The toxic effects of pollutants on the mineralization of acetate in subsoil microcosms

Abstract

The mineralization of [14C]acetate or [14C]glucose was studied in aseptic subsoil samples at 10°C. At 1 μg/L and 100 μg/L, the mineralization showed first‐order kinetics preceded by a short lag phase. For the sandy subsurface soils, the half‐lives of acetate or glucose varied from 0.5 until 21 d, whereas a peat subsoil and a sandy surface soil produced half‐lives of 11 and 1.6 h, respectively. The EC50 is the toxicant concentration that causes 50% reduction of the percentage substrate mineralized at a certain incubation time and increases with time. The time‐independent toxicant concentration IC50 gives 50% inhibition of the mineralization rate and is equal to the EC50 at short incubation times. The IC50 of pentachlorophenol varied between 0.1 and 880 mg/kg subsoil. The aerobic mineralization of acetate in surface sand showed an IC50 of 28 mg pentachlorophenol/kg. Hydrochloric acid showed IC50 values ranging from pH 6.2 to pH 2.8. For chlorite and cadmium, the IC50 values were 34 and 59 mg/kg, respectively. When soils were incubated with 100 mg acetate/L, exponential growth kinetics were observed. Under these conditions the inhibitory effect of pentachlorophenol was relatively small because the resistant part of the acetate‐mineralizing microflora could grow and replace the inhibited part. The kinetics show why toxicity tests using high substrate concentrations are less sensitive than tests using more natural low substrate concentrations. The model predicts that slow‐growing microorganisms, which are vital to the functioning of the ecosystem, would recover only very slowly from the toxic effects of environmental pollution.

Toxic Effects of Pollutants on the Mineralization of 4-Chlorophenol and Benzoate in Methanogenic River Sediment

“…For comparison of the data, however, we used the calculated IC50 and IC10, the toxicant concentrations inhibiting the mineralization rate for 50 or 1070, respectively. In contrast with the EC50 and EC10, the IC50 and IClO do not become higher at prolonged incubation times [5]. The 4CP mineralization is slightly less sensitive than the benzoate mineralization.…”

“…All sediment samples were taken with a core sampler at the same location (Gorinchem, The Netherlands) in a har bor in the estuary of the Rhine River eastern longitude 4'57 50", northern latitude 5 1'49'46'' Introduction of oxy gen to the sediment was minimized The core sampler consisted of a plastic sample tube with a metal jacket that was pushed into the sediment The upper part of the jacket was then closed so that the sample could be pulled out Both sides of the sample tube were closed, and the sample was transported to an anaerobic glove box immediately The used anaerobic techniques were described previously [25] Depth of sampling was at 0 5 to 1 m below the sediment-water interface The sediment was composed primarily of methanogenic mud, sample properties are presented in Table 1 Methods of sediment analysis were described previously [5] Fresh sediment samples were used in all experiments previously [5] Preparation of microcosms…”

“…Bottles were heated, acidified with H2SO4, and flushed with nitrogen gas to trap formed I4CO2 in scintillation vials with absorbent (Carbosorb" from Packard Instrument Co , Downers Grove, IL) [5,25] The extraction efficiency of car bon dioxide from the sediment slurry was go%, which was measured by extraction of NaHi4C03 1251…”

“…A nonlinear least-squares method was used to determine the best fitting curve through the data For the mineralization experiments both the carbon dioxide and the substrate curve were fitted with a first-order kinetic model described by van Beelen et a1 [5] The tomcant concentrations that give a 10 or 50% decrease in the amount of I4CO2 formed after a specific incubation time are called the EClO and EC50, re spectively They were estimated by fitting a logistic doseresponse curve through the data [5] The effect of a pollutant on the mineralization of a substrate seems to decrease at long incubation times because the uninhibited part of the microflora continues to degrade the substrate This occurs when the incubation time is much longer than the substrate half-life Time-independent inhibiting concentrations (IC50 and IC10) were calculated because the effect concentrations (ECSO and EClO) are dependent on both the incubation time of the toxicity experiment and the half life of the substrate mineralization A mathematical der ivation and formulas are described in detail by van Beelen et a1 [5] The IClO is defined as the toxicant concentration that causes a 10% decrease of the mineralization rate To determine the half-life of the substrate mineralization, toxic ity experiments were accompanied by a mineralization-rate experiment from the same sediment sample Because of the differences in substrate recoveries, only the C02 data were used to calculate inhibiting concentrations Because measurements from duplicate bottles were performed in all experiments, it is possible to consider one complete data set as consisting of two separate experiments This method was used to calculate the standard deviation by a method described previously [ 5 ]…”

“…For this study the anaerobic mineralization of low concentrations of benzoic acid and 4-chlorophenol (4CP) was monitored under environmentally relevant conditions. It was shown previously that the use of high substrate concentrations in microbial toxicity tests can lead to a large underestimation of the sensitivity of the natural process [ 5 ] .…”