Isotope fractionation of selenium by biomethylation in microcosm incubations of soil

The natural abundance of stable Se isotopes in methylselenides reflects sources and formation conditions of methylselenides. We tested the effects of (i) different inorganic Se species spiked to soils and (ii) different soil samples on the extent of fungal biomethylation of Se and the Se isotope ratios (δ82/76Se) in methylselenides. Furthermore, we assessed the decrease of dissolved, bioavailable Se during three days of equilibration of the soils with Se-enriched solutions. We conducted closed microcosm experiments containing soil spiked with Se(IV) or Se(VI), a growth medium, and the fungus species Alternaria alternata for 11 d. The concentrations and isotope ratios of Se were determined in all components of the microcosm with multicollector ICP-MS. The equilibration of the spiked Se(IV) and Se(VI) for 3 d resulted in a decrease of dissolved, bioavailable Se concentrations by 32 to 44% and 8 to 14%, respectively. Very little isotope fractionation occurred during this phase, and it can be attributed to mixing of the added Se with the pre-existing Se in the soils and minor Se(IV) reduction in one experiment. In two of the incubated soils – moderately acidic roadside and garden soils – between 9.1 and 30% of the supplied Se(IV) and 1.7% of the supplied Se(VI) were methylated while in a strongly acidic forest soil no Se methylation occurred. The methylselenides derived from Se(IV) were strongly depleted in 82Se (δ82/76Se = − 3.3 to − 4.5‰) compared with the soil (0.16–0.45‰) and the added Se(IV) (0.20‰). The methylselenide yield of the incubations with Se(VI) was too small for isotope measurements. Our results demonstrate that Se source species and soil properties influence the extent of Se biomethylation and that the produced methylselenides contain isotopically light Se.

El valor estructural de la magia en el universo pastoril de Lope de Vega

The different aspects of magic became an essential ingredient of bucolic universe since the origin of pastoral genre. Within an idealized and utopic frame, magic built a bridge towards transcendence, and balanced the disharmony created by human love emotions within Arcadian microcosm. This concept of magic became infused to Sixteenth century pastoral novel through Neoplatonism and Hermetic tradition. From a narratological point of view, magic episodes became indispensable to complete the philosophical meaning of the works. Lope de Vega, in La Arcadia, published in 1598, fully participated from this tradition, though he will approach the convention from a particular dramatic-like perspective. And it was precisely in pastoral comedias where the Fénix ends up deconstructing bucolic code to remodel it as a mere vestige of the tradition and treat magic as a humorous ingredient.