Size-selective predation on groundwater bacteria by nanoflagellates in an organic-contaminated aquifer

Abstract

Time series incubations were conducted to provide estimates for the size selectivities and rates of protistan grazing that may be occurring in a sandy, contaminated aquifer. The experiments involved four size classes of fluorescently labeled groundwater bacteria (FLB) and 2- to 3-microns-long nanoflagellates, primarily Spumella guttula (Ehrenberg) Kent, that were isolated from contaminated aquifer sediments (Cape Cod, Mass.). The greatest uptake and clearance rates (0.77 bacteria.flagellate-1.h-1 and 1.4 nl.flagellate-1.h-1, respectively) were observed for 0.8- to 1.5-microns-long FLB (0.21-microns3 average cell volume), which represent the fastest growing bacteria within the pore fluids of the contaminated aquifer sediments. The 19:1 to 67:1 volume ratios of nanoflagellate predators to preferred bacterial prey were in the lower end of the range commonly reported for other aquatic habitats. The grazing data suggest that the aquifer nanoflagellates can consume as much as 12 to 74% of the unattached bacterial community in 1 day and are likely to have a substantive effect upon bacterial degradation of organic groundwater contaminants.

FIG. 1

Uptake of FLB by 2- to 3-μm aquifer nanoflagellates during the grazing experiments. Error bars represent standard errors of the mean and total experimental variability. Coefficients of determination (r² values) for linear regressions fitted to 0- to 4-h data were 0.80 (for the 0.1- to 0.5-μm size class), 0.87 (0.5 to 0.8 μm), 0.98 (0.8 to 1.5 μm), and 0.93 (>1.5 μm). The inset shows clearance rates for each size class of FLB based on uptake rates observed during the first 4 h of the grazing experiments. prot, protist.

FIG. 2

Cells of S. guttula from liquid cultures from the MMR aquifer. Micrographs b to f are still images of live cells extracted from digital video clips. (a) Transmission electron micrograph of whole-mount shadow-cast preparation, showing the long flagellum (LF) with flagellar hairs and the short naked flagellum (SF). Scale bar, 2.5 μm. (b) Cell swimming to the rapid beat of the long flagellum; scale bar, 10 μm. (c) Cell which has just become detached from sediment particles (not visible); attachment was achieved by means of a thin posterior protoplasmic filament (arrow). Scale bar, 10 μm. (d) Cell attached to sediment particles. The attachment filament (not visible) arises from the pointed cell posterior (arrow). Scale bar, 10 μm. (e and f) Sequential frames of cell with actively beating long flagellum (arrow). Numerous food bacteria are visible in the background. Scale bar, 10 μm.