Responses of antioxidant defenses to Cu and Zn stress in two aquatic fungi

Abstract

Aquatic hyphomycetes are fungi that play a key role in plant litter decomposition in streams. Even though these fungi occur in metal-polluted streams, the mechanisms underlying their tolerance to metals are poorly documented. We addressed the effects of Zn and Cu in Varicosporium elodeae and Heliscus submersus by examining metal adsorption to cell walls, plasma membrane integrity and production of reactive oxygen species at metal concentrations inhibiting biomass production in 50% or 80%. The activity of the enzymes catalase, superoxide dismutase and glucose-6-phosphate dehydrogenase was measured to elucidate their role in coping with oxidative stress induced by metals at short- (14 h) and long- (8 days) term exposure. Results show that V. elodeae was more susceptible to the toxic effects induced by Cu and Zn than H. submersus, as indicated by more extensive inhibition of biomass production. Both metals, particularly Cu, induced oxidative stress in the two fungal species, as shown by the noticeable recovery of biomass production in the presence of an antioxidant agent. In both fungi, Cu induced a more severe disruption of plasma membrane integrity than Zn. Our studies on antioxidant defenses showed that catalase had a greater role alleviating stress induced by Zn and Cu than superoxide dismutase. Chronic metal stress also stimulated the production of NADPH, via the pentose phosphate pathway by increasing the activity of glucose-6-phosphate dehydrogenase. Our results suggest that the tolerance of aquatic hyphomycetes to Cu and Zn is associated with the ability of these fungi to initiate an efficient antioxidant defense system.

Copper and zinc affect the activity of plasma membrane H+-ATPase and thiol content in aquatic fungi

“…It was previously demonstrated that the generation of ROS contributes noticeably to copper and zinc toxicity in aquatic hyphomycetes (Azevedo et al, 2007(Azevedo et al, , 2009Pradhan et al, 2015). The interaction of ROS with biological membranes results in a variety of functional alterations due to direct interaction with the molecular cell machinery and/ or oxidative modification of biological macromolecules (Stark, 2005).…”

“…Specific Cys residues of the H + -ATPase are targets for iron and copper Fenton reagents, leading to enzyme inactivation (Stadler et al, 2003). Consistently, reducedthiol groups were shown to be essential for maintaining H + -ATPase activity under iron stress in wheat root plasma membranes (Yang et al, 2003).We previously reported that exposure of aquatic hyphomycetes to copper and zinc led to intracellular ROS accumulation and plasma membrane disruption (Azevedo et al, 2007). In the hyphomycetes, metal tolerance was associated with increased levels of thiol compounds in cells (Miersch et al, 2001; Braha et al, 2007; Guimarães-Soares et al, 2007), and there is evidence pointing to glutathione, phytochelatins and metallothioneins as putative metal sequesters or ROS scavengers (Jaeckel et al, 2005; Guimarães-Soares et al, 2006).…”

“…We previously reported that exposure of aquatic hyphomycetes to copper and zinc led to intracellular ROS accumulation and plasma membrane disruption (Azevedo et al, 2007). In the hyphomycetes, metal tolerance was associated with increased levels of thiol compounds in cells (Miersch et al, 2001; Braha et al, 2007; Guimarães-Soares et al, 2007), and there is evidence pointing to glutathione, phytochelatins and metallothioneins as putative metal sequesters or ROS scavengers (Jaeckel et al, 2005; Guimarães-Soares et al, 2006).…”

“…Metal toxicity in fungi may result from direct interaction between metal ions and biomolecules or from mechanisms related to the ability of metals to generate reactive oxygen species (ROS) (Stohs & Bagchi, 1995;Azevedo et al, 2007). Transition metals, such as copper and iron, greatly increase ROS production through the Fenton reaction (Bai et al, 2003).…”

“…Metal tolerance in fungi can be achieved by several complex mechanisms, including extracellular precipitation, biosorption, controlled uptake and intracellular sequestration and/or compartmentalization, whose relative contributions to metal detoxification can vary with metal type and fungal species (Krauss et al, 2011). Therefore, we are still far from fully understanding the mechanisms underlying metal tolerance/resistance in fungi, despite the large amount of information on the effects of metals in living organisms.Metal toxicity in fungi may result from direct interaction between metal ions and biomolecules or from mechanisms related to the ability of metals to generate reactive oxygen species (ROS) (Stohs & Bagchi, 1995;Azevedo et al, 2007). Transition metals, such as copper and iron, greatly increase ROS production through the Fenton reaction (Bai et al, 2003).…”