Organic matter decomposition by fungi in a Mediterranean forested stream : contribution of streambed substrata

In forested streams where light is a limiting factor for primary production, energy sources are mainly allochthonous (leaf litter), the metabolic processes being typically heterotrophic (Fisher & Likens 1973, Minshall et al. 1983). In Mediterranean systems, the greatest input of plant material occurs in autumn, thereby providing the potential for decomposition processes. The initial breakdown of leaves in the stream is mainly carried out by aquatic hyphomycetes (Suberkropp & Klug 1976, Findlay & Arsuffi 1989, Gessner & Chauvet 1994, Mathuriau & Chauvet 2002), while the contribution of bacteria is lower (Hieber & Gessner 2002). This group of fungi is crucial in decomposition process as it breaks down lignified carbohydrates, which constitute a natural protection of polysaccharide components against enzymatic attack (Griffin 1994). After cellulose, lignin is the most abundant form of aromatic carbon in the world. Lignin degradation does not provide a primary source of carbon and energy for fungal growth, but decay process and utilization of carbohydrates for fungal growth can occur only with the coordinate degradation of this carbon (Griffin 1994).