Bacterivory rate estimates and fraction of active bacterivores in natural protist assemblages from aquatic systems

Abstract

Unlike the fraction of active bacterioplankton, the fraction of active bacterivores (i.e., those involved in grazing) during a specified time period has not been studied yet. Fractions of protists actively involved in bacterivory were estimated assuming that the distributions of bacteria and fluorescently labeled bacteria (FLB) ingested by protists follow Poisson distributions. Estimates were compared with experimental data obtained from FLB uptake experiments. The percentages of protists with ingested FLB (experimental) and the estimates obtained from Poisson distributions were similar for both flagellates and ciliates. Thus, the fraction of protists actively grazing on natural bacteria during a given time period could be estimated. The fraction of protists with ingested bacteria depends on the incubation time and reaches a saturating value. Aquatic systems with very different characteristics were analyzed; estimates of the fraction of protists actively grazing on bacteria ranged from 7 to 100% in the studied samples. Some nanoflagellates appeared to be grazing on specific bacterial sizes. Evidence indicated that there was no discrimination for or against bacterial surrogates (i.e., FLB); also, bacteria were randomly encountered by bacterivorous protists during these short-term uptake experiments. These analyses made it possible to estimate the ingestion rates from FLB uptake experiments by counting the number of flagellates containing ingested FLB. These results represent the first reported estimates of active bacterivores in natural aquatic systems; also, a proposed protocol for estimating in situ ingestion rates by protists represents a significant improvement and simplification to the current protocol and avoids the tedious work of counting the number of ingested FLB per protist.

FIG. 1

Comparison of the percentage of protists, both flagellates (A) and ciliates (B), with ingested FLB obtained experimentally and estimated from Poisson distributions (equation 2 [see Materials and Methods]). Data are from different aquatic systems: Sapelo Island (□), Butron River (▵), Salvaje Beach (◊), and off the Oregon coast (×). Lines represent the 1:1 slope.

FIG. 2

Ten representative examples of the fraction of active bacterivorous nanoflagellates versus incubation time. Saturating curves fitting the plotted data are also shown. Samples of experiments were from the Butron River (Spain) (panels A to D) and from Sapelo Island (panels E to J). Results presented in panels G and I were from samples inoculated with <0.6-μm-diameter FLB, and panels H and J represent the corresponding experiments with FLB made from the whole natural bacterial community. All other experiments were performed with FLB prepared from the whole natural bacterial population.

FIG. 3

Comparison of bacterivory rates obtained from the standard protocol (according to the method of Sherr et al. [36]) and from estimates from equation 3 (proposed protocol). Data are from flagellates (A) and ciliates (B) and from different aquatic systems as follows: Sapelo Island (□), Butron River (▵), Salvaje Beach (◊), and off the Oregon coast (×). Lines represent the 1:1 slope. Bacterivory rates are expressed in bacteria protist⁻¹ minute⁻¹.