Isotope Fractionation of Selenium During Fungal Biomethylation by Alternaria alternata

Abstract

The natural abundance of stable Se isotopes may reflect sources and formation conditions of methylated Se. We aimed at (1) quantifying the degree of methylation of selenate [Se(VI)] and (hydro)selenite [Se(IV)] by the fungus Alternaria alternata at pH 4 and 7 and (2) determining the effects of these different Se sources and pH values on 82Se/76Se ratios (δ82/76Se) in methylselenides. Alternaria alternata was incubated with Se(VI) and Se(IV) in closed microcosms for 11-15 days and additionally with Se(IV) for 3-5 days at 30 °C. We determined Se concentrations and δ82/76Se values in source Se(VI) and Se(IV), media, fungi, and trapped methylselenides. In Se(VI) incubations, methylselenide volatilization ended before the 11th day, and the amounts of trapped methylselenide were not significantly different among the 11-15 day incubations. In 11-15 days, 2.9-11% of Se(VI) and 21-29% of Se(IV) were methylated, and in 3-5 days, 3-5% of Se(IV) was methylated. The initial δ82/76Se values of Se(VI) and Se(IV) were -0.69±SD 0.07‰, and -0.20±0.05‰, respectively. The δ82/76Se values of methylselenides differed significantly between Se(VI) (-3.97‰ to -3.25‰) and Se(IV) (-1.44‰ to -0.16‰) as sources after 11-15 days of incubation; pH had little influence on δ82/76Se values. Thus, the δ82/76Se values of methylselenide indicate the source species of methylselenides used in this study. The strong isotope fractionation of Se(VI) is probably attributable to the different reduction steps of Se(VI) to Se(-II) which were rate-limiting explaining the low methylation yields, but not to the methylation itself. The shorter incubation of Se(IV) for 3-5 days showed a large Se isotope fractionation of at least -6‰ before the biomethylation reaction reached its end. This initial Se isotope fractionation during methylation of Se(IV) is much larger than previously published.

Selenium as paleo-oceanographic proxy: A first assessment

“…The figure also includes dimethyl selenide (DMSe) efflux as a possible pathway for the removal of Se from the oceans (Amouroux et al, 2001). This process is unlikely to impart a significant isotopic shift, as little fractionation is associated with volatilization (Schilling et al, 2011). The transport of Se to the sediments is dominantly in the form of settling organic matter in all three redox regimes, with some input of oxidized Se species in the oxic and OMZ water column scenarios and elemental Se and FeSe in the anoxic/ euxinic scenario.…”

“…The analytical data were first corrected for interferences (for detailed discussion, see Electronic Annex Zhu et al, 2008;Clark and Johnson, 2010;supporting information;Schilling et al, 2011;supporting information). The different isotopic ratios ( 74 Se/ 78 Se, 76 Se/ 78 Se, 77 Se/ 78 Se, 80 Se/ 78 Se, and 82 Se/ 78 Se) were then entered into an iterative data reduction procedure that extracted the sample's 82 Se/ 76 Se and 82 Se/ 78 Se ratios from the measurements on the sample-spike mixture.…”

Selenium Isotopes as a Biogeochemical Proxy in Deep Time

“…Hence marine phytoplankton may record the composition of seawater (Mitchell et al 2012). Volatilization of methylated selenium gases is associated with moderate fractionations of 2-4‰ (Schilling et al 2011b;, but given the short residence time of these gases in the atmosphere, this process is likely insignificant over geological timescales.…”

“…Selenium associated with sulfate evaporites appears to be minor (Hagiwara 2000). The major forms of selenium in siliciclastic sediments are organic-and pyrite-bound Se(-II) ( Schilling et al 2011b); however, they have a short lifetime of only a few hours because they are rapidly oxidized in the modern oxic atmosphere (Wen and Carignan 2007). Volcanic processes may produce H2Se gas, but this rapidly oxidizes to Se(0) today (Suzuoki 1965).…”

Isotope Fractionation Processes of Selected Elements

“…Selenium is an essential trace element for animals and humans having a narrow concentration range between sufficiency and toxicity (Schilling et al 2011). It occurs in four oxidation states that differ in their nutritional and toxic behaviour.…”