Fungal secondary metabolites
Secondary metabolites are distinguished from primary metabolites that are produced and distributed almost universally by the intermediary metabolism of living organisms. Secondary metabolites are often bioactive, usually of low molecular weight, and are produced as families of related compounds at restricted stages in the life cycle, with production often being correlated with a specific stage in the fungal development or morphological differentiation (Calvo et al., 2002). Secondary metabolites are dispensable for the producing fungus and have restricted taxonomic distribution with only a small group of organisms producing each metabolite (Bennett and Bentley, 1989; Keller et al., 2005). Filamentous fungi produce an enormous variety of secondary metabolites in pure culture but also when growing on natural substrates such as brewing cereals and malt. Nielsen and Smedsgaard (2003) provided a list of 474 individual and structurally well-characterized substances isolated from extracts of pure liquid cultures of filamentous fungi. To date, the most recent version (2012) of the AntiBase database (Wiley-VCH, Weinheim, Germany) used for the LC-MS based identification of microbial secondary compounds contains 3000 fungal secondary metabolites (Klitgaard et al., 2014). However, this number is likely to represent only a fraction of the total secondary metabolites produced by filamentous fungi. Fungal secondary metabolites can have a diversity of physiological and ecological functions (Vining 1990) but in the majority of cases their functions are obscure. However, it can be assumed that they are bound to play an important role in the fungal life cycle and in the interaction with the environment since their production is highly regulated and complex in most cases. Regulatory links between secondary metabolism, light and sexual/asexual reproduction have been established, which might explain their function to a wider extent (Fox and Howlet, 2008).