Analysis of the distribution of ingested bacteria in nanoflagellates and estimation of grazing rates with flow cytometry

The distribution of ingested bacteria in nanoflagellates was assessed to suggest whether or not there are subgroups of grazers with different grazing rates. Several discrete random distributions were compared to the distribution of ingested fluorescently labeled bacteria (FLB) in cultures of Rhynchomonas nasuta and Paraphysomonas vestita. Sample distributions typically fit both the Poisson with extra zeros (Poisson EZ, tested as a truncated Poisson) and negative binomial, but only occasionally fit a Poisson. Both the Poisson EZ and the negative binomial distributions suggest a heterogeneous population composed of subgroups of flagellates with different grazing rates. Although these models provide acceptable mathematical descriptions, their specific biological implications with regard to the number of flagellate subgroups remain to be proven. Based on fit of the distribution of ingested prey to a Poisson EZ, a rapid cytometric method for estimation of grazing rates on FLB is presented. The method uses changes in the probability of grazers not ingesting FLB during short incubations (ca 15 min) to estimate the Poisson parameter and the fraction of extra zeros, from which the average grazing rate is calculated. Grazing rates determined by microscopy and by this cytometry method were similar. Frequency distributions of cytometric histograms of fluorescent microspheres in grazers suggest that both the Poisson EZ and negative binomial models are simplifications of a more complex distribution of grazing rates.