Aquatic hyphomycetes are the major microbial decomposers of plant litter in streams. We selected three aquatic hyphomycete species with different abilities to tolerate, adsorb and accumulate copper and zinc, and we investigated the effects of these metals on H + -ATPase activity as well as on the levels of thiol (SH)-containing compounds. Before metal exposure, the species isolated from a metal-polluted stream (Heliscus submersus and Flagellospora curta) had higher levels of thiol compounds than the species isolated from a clean stream (Varicosporium elodeae). However, V. elodeae rapidly increased the levels of thiols after metal exposure, emphasizing the importance of these compounds in fungal survival under metal stress. The highest amounts of metals adsorbed to fungal mycelia were found in the most tolerant species to each metal, i.e. in H. submersus exposed to copper and in V. elodeae exposed to zinc. Short-term (10 min) exposure to copper completely inhibited the activity of H + -ATPase of H. submersus and V. elodeae, whilst zinc only led to a similar effect on H. submersus. However, at longer exposure times (8 days) the most metal-tolerant species exhibited increased H + -ATPase activities, suggesting that the plasma membrane proton pump may be involved in the acclimation of aquatic hyphomycetes to metals.
INTRODUCTIONThere is a growing interest in understanding the ecological, physiological and biochemical properties that allow some fungi to colonize and live in metal-polluted habitats. Fungi play a critical role in organic matter turnover in streams. Amongst fungi, aquatic hyphomycetes appear to have the greatest ecological role as decomposers of plant detritus in streams (Baldy et al., 2002; Pascoal & Cássio, 2004). Even though metal pollution lowers the biodiversity and activity of aquatic hyphomycetes, the occurrence of these groups of fungi has been consistently reported in metal-polluted streams (Pascoal et al., 2005a;Sridhar et al., 2005). This means that fungi, similar to other living organisms, have to tightly regulate the intracellular metal concentration in such a way that safe uptake of the required metal ions in the cytosol and organelles can occur without cellular damage due to metal toxicity (Kneer et al., 1992). 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 coppe...
