could be distributed along an oligotrophic gradient.

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