Comparison of flow cytometric and epifluorescent counting methods for marine heterotrophic bacteria

Abstract

          Two direct heterotrophic bacterioplankton counting methods, epifluorescence microscopy (EM) and flow cytometry (FCM) were compared using samples collected in two geographically different oceanic regions, the Adriatic Sea and the English Channel. A statistically significant correlation was found between results obtained by these two methods for samples collected in the Adriatic Sea (r =0.61, n =919, P <0.001) and in the English Channel (r =0.64, n =33, P <0.001). Samples from the Adriatic Sea showed on average 1.16 times higher values obtained by flow cytometry than values estimated by epifluorescence microscopy, while samples from the English Channel showed on average a 0.74 ratio between flow cytometry and epifluorescence microscopy counts. The overall coefficient of variation for epifluorescence microscopy data for samples from the Adriatic Sea and the English Channel was 15.91% and 12.89%, respectively. The flow cytometry method had lower overall coefficient of variation value; for samples collected in the Adriatic Sea it was 3.64%, while for samples collected in the English Channel it was 1.88%