Impact of uranium exposure on marine yeast, Yarrowia lipolytica: Insights into the yeast strategies to withstand uranium stress

A marine yeast, Yarrowia lipolytica, was evaluated for morphological, physiological and biochemical responses towards uranium (U) exposure at pH 7.5. The yeast revealed biphasic U binding - a rapid biosorption resulting in similar to 35% U binding within 15-30 min followed by a slow biomineralization process, binding up to similar to 45.5% U by 24 h on exposure to 50 mu M of uranyl carbonate. Scanning electron microscopy coupled with Energy Dispersive X-ray spectroscopy analysis of 24 h U challenged cells revealed the deposition of uranyl precipitates due to biomineralization. The loss of intracellular structures together with surface and subcellular localization of uranyl precipitates in 24 h U exposed cells were visualized by transmission electron microscopy. Cells treated with 50 mu M U exhibited membrane permeabilization which was higher at 200 mu M U. Enhanced reactive oxygen species (ROS) accumulation and lipid peroxidation, transient RNA degradation and protein oxidation were observed in U exposed cells. High superoxide dismutase levels coupled with uranium binding and bioprecipitation possibly helped in counteracting U stress in 50 mu M U treated cells. Resistance to U toxicity apparently developed under prolonged uranyl (50 mu M) incubations. However, cells could not cope up with toxicity at 200 mu M U due to impairment of resistance mechanisms.