Interactions between a polycyclic aromatic hydrocarbon mixture and the microbial communities in a natural freshwater sediment.

The toxicity of a polycyclic aromatic hydrocarbon (PAH) mixture was assessed on the indigenous microbial communities of a natural freshwater sediment. The fate and effects of the PAH mixture (phenanthrene, fluoranthene and benzo(k)fluoranthene) were studied over 28 days. Bacterial communities were described by bacterial counts (total bacteria and viable bacteria), and by some hydrolytic enzyme activities (beta-glucosidase and leucine-aminopeptidase), PAH concentrations were measured in the overlying waters and in the sediments. No effect of PAH was detected at 30 mg/kg for all bacterial parameters. At 300 mg/kg, the quantity of total bacteria and the proportion of viable bacteria markedly decreased, compared to the control (0 mg PAH/kg). At 300 mg/kg, an increase of the beta-glucosidase activity and a decrease of the leucine-aminopeptidase activity were observed. For all treatments, the benzo(k)fluoranthene concentration in the sediment was stable over 28 days whereas, in the same time, only 3-6% of the initial concentrations of phenanthrene and fluoranthene remained. This study shows that (1) PAH induce perturbations of sediment microbial communities in terms of density and metabolism (but not always as an inhibition), (2) indigenous bacteria of sediments might be used for toxicity assessment of specific organic pollutants, (3) native microorganisms of sediment seem to have a high capacity for PAH degradation, depending on the physico-chemical properties and the bioavailability of the substance encountered.

Assessment of Three Methods for Detection and Quantification of Nitrite-Oxidizing Bacteria and Nitrobacter in Freshwater Sediments (MPN-PCR, MPN-Griess, Immunofluorescence).

> Abstract Nitrification in freshwater, a key process in the nitrogen cycle, is now well known to take place predominantly on suspended particles and in sediment. Nitrobacter is the most commonly isolated nitrite oxidizing bacteria from water environments. Three methods for counting nitrite oxidizing communities (especially Nitrobacter) in sediment were investigated: MPN-Griess, fluorescent antibodies (immunofluorescence), and a more recent molecular method coupling specific DNA amplification by PCR and statistical MPN quantification. After preliminary adjustments of the MPN-PCR technique, the detection level and the yield of each method were determined by inoculating a sediment with a pure Nitrobacter culture. The best recovery yield was obtained with the immunofluorescence technique (21.3%) and the lowest detection level was reached with the MPN-Griess method (10(3) Nitrobacter/g dry weight sediment). The MPN-PCR method resulted in the lowest recovery yields and needs further adaptation to become a reliable and precise tool for investigations of nitrifying bacteria in sediment.

Impact of wastewater treatment plant discharge on enzyme activity in freshwater sediments.

The biological self-purification processes are a central point for the ecological states of rivers. The degradation efficiency of pollutants is mainly due to the microflora and can be detected with enzyme-activity tests. Extracellular-enzyme activity of freshwater sediments was measured, in a microcosm, versus different pollution levels caused by organic wastewater treatment plant (WTP) discharges. Biochemical evaluation of Vmax values for each exoenzyme appears to be a function of the organic matter content of the WTP effluent (Fig. 2). Glucosidase and peptidase reveal a significant negative correlation (respectively, r = 0.99, with P < 0.02; and r = 0.853, with P < 0.08) of Vmax versus DOC concentration (i.e., an inhibition effect). The same relationships were observed with K(m) values, beta-Glucosidase and aminopeptidase activity are well described by the Michaëlis-Menten equation, but linearization with the Lineweaver-Burk equation does not fit with a simple type of inhibition. Two sediments (sand and silt) have been tested, and the differences in the exoenzyme activity of the two sediments after WTP effluent input could be explained by their physicochemical differences. The effects of WTP effluents on a freshwater sediment indicate that, in the current experiments, the microbial potential exoenzyme activity does not increase: this finding implies that, in rivers, the global hydrolytic potential could remain steady down-stream of a discharge point.