Flow cytometric enumeration of DNA-stained oceanic planktonic protists

Zubkov, M.V., Burkill, P.H. and Topping, J.N. (2007) Flow cytometric enumeration of DNA-stained oceanic planktonic protists. Journal of Plankton Research, 29 (1), 79-86. (doi:10.1093/plankt/fbl059).

Abstract

The aim of this study was to test the practicality of enumerating fixed, DNA-stained heterotrophic protists (H) and phototrophic protists (P) in contrasting regions of the Atlantic Ocean. Oceanic protists were enumerated using a standard flow cytometer (FACSort, BD) at an enhanced flow rate of up to 1.0 mL min–1 to increase numbers of counted cells. The enumeration error of protists decreased hyperbolically from 30–40 to < 5% corresponding to the number (<100 to > 2000) of enumerated cells. H and P were discriminated using the extra red chlorophyll-derived plastidic fluorescence of the latter. The relationship between counts of stained and unstained fixed and unfixed P was statistically close to 1:1, confirming the accuracy of stained protist counting by flow cytometry and adequate discrimination of P from H cells. The estimated average abundance of H in the surface mixed layer of the southern and northern oligotrophic gyres was remarkably similar, with 400 ± 140 and 450 ± 60 cells mL–1, respectively, adding further evidence to the suggestion that these regions are in steady state. In agreement with earlier studies in more productive aquatic environments, a significant correlation (correlation coefficient 0.84, P < 0.0001) was found between the H and the total bacterioplankton numbers, with an average ratio of 1300 prokaryotes to 1 H cell, suggesting a relatively constant trophic interaction between these two groups. This study demonstrates that flow cytometric enumeration of protists is 100 times faster compared with microscopy and, thus, represents a major improvement for quantifying protists in ocean waters, including oligotrophic gyres.

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