Study on the function of carbon transport by heterotrophic dinoflagellates

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Diatoms play an important role jn the carbon sequestration to the deep sea, i.e., biological pump. The fate of diatom bloom is largely influenced by the cell sinking, sinking as faecal pellet of diatom grazer, or remineralization in the epipelagic layer. In order to understand the fate, many studies on the macrozooplankton feeding on diatoms has been carried out. On the other hand, biology and ecology of the other diatom feeder, heterotrophic dinoflagellates, are limited, and the role of the dinoflagellates on carbon sequestration is unknown. In this study, we investigated on the diatom grazing by dinoflagellates and the role on carbon sequestration.We found 4 diatom-feeding heterotrophic dinoflagellates, such as Gyrodinium sp. , in the genus of Gyrodinium and Gymnodinium in the subarctic Pacific. Their contribution in microzooplankton assemblage is generally low. , and often absent. The high biomass was observed in the diatom blooming station. The contribution of the heterotrophic din … More oflagellates reached 20% of total microzooplankton in number in the Oyashio region and 60% in the carbon biomass. In the central region of the western subarctic gyre, their contribution was 10% in weight at the non-bloom status, and it increased to 50%(20.1 mgC m^<-3>) during the bloom. These indicate that the dynamics of the heterotrophic dinoflagellates is largely dependent on the diatom density. The maximum ingestion rate observed was 532 mgC m^<-3> d^<-1>.The mathematical simulation showed that the heterotrophic dinoflagellates had potential to graze down all the diatom production during diatom bloom in the western subarctic Gyre. The importance of the grazing on the carbon transport is largely influenced by the initial cell density of the dinoflagellate and the predator density. In the case of the low initial density of the dinoflagellates or high predator density, the biomass of the dinoflagellates does not reach to high enough to impact the diatom bloom within the general duration of the event of 1-4 weeks It is suggested that the selective feeding of copepods on the dinoflagellates decrease the population size when the timing of copepod growth period initiate earlier than the diatom bloom. These results indicate that the diatom, carbon and nutrient dynamics in spring are crucially influenced by the food-web components.This study demonstrate the ecological characteristics of the heterotrophic dinoflagellates and their influence of carbon and biogenic elemental cycling which are little know previously. It is, however, still unclear the roles on the food-web dynamics and biogeochemical cycling due to their large horizontal and temporal variations in their abundance. We need further studies on physiological and biological characteristics of the heterotrophic dinoflagellates in order to develop the ecosystem model including the dinoflagellate and to understand their role in the ecosystem and biogeochemical cycling. Less