**2. Life styles of aquatic fungi**

Aquatic habitats are heterogeneous in time and space and greatly differ in their physicochemical features. Consequently, composition and abundance of aquatic fungi should differ

(Czeczuga et al., 2005; Nechwatal et al., 2008). Slime moulds (*Amoebozoa*; Adl et al., 2005) are also found in freshwater habitats. Although they are relatively easy to isolate from plant detritus submerged in ponds and lakes, their ecology is little known and requires further

Aquatic "true fungi" are osmoorganotrophs, absorbing nutrients across their cell wall. Most of them have a filamentous growth stage during their life cycle. This morphology enables them to invade deep into substrates and to directly digest particulate organic matter (POM) to acquire nutrients for growth and reproduction. Fungal filaments vary in length from several micrometers for the "rhizoids" of *Chytridiomycetes* to several millimetres or metres for hyphae or hyphal networks, e.g. of hyphomycetes colonising leaves, wood, and soil. However, there are always exceptions, such as unicellular yeasts, which lost filamentous growth during their evolution. Here, we will focus on diversity and function of fungi in

Aquatic habitats are characterised by a unique balance of allochthonous (external) and autochthonous (internal) organic matter supply, which is controlled largely by watershed characteristics, surface area and location. For example, headwater streams and small ponds receive most of their organic matter from terrestrial riparian vegetation, whereas large lakes are mainly supplied with organic matter internally from algal primary producers. Organic carbon derived from terrestrial vegetation varies substantially from that of algae. Plant remains contain large fractions of lignin, hemicelluloses and cellulose, which prolong microbial decomposition to several months, whereas algae contain much fewer recalcitrant polymers and thus are rapidly mineralised, usually within a few days. In small or shallow lentic systems submerged and emergent aquatic macrophytes often dominate the primary production, representing the most productive non-marine ecosystem worldwide. Aquatic fungi, being heterotrophs, are reliant upon photosynthetically produced organic matter. In order of decreasing biodegradability, the fungal community consumes microscopic algae, macroscopic aquatic macrophytes and terrestrial plant litter (including wood). On localised spatial scales or short-term temporal scales, carbon and nutrients from other sources may gain high importance. Resources derived from animals include fish, fish eggs, carcasses, excuviae, living zooplankton, insects, feathers and hair, while other plant-derived resources include pollen, spores, seeds and fruits (Cole et al., 1990). Interestingly, it seems to be nearly impossible to find a natural organic source that cannot be utilized by aquatic fungi (Sparrow, 1960). This notion points to either a high functional redundancy of a limited set of fungal species or to a high biodiversity of fungal specialists. Most likely, in natural systems both cases occur at the same time. Another interesting feature of aquatic habitats is the coupling of aquatic systems to terrestrial environments via animals, mainly insects, which are able to export nutrients from the aquatic ecosystem (Vander Zanden & Gratton, 2011). It will be shown later, that fungi are often closely associated with insects, which can be key organisms in aquatic freshwater systems. Although often overlooked, fungi represent a common and important component of almost every trophic level of any aquatic ecosystem.

Aquatic habitats are heterogeneous in time and space and greatly differ in their physicochemical features. Consequently, composition and abundance of aquatic fungi should differ

**1.1 Characteristics of the aquatic habitats influencing fungal life** 

investigation (Lindley et al., 2007).

various aquatic systems.

**2. Life styles of aquatic fungi** 

significantly between these habitats (Wurzbacher et al., 2010). Whereas Wurzbacher et al. (2010) have recently reviewed the ecology of fungi in lake ecosystems, and present a thorough discussion on fungal communities within the different water bodies, in this book chapter we want to present a concise overview on fungal life-forms and diversity in various water bodies.
