Selenium Metabolism in Two Selenophilic Alternaria Fungi: An Examination of Selenium–Sulfur Interactions

Post, Sarah L.
Zukauckas, Kelsea N.
Selenium (Se) is known to be an essential nutrient for animals and bacteria, but not for plants and fungi. Despite this, certain plants and their rhizosphere fungi are capable of accumulating high levels of Se without apparent toxic effects. The purpose of this experiment was to test if two selenophilic fungi, Alternaria tenuissima (A2) and Alternaria astragali (A3), utilize the sulfur (S) assimilation pathway to accumulate Se. Additionally, we examined if Se uptake in these fungi was correlated with their original isolation location on the plant (seed, A2 or root, A3). Fungi were grown in liquid media containing combinations of sulfate (SO42-) with either selenate (SeO42-) or selenocysteine (SeCys). After seven days, the cultures were harvested, lyophilized, acid digested and analyzed using TXRF. Our results demonstrate a positive correlation between Se provided and accumulated with pronounced levels of Se in the fungi, ranging from 1–2 orders of magnitude beyond what was supplied. In addition, with increasing SO42- provided, both fungi displayed a decrease in overall SeO42- accumulation suggesting the presence of and competition for, a possibly novel Se transporter. However, increasing SeO42- provided did not result in decreasing SO42- accumulated, implying Se does not compete with S for the SO42- transporter. Furthermore, there was no evidence of any competition with SO42- when the fungi were supplied with SeCys. Our data provides further evidence suggesting Se specific metabolism is occurring outside of the S assimilation pathway in these selenophilic organisms.
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