Bacterioplankton community structure: Protists control net production and the proportion of active bacteria in a coastal marine community

Abstract

A series of dialysis experiments was performed to study the relative importance of substrate limitation and grazing in controlling the proportion of active cells of coastal marine bacterioplankton. The grazer community was manipulated by filling dialysis bags with unfiltered water and water serially passed through 150-, 40-, and O.&pm pore-size filters. The total number of bacteria, the number of metabolically active cells, bacterial loss rates, and the abundances of heterotrophic nanoflagellates were measured immediately and at 3 and 6 d. Gross growth rates were similar in all treatments, suggesting that ambient nutrient concentrations set an upper limit to the maximum growth rates, whereas grazing determined the net growth rates and the final number of bacteria. Bacterial loss rates, measured as the disappearance of fluorescently labeled minicells, correlated well with the initial density of heterotrophic nanoflagellates in the different treatments. The number of active cells at the end of the experiments varied widely among treatments and reached 2.0 x lo6 ml-l, or over 55% of the total final density in dialysis bags, with little or no grazing by nanoflagellates. The final proportion of active cells was negatively correlated to both the loss rates and the initial nanoflagellate density, and it was estimated that grazing rates on metabolically active bacteria were four or more times higher than those on inactive bacteria. Heterotrophic nanoflagellates thus seemed to control bacterial density by skimming newly growing cells rather than by cropping the standing stock of bacteria.

Size Selective Feeding in Cyclidium glaucoma (Ciliophora, Scuticociliatida) and Its Effects on Bacterial Community Structure: A Study from a Continuous Cultivation System

“…3, Table 1). In addition, we observed that uptake rates of FLB with a mean size of 0.94 × 0.58 µm (volume = 0.230 µm 3 ) were very similar to ingestion rates of 0.6 µm spherical beads (volume = 0.113 µm 3 , Fig. 7).…”

“…This contradiction is probably explained by a misinterpretation of data derived from using spherical latex beads as food surrogates. For example, a spherical latex bead with a diameter of 0.6 µm corresponds to a volume of 0.113 µm 3 . A filter feeding ciliate will most probably select for the diameter of a spherical prey or for the width of a rod-shaped particle, but not for the prey volume.…”

“…In contrast, bacteria in II-ciliate increased their MCV B up to a maximum value of 0.246 µm 3 . During the first week, bacteria had an average MCV B of 0.089 µm 3 and an average of 0.171 µm 3 from day 8 until the end of the study. About 80% of bacteria in II-control as well as in II-ciliate were between 0.5 and 2.5 µm long (Fig.…”

“…Hourly uptake rates were calculated from 5 to 10 min feeding periods. The mean cell size of FLBs were 0.94 × 0.58 µm resulting in a mean cell volume of 0.224 µm 3 . Total grazing rates (TGR) were calculated as ciliate abundance times hourly uptake rates.…”

“…Until now, most studies in this field focused on the size selective feeding of ciliates and flagellates and its various effects on bacteria. Not only does the uptake of distinct prey size classes change the size distribution of a BC [23,40], but feeding on larger, i.e., more active or dividing, cells affects the productivity of the whole community [2,3,32]. On the one hand, grazing controls the bacterial production rather than the standing stock.…”