Donald L. Johnstone

In situ measurement of microbial activity and controls on microbial CO2 production in the unsaturated zone

Carbon dioxide COncentrations were measured at various depths and times in the unsaturated zones of two hydraulically and geochemically contrasting field sites, one in southeastern Washington state, and the other in south central Saskatchewan. In situ CO, production rates were calculated from a mass balance that accounted for diffusive fluxes and partitioning of CO[sub 2] into an advecting aqueous phase. Production rates were compared with (1) microbial abundance and (2) subsurface temperature to determine whether subsurface CO[sub 2] production rates could be expressed as a simple function of these two variables. At the Washington site, subsurface production was successfully expressed as a function of microbial abundance and temperature for a large portion of the year, but not near the end of the growing season. Although subsurface microbes and organic carbon were more abundant at the Saskatchewan site, subsurface CO[sub 2], production rates were generally several orders of magnitude lower than at the Washington site, and no correlation could be established between microbial numbers, temperature, and production rate. The cases where production rates could not be expressed as a function of microbial numbers and temperature suggested conditions in which some other factor, such as nutrient limitations, was controlling.

Bacterial reduction of hexavalent chromium by Enterobacter cloacae strain HO1 grown on sucrose

Chromium(VI) was reduced by Enterobacter cloacae strain HO1 grown with sucrose as a carbon source and nitrate as the initial terminal electron acceptor. Under excess substrate conditions, the Cr(VI) concentration, initially at 5 and 10 mg/l, was reduced to less than 100 mg/l.

Selenium reduction by a denitrifying consortium

A denitrifying bacterial consortium obtained from the Pullman, Washington wastewater treatment facility was enriched under denitrifying conditions and its ability to reduce selenite and selenate was studied. Replicate experiments at two different experimental conditions were performed. All experiments were performed under electron-acceptor limiting conditions, with acetate as the carbon source and nitrate the electron acceptor. In the first set of experiments, selenite was present, whereas, in the second set, selenate was added. A significant lag period of approximately 150 h was necessary before selenite or selenate reduction was observed. During this lag period, nitrate and nitrite use was observed. Once selenite or selenate reduction had started, nitrate and nitrite reduction was concomitant with selenium species reduction. Trace amounts of selenite were detected during the selenate reduction study. Analysis of the data indicates that, once selenium species reduction was induced, the rate of reduction was proportional to the selenium species concentration and to the biomass concentration. Furthermore, at similar biomass and contaminant concentrations, selenite reduction is approximately four times faster than selenate reduction. Copyright 1999 John Wiley & Sons, Inc.

Hydrogeologic parameters affecting vadose‐zone microbial distributions

Abstract The vadose zone and its contaminant‐attenuating processes are physically interposed between surface contamination and groundwater supplies. Given the potential role of microorganisms in mediating vadose‐zone chemical processes, it is vital to understand vadose microbial distributions and factors controlling those distributions. Vadose and shallow saturated zone sediments obtained from cores drilled to approximately 8 m below the surface at two hydrogeologically contrasting sites, named Dalmeny and Washington State University (WSU), were examined for culturable heterotrophic bacteria, total organic carbon (TOC), and sediment texture. Pore‐water elutions were analyzed for dissolved organic carbon, sulfate, and inorganic nitrogen species. Numbers of cultured bacteria (103‐107 g−1) generally decreased with depth at both sites. The TOC decreased uniformly with depth at WSU where soil processes are the sole carbon source; at Dalmeny, where both soil and kerogen carbon are present, TOC was higher and re...

Bacterial Reduction of Chromium

A mixed culture was enriched from surface soil obtained from an eastern United States site highly contaminated with chromate. Growth of the culture was inhibited by a chromium concentration of 12 mg/L. Another mixed culture was enriched from subsurface soil obtained from the Hanford reservation, at the fringe of a chromate plume. The enrichment medium was minimal salts solution augmented with acetate as the carbon source, nitrate as the terminal electron acceptor, and various levels of chromate. This mixed culture exhibited chromate tolerance, but not chromate reduction capability, when growing anaerobically on this medium. However, this culture did exhibit chromate reduction capability when growing anaerobically on TSB. Growth of this culture was not inhibited by a chromium concentration of 12 mg/L. Mixed cultures exhibited decreasing diversity with increasing levels of chromate in the enrichment medium. An in situ bioremediation strategy is suggested for chromate contaminated soil and groundwater.

Equilibrium sorption of Cr6+ by a consortia of denitrifying bacteria

SummaryThe accumulation of metal ions on to biological materials is a promising method for removing these toxic materials from waste or ground-waters. A fractional factorial statistical experimental design was used to ascertain which experimental parameters affect the sorption of Cr6+ by a consortium of denitrifying bacteria using a minimum number of experiments. Analysis of this set of experiments found that the steady state sorption behavior could be described by Langmuir isotherms. Observed variations in the isotherm parameters were due to the imposed changes in the experimental variables. In particular, solution pH and the state of the biomass were found to have the greatest influence upon the amount of chromium that was taken up, while other factors had less significance, but were still important.

Effects of slight variations in nutrient loadings on pore plugging in soil columns

The high nutrient concentrations that would exist near the nutrient injection well during the application of cometabolicin situ bioremediation may lead to the development of significant quantities of biomass at this point in the subsurface. This biomass can decrease the porosity of the soil to such an extent that nutrient injection is no longer possible. In this work, experiments were conducted using a porous media biofilm reactor, operated under constant substrate loading conditions, such that the pressure drop across the reactor was allowed to increase to maintain a constant volumetric flow rate through the reactor. Results suggest that biomass production, and hence biofilm thickness, near the injection feed port is highly sensitive to substrate loading. In addition, these variations in biofilm thickness produce dramatic differences in the pressure drop that is attained across the reactor. Use of the Kozeny-Carman equation can be used to predict that once a critical depth has been exceeded, the pressure drop across the bed will increase exponentially within biofilm depth. This result means that pressure is not a reliable indicator of the onset of pore plugging.

Long-term intracellular chromium partitioning with subsurface bacteria

Subsurface bacteria were used to study the kinetics of chromate uptake and the internal distribution of the chromium that is taken up by these cells using two equilibration periods (1 and 50 days). Cells that were exposed to chromate for 50 days (to simulate in-situ conditions) were able to sequester up to 200% more chromium per unit mass of cells than were cells that were exposed for only 1 day. Chromium distributions showed an increase in chromium sorption by the cell wall, by the membrane and ribosome, and by the soluble fraction after a 50-day equilibration period compared to after 1 day of equilibration. Killed cell controls suggest that active transport of chromate is the method of intracellular accumulation during the 50-day equilibration period.

Comparison of chromium adsorption to starved and fresh subsurface bacterial consortium

The adsorption of chromium (Cr+6) to a denitrifying consortium was investigated for starved and fresh cells under three pH values (pH 6.0, 7.5 and 9.0). Cells starved 50 days adsorbed approximately 10–15% more Cr+6 than fresh (0 day) cells at those three pH conditions.

Chromium distribution in subcellular components between fresh and starved subsurface bacterial consortium

SummaryThe distribution of chromium in subcellular components was examined with a fresh and starved denitrifying consortium by performing Cr+6 equilibration and cell fractionation tests. The cell wall fraction of 50 day starved cells adsorbed approximately 100% more chromium than did the cell wall fraction in fresh cells. The soluble fraction of 50 day old cells showed less affinity for chromium than fresh cells.