Reductive dechlorination of tetrachloroethene by a stepwise catalysis of different organohalide respiring bacteria and reductive dehalogenases.

The enrichment culture SL2 dechlorinating tetrachloroethene (PCE) to ethene with strong trichloroethene (TCE) accumulation prior to cis-1,2-dichloroethene (cis-DCE) formation was analyzed for the presence of organohalide respiring bacteria and reductive dehalogenase genes (rdhA). Sulfurospirillum-affiliated bacteria were identified to be involved in PCE dechlorination to cis-DCE whereas "Dehalococcoides"-affiliated bacteria mainly dechlorinated cis-DCE to ethene. Two rdhA genes highly similar to tetrachloroethene reductive dehalogenase genes (pceA) of S. multivorans and S. halorespirans were present as well as an rdhA gene very similar to the trichloroethene reductive dehalogenase gene (tceA) of "Dehalococcoides ethenogenes" strain 195. A single strand conformation polymorphism (SSCP) method was developed allowing the simultaneous detection of the three rdhA genes and the estimation of their abundance. SSCP analysis of different SL2 cultures showed that one pceA gene was expressed during PCE dechlorination whereas the second was expressed during TCE dechlorination. The tceA gene was involved in cis-DCE dechlorination to ethene. Analysis of the internal transcribed spacer region between the 16S and 23S rRNA genes revealed two distinct sequences originating from Sulfurospirillum suggesting that two Sulfurospirillum populations were present in SL2. Whether each Sulfurospirillum population was catalyzing a different dechlorination step could however not be elucidated.

Effect of freezing and thawing on microbial activity and glyphosate degradation in two Norwegian soils.

Little research has been done on pesticide dissipation in cold climates and there is a need to focus on the influence of climate on pesticide degradation in soil. Glyphosate, N-(phosphonomethyl)glycine, is a herbicide frequently used for controlling perennial weeds through application after harvest and was used as a model compound for this study. The effect of freeze-thaw activity on the availability of glyphosate in soil, and consequently its mineralization by soil microorganisms, was studied through laboratory incubations of repacked soil cores treated with 14C-labelled glyphosate and subjected to different freeze-thaw treatments. Winter simulation regimes applied were constant thaw (+5 degrees C), constant freezing (-5 degrees C), unstable conditions with short fluctuations (24 h of -5 degrees C followed by 24 h of +5 degrees C), and long duration fluctuations (3 weeks of -5 degrees C followed by 3 weeks of +5 degrees C). Distribution of 14C-glyphosate was followed during the incubation through measurements of the mineralized fraction (14CO2), soil water fraction, KOH extractable fraction, and non-extractable fraction. Microbial parameters used to characterize the soils were estimates of size of microbial biomass, overall microbial activity and microbial diversity. The constant freezing treatment exhibited the lowest amount of glyphosate mineralization. The constant thawed treatment and the treatments with fluctuating temperature exhibited significantly increased mineralization. These results were in accordance with the observed concentration of glyphosate in soil water; the higher the activity, the lower the concentration. The amount of glyphosate extractable with KOH and the resulting non-extractable fraction, however, were not significantly affected by soil type or temperature regime. The glyphosate mineralization pattern was comparable with the overall microbial activity in the soils. Observed different levels of diversity might explain some of the difference in total glyphosate mineralization between soils.

Spatial variability in 14C-herbicide degradation in surface and subsurface soils.

The spatial variability in mineralization of atrazine, isoproturon and metamitron in soil and subsoil samples taken from a 135-ha catchment in north France was studied. Fifty-one samples from the top layer were taken to represent exhaustively the 31 agricultural fields and 21 soil types of the catchment. Sixteen additional samples were collected between depths of 0.7 and 10 m to represent the major geological materials encountered in the vadose zone of the catchment. All these samples were incubated with 14C-labelled atrazine under laboratory conditions at 28 degrees C. Fourteen selected surface samples which exhibited distinctly different behaviour for atrazine dissipation (including sorption and mineralization) were incubated with 14C-isoproturon and 14C-metamitron. Overall soil microbial activity and specific herbicide degradation activities were monitored during the incubations through measurements of total carbon dioxide and 14C-carbon dioxide respectively. At the end of the incubations, extractable and non-extractable (bound) residues remaining in soils were measured. Variability of herbicide dissipation half-life in soil surface samples was lower for atrazine and metamitron (CV < 12%) than for isoproturon (CV = 46%). The main contributor to the isoproturon dissipation variability was the variability of the extractable residues. For the other herbicides, spatial variability was mainly related to the variability of their mineralization. In all cases, herbicide mineralization half-lives showed higher variability than those of dissipation. Sorption or physicochemical soil properties could not explain atrazine and isoproturon degradation, whose main factors were probably directly related to the dynamics of the specific microbial degradation activity. In contrast, variability of metamitron degradation was significantly correlated to sorption coefficient (K(d)) through correlation with the sorptive soil components, organic matter and clay. Herbicide degradation decreased with depth as did the overall microbial activity. Atrazine mineralization activity was found down to a depth of 2.5 m; beyond that, it was negligible.