Protistan communities in aquifers: a review

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

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.

Effect of flagellates on free-living bacterial abundance in an organically contaminated aquifer

Little is known about the role of protists in the saturated subsurface. Porous media microcosms, containing bacteria and protists, were used to determine whether flagellates from an organically contaminated aquifer could substantively affect the number of free-living bacteria (FLB). When flagellates were present, the 3-40% maximum breakthrough of fluorescently labelled FLB injected into the microcosms was much lower than the 60-130% observed for killed controls. Grazing and clearance rates (3-27 FLB flag-1 h-1 and 12-23 nl flag-1 h-1, respectively) calculated from the data were in the range reported for flagellates in other aqueous environments. The data provide evidence that flagellate bacterivory is an important control on groundwater FLB populations.