Flow cytometric determination of bacterial abundance in lake plankton with the green nucleic acid stain SYTO 13

We have developed a protocol for the cytometric enumeration of bacterioplankton in lakes with the nucleic acid stain SYTO 13; our protocol can be used with any standard bench‐top flow cytometer equipped with an argon laser with excitation at 488 nm. The protocol performs well in the range of bacterial densities from 10 5 to 10 6 ml −1 . Estimates of bacterial abundance using the cytometric technique ranged from 72 to 141% of the epifluorescence estimates with DAPI staining. The average C.V. of replicate cytometric counts was 2.4%, and the average C.V. for cytometric counts in replicate water samples was 6.7%. The precision of cytometric counts was 10‐fold higher than corresponding epifluorescence bacterial counts, and the sample processing time is decreased by 10–15‐fold relative to the epifluorescence technique. This cytometric protocol can be applied to samples from a wide variety of lakes and offers an accurate, precise, and fast alternative to standard epifluorescence bacterial counts.