Active flagellates grazing on prokaryotes in high salinity waters of a solar saltern

Heterotrophic nanoflagellates (HNF) are known to be active grazers of prokaryotes and frequently major agents of prokaryotic mortality in various marine environments. However, studies on HNF occurrences and activities in high salinity (> 300‰) waters have shown contradictory results. Further, measurements of active HNF grazing on prokaryotes have not been reported in high salinity waters. Interestingly, in high salinity waters of a solar saltern located on the west coast of Korea, the flagellate grazing rates determined by uptake of fluorescently labeled prokaryotes (FLP) ranged from 1.4 to 13.0 x 10⁸ cells l^-1 h^-1, turning over prokaryotes in 9 to 48 d. The occurrences of active grazing in high salinity waters were further independently confirmed through an FLP disappearance approach. Prokaryotes in high salinity waters seemed to be strongly controlled by flagellates grazing in summer in the saltern. Naturally, high occurrences of flagellates (7 to 28 x 10⁶ HNF l^-1) were observed in high salinity waters. The flagellates were 5 to 12 μm in length, devoid of autofluorescence and actively ingested prokaryotes. In cultures maintained in the dark for 4 mo after field samples were diluted to contain a single HNF per culture tube, active flagellate growth continued to be observed. It seems that heterotrophic and extremely halophilic eukaryotic grazers actively ingest prokaryotes in high salinity waters.