Plasma membrane H+ pump at a crossroads of acidic and iron stresses in yeast-to-hypha transition†

Iron is an essential nutrient but is toxic in excess mainly under acidic conditions. Yeasts have emerged as low cost, highly efficient soil inoculants for the decontamination of metal-polluted areas, harnessing an increasing understanding of their metal tolerance mechanisms. Here, we investigated the effects of extracellular iron and acid pH stress on the dimorphism of Yarrowia lipolytica. Its growth was unaffected by 1 or 2 mM FeSO₄, while a strong cellular iron accumulation was detected. However, the iron treatments decreased the hyphal length and number, mainly at 2 mM FeSO₄ and pH 4.5. Inward cell membrane H⁺ fluxes were found at pH 4.5 and 6.0 correlated with a pH increase at the cell surface and a conspicuous yeast-to-hypha transition activity. Conversely, a remarkable H⁺ efflux was detected at pH 3.0, related to the extracellular microenvironment acidification and inhibition of yeast-to-hypha transition. Iron treatments intensified H⁺ influxes at pH 4.5 and 6.0 and inhibited H⁺ efflux at pH 3.0. Moreover, iron treatments inhibited the expression and activities of the plasma membrane H⁺-ATPase, with the H⁺ transport inhibited to a greater extent than the ATP hydrolysis, suggesting an iron-induced uncoupling of the pump. Our data indicate that Y. lipolytica adaptations to high iron and acidic environments occur at the expense of remodelling the yeast morphogenesis through a cellular pH modulation by H⁺-ATPases and H⁺ coupled transporters, highlighting the capacity of this non-conventional yeast to accumulate high amounts of iron and its potential application for bioremediation.