Enrichment of specific protozoan populations during in situ bioremediation of uranium-contaminated groundwater

Abstract

The importance of bacteria in the anaerobic bioremediation of groundwater polluted with organic and/or metal contaminants is well recognized and in some instances so well understood that modeling of the in situ metabolic activity of the relevant subsurface microorganisms in response to changes in subsurface geochemistry is feasible. However, a potentially significant factor influencing bacterial growth and activity in the subsurface that has not been adequately addressed is protozoan predation of the microorganisms responsible for bioremediation. In field experiments at a uraniumcontaminated aquifer located in Rifle, CO, USA, acetate amendments initially promoted the growth of metal-reducing Geobacter species, followed by the growth of sulfate reducers, as observed previously. Analysis of 18S rRNA gene sequences revealed a broad diversity of sequences closely related to known bacteriovorous protozoa in the groundwater before the addition of acetate. The bloom of Geobacter species was accompanied by a specific enrichment of sequences most closely related to the ameboid flagellate, Breviata anathema, which at their peak accounted for over 80% of the sequences recovered. The abundance of Geobacter species declined following the rapid emergence of B. anathema. The subsequent growth of sulfate-reducing Peptococcaceae was accompanied by another specific enrichment of protozoa, but with sequences most similar to diplomonadid flagellates from the family Hexamitidae, which accounted for up to 100% of the sequences recovered during this phase of the bioremediation. These results suggest a preypredator response with specific protozoa responding to increased availability of preferred prey bacteria. Thus, quantifying the influence of protozoan predation on the growth, activity and composition of the subsurface bacterial community is essential for predictive modeling of in situ uranium bioremediation strategies.

Evidence of Geobacter-associated phage in a uranium-contaminated aquifer

“…Previous studies have shown that these reduced products are formed biologically by dissimilatory Fe(III) and sulfate-reducing bacteria primarily from the genera Geobacter, Desulfobacter and Desulfosporosinus (Anderson et al, 2003;Holmes et al, 2013b).…”

“…Furthermore, rates of subsurface Geobacter growth were significantly lower than the rates predicted by genome-scale metabolic models (Scheibe et al, 2009;Fang et al, 2011;Lovley et al, 2011;Zhuang et al, 2011). Protozoa grazing on Geobacter species was one possible explanation for this discrepancy (Holmes et al, 2013b), as studies have shown that the specific growth rate of bacteria is higher in the presence of protozoa (Bloem et al, 1988;Verhagen et al, 1995;Strauss and Dodds, 1997;Biagini et al, 1998). In addition to protozoan grazing, it is also likely that phage activity has a substantial impact on bacterial growth in uranium-contaminated aquifers.…”

Molecular Analysis of theIn SituGrowth Rates of Subsurface Geobacter Species

“…Studies at the Rifle site have shown that protozoa become abundant in the groundwater when dissimilatory metal reduction is stimulated by the addition of acetate (64). The increase in protozoan abundance is significant because protozoa can exploit substantial quantities of bacterial biomass production (52), and grazing can significantly stimulate microbial metabolic activity and growth (53,54).…”

Protozoan grazing reduces the current output of microbial fuel cells

“…agilis, H. inflata, and B. anathema were chosen as potential G. sulfurreducens grazers because they are related to protozoa previously detected in Geobacter-enriched groundwater (Holmes et al, 2013) and Heteromita strain DH-1 is a protozoan isolate from those groundwaters (D.E. Holmes et al, manuscript in preparation).…”

“…For example, when the growth of Geobacter species in the subsurface was stimulated with the addition of high concentrations of electron donor in the form of acetate, a bloom of protozoa accompanied increases in Geobacter growth and substantially lowered the accumulation of Geobacter biomass below that expected in the absence of protozoan grazing (Holmes et al, 2013). An anode biofilm also represents a substantial enrichment of Geobacteraceae likely to enhance grazing opportunities for protozoa.…”