Verónica Ferreira , Vladislav Gulis , Cláudia Pascoal and Manuel

Aquatic hyphomycetes are a group of freshwater fungi that includes asexual stages of ascomycetes and basidiomycetes. Even though aquatic hyphomycetes are a phyllogenetically heterogeneous group, there are common characteristics to most species: (i) they are saprotrophs, using submerged leaves and wood as substrates and sources of carbon (Gulis et al. 2006b), (ii) they can obtain nutrients from both the water column and from organic substrates (Suberkropp 1995), (iii) they inhabit mostly well aerated and turbulent waters (Medeiros et al. 2009), and (iv) they produce large numbers of conidia (asexual spores) with distinct shapes and sizes (Gessner and Chauvet 1994; Gulis and Suberkropp 2003c; Ferreira et al. 2006a) (Fig. 18.1). Aquatic hyphomycetes are important players in small woodland streams, where they facilitate a fundamental ecosystem-level process—the decomposition of allochthonous plant litter entering streams from the riparian zone, mostly as leaves (Webster et al. 1997). Once in water, leaves leach soluble compounds, particularly during the first 24–48 h of submersion and are colonized by a range of microbes. In temperate streams, aquatic hyphomycetes are often the main microbial colonizers of submerged litter contributing >90% to the total microbial biomass and production (Weyers and Suberkropp 1996; Hieber and Gessner 2002; Pascoal and Cássio 2004). After attachment to the leaf litter surface, aquatic hyphomycete conidia germinate (Treton et al. 2004) and mycelia penetrate the leaf mesophyll (Canhoto and Graça 1999). Aquatic hyphomycetes macerate the leaf matrix with extracellular enzymes (Chamier and Dixon 1982; Arsuffi and Suberkropp 1984, 1988), mineralize organic carbon (Gulis et al. 2006b), or convert it into fungal biomass (Gessner and Chauvet 1994; Gulis et al. 2006b), and release large numbers of conidia (Gessner and Chauvet 1994; Gulis et al. 2006b; Ferreira et al. 2006a). The activities of aquatic hyphomycetes on leaf litter have been shown to correlate well with litter mass loss (Gessner and Chauvet 1994; Niyogi et al. 2003), and they can directly account for the loss of up to 39% of initial litter mass under field conditions (Hieber and Gessner 2002; Pascoal and Cássio 2004). The accumulation of fungal biomass and the maceration of litter also increase litter quality and palatability to invertebrate detritivores whose feeding activities lead to further litter mass loss (Canhoto and Graça 2008). Also, conidia released into the water column are used as food by the filter feeders (Bärlocher and Brendelberger