Vertical stratification of microbial communities and isotope geochemistry tie groundwater denitrification to sampling location within a nitrate-contaminated aquifer

Nitrate pollution is a major threat to groundwater quality in agricultural areas. Natural attenuation of nitrate in contaminated aquifers is mediated by denitrifying microbial populations in anoxic environments. Vertical distribution of denitrifying microbial communities in aquifers is greatly influenced by groundwater redox conditions, local hydrogeological parameters, and seasonal variability in groundwater flow and recharge. In this study, we investigated groundwater geochemistry and the composition of bacterial and archaeal communities with increasing depth in a shallow nitrate-contaminated aquifer in British Columbia, Canada. High-resolution passive diffusion sampling was conducted to collect groundwater at 10-cm intervals from 4 to 20 m below ground surface (mbgs) in the aquifer. Geochemical analyses of major ions indicated a general shift in the groundwater chemistry below 16 mbgs including decreasing chloride concentrations that suggest two-end member mixing of shallow and deep groundwater with different chemistries. A redoxcline was further observed within a 2 m transition zone at 18-20 mbgs characterized by sharp declines in nitrate concentrations and increases in sulfate and total inorganic carbon. Excursions in δ¹⁵N-NO₃^- and δ¹⁸O-NO₃^- in the same depth interval are consistent with denitrification, and a concomitant decrease in δ³⁴S-SO₄^2- suggested that denitrification was coupled to sulfide or sulfur oxidation. Microbial communities within this depth interval were significantly dissimilar to those above and below, featuring putative lithotrophic denitrifying bacteria belonging to the genera Sulfurifustis, Sulfuritalea and Sulfuricella. These lineages were detected in greatest abundance at 19 mbgs while the abundances of putative heterotrophic sulfate-reducing bacteria belonging to the genus Desulfosporosinus were greatest at 20 mbgs. In addition to help distinguish denitrification from mixing-induced changes in groundwater chemistry, the above observed vertical stratification of the microbial key players connects nitrate removal to the locations of the aquifer sampled.

A Field-Scale Approach to Estimate Nitrate Loading to Groundwater

The quantification of groundwater NO loading associated with a specific field or set of management practices so that groundwater quality improvements can be objectively assessed is a major challenge. The magnitude and timing of NO export from a single agricultural field under raspberry ( L.) production were investigated by combining high-resolution groundwater NO concentration profiles (sampled using passive diffusion samplers) with Darcy's flux estimation at the field's down-gradient edge (based on field-measured hydraulic gradients and laboratory-estimated hydraulic conductivity). Annual recharge estimated using Darcy's law (1002 mm) was similar to that obtained using two other approaches. The similarity in the rate of Cl applied to the field and the estimated export flux over the 1-yr monitoring period (51 vs. 56 kg Cl ha) suggested the mass flux estimation approach was robust. An estimated 80 kg NO-N ha was exported from the agricultural field over the 1-yr monitoring period. The greatest monthly groundwater mass flux exported was observed in February and March (∼11 kg NO-N ha), and was associated with NO leached from the soil zone during the onset of precipitation in the previous autumn. Provided the groundwater recharged from the field of interest can be isolated within a vertical profile, this approach is an effective method for obtaining spatially integrated estimates of the magnitude and timing of NO loading to groundwater.

Phytoalexins from Thlaspi arvense, a wild crucifer resistant to virulent Leptosphaeria maculans: structures, syntheses and antifungal activity

Phytoalexins are inducible chemical defenses produced by plants in response to diverse forms of stress, including microbial attack. Our search for phytoalexins from cruciferous plants resistant to economically important fungal diseases led us to examine stinkweed or pennycress (Thlaspi arvense), a potential source of disease resistance to blackleg. We have investigated phytoalexin production in leaves of T. arvense under abiotic (copper chloride) and biotic elicitation by Leptosphaeria maculans (Desm.) Ces. et de Not. [asexual stage Phoma lingam (Tode ex Fr.) Desm.], and report here two phytoalexins, wasalexin A and arvelexin (4-methoxyindolyl-3-acetonitrile), their syntheses and antifungal activity against isolates of P. lingam/L. maculans, as well as the isolation of isovitexin, a constitutive glycosyl flavonoid of stinkweed, having antioxidant properties but devoid of antifungal activity.

Enantiomeric excess of a cruciferous phytoalexin, spirobrassinin, and its enantiomeric enrichment in an achiral HPLC system

Enantiomeric purity of a cruciferous phytoalexin, spirobrassinin (1), was determined by chiral HPLC analysis. The enantiomeric excesses of two natural spirobrassinin fractions separated by nonchiral chromatography were considerably different. A significant enantiomeric enrichment was observed during the nonchiral chromatographic separation of an artificial enantiomeric mixture of 1.

Restoration of the self-splicing activity of a defective group II intron by a small trans-acting RNA

The yeast mitochondrial group II intron bI1 is self-splicing in vitro. We have introduced a deletion of hairpin C1 within the structural domain 1 that abolishes catalytic activity of the intron in the normal splicing reaction in cis, but does less severely affect a reaction in trans, the reopening of ligated exons. Since exon reopening is supposed to correspond to a reverse 3' cleavage this suggests that the deletion specifically blocks the first reaction step. The intron regains its activity to self-splice in cis by intermolecular complementation with a small RNA harbouring sequences lacking in the mutant intron. These results demonstrate the feasibility to reconstitute a functionally active structure of the truncated intron by intermolecular complementation in vitro. Furthermore, the data support the hypothesis that group II introns are predecessors of nuclear pre-mRNA introns and that the small nuclear RNAs of the spliceosome arose by segregation from the original intron.

Insect hormone activity of p-(1,5-dimethylhexyl)benzoic acid derivatives in Dysdercus species

Derivatives of p-(1,5-dimethylhexyl)benzoic acid are juvenile hormone analogs with selective action on the hemipteran insects of the family Pyrrhocoridae. Their juvenile hormone activity is constant on five species of Dysdercus; it is about ten times lower on Pyrrhocoris, and no activity has been detected on hemipterans of some other families. Absence of profound species-specific variations in the activity suggests that the most active compounds of this type can be used as selective pesticides against all species of Dysdercus.