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

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.