Flow cytometric enumeration of DNA-stained oceanic planktonic protists

The aim of this study was to test the practicality of enumeratingfixed, DNA-stained heterotrophic protists (H) and phototrophicprotists (P) in contrasting regions of the Atlantic Ocean. Oceanicprotists were enumerated using a standard flow cytometer (FACSort,BD) at an enhanced flow rate of up to 1.0 mL min^–1 toincrease numbers of counted cells. The enumeration error ofprotists decreased hyperbolically from 30–40 to < 5%corresponding to the number (<100 to > 2000) of enumeratedcells. H and P were discriminated using the extra red chlorophyll-derivedplastidic fluorescence of the latter. The relationship betweencounts of stained and unstained fixed and unfixed P was statisticallyclose to 1:1, confirming the accuracy of stained protist countingby flow cytometry and adequate discrimination of P from H cells.The estimated average abundance of H in the surface mixed layerof the southern and northern oligotrophic gyres was remarkablysimilar, with 400 ± 140 and 450 ± 60 cells mL^–1,respectively, adding further evidence to the suggestion thatthese regions are in steady state. In agreement with earlierstudies in more productive aquatic environments, a significantcorrelation (correlation coefficient 0.84, P < 0.0001) wasfound between the H and the total bacterioplankton numbers,with an average ratio of 1300 prokaryotes to 1 H cell, suggestinga relatively constant trophic interaction between these twogroups. This study demonstrates that flow cytometric enumerationof protists is 100 times faster compared with microscopy and,thus, represents a major improvement for quantifying protistsin ocean waters, including oligotrophic gyres.