**1. Introduction**

226 The Dynamical Processes of Biodiversity – Case Studies of Evolution and Spatial Distribution

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Seventy-one percent of our planet's surface consist of water, but only 0.6% are lentic and lotic freshwater habitats. Often taken for granted, freshwaters are immensely diverse habitats and host >10% of all animal and >35% of all vertebrate species worldwide. However, no other major components of global biodiversity are declining as fast and massively as freshwater species and ecosystems. Urbanisation, economic growth, and climate change have increased pressure on freshwater resources, whilst biodiversity has given way to the increasing demands of a growing human population. The adverse impacts on aquatic ecosystems include habitat fragmentation, eutrophication, habitat loss, and invasion of pathogenic as well as toxic species. Although there is increasing evidence that freshwater fungal diversity is high, the study of the biodiversity of freshwater fungi is still in its infancy. In light of the rapid decline in freshwater biodiversity, it is timely and necessary to increase our efforts to evaluate the diversity and potential ecological function of this fascinating and diverse group of freshwater organisms.

Hyde et al (2007) have estimated that there are approximately 1.5 million fungal species on earth. Of these, only around 3000 species are known to be associated with aquatic habitats and only 465 species occur in marine waters (Shearer et al., 2007). This small proportion of aquatic fungal taxa is surprising because the aquatic environment is a potentially good habitat for many species. Based on this notion we assume that the "real" number of aquatic fungi is much larger than 3000 and includes a large variety of hitherto undescribed species with unknown ecological function.

Aquatic fungi are usually microscopic organisms, which do not produce visible fruiting bodies but grow asexually (anamorphic fungi). Their occurrence in water is rather subtle and specialised methods are needed to examine their diversity, population structure and ecological function. Water associated fungi have been known historically as "phycomycetes", a functionally defined group consisting of "true fungi" (*Eumycota*) and "analogously evolved fungus-like organisms" belonging to *Chromista* (*Oomycetes*, *Thraustochytridiomycetes*). Other groups formerly placed in the fungal kingdom include slime moulds (*Amobae*), *Ichthyosporae (Mesomycetozoea)* and *Actinomycetes (Bacteria)*, which are now recognised as distinct taxa. While the "true fungi" are a sister group to animals, *Oomycetes* are biochemically distinct from fungi while having similar morphology, size and habitat usage (Money, 1998). Colloquially known as "water moulds", they comprise approx. 200 species inhabiting freshwater, mud and soil. Many of these are saprobes or parasites

Aquatic Fungi 229

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.

**2.1 The role of fungi as decomposers, predators, endophytes, symbionts, parasites,** 

Aquatic fungi are heterotrophs, i.e. they *sensu stricto* depend on external organic matter, which may be dead or alive. Aquatic systems harbour a wealth of organisms that can serve as suitable hosts: algae from different phyla, cyanobacteria, protists, zooplankton, fish, birds, mussels, nematodes, crayfish, mites, insects, amphibians, mammals, plants and other fungi (Sparrow, 1960; Ellis & Ellis, 1985). Fungi are omnipresent and therefore associated with almost every organism, often as parasites, sometimes as symbionts and of course as

Parallel to fungi in soil, aquatic fungi act as prominent decomposers of POM: foremost coarse particulate organic matter (CPOM) including plant and animal debris. Filamentous growth habit is a key feature of many aquatic fungi, and this feature is responsible for their superiority to heterotrophic bacteria as pioneer colonisers. Hyphae allow fungi to actively penetrate plant tissues and tap internal nutrients. Therefore, Gessner & Van Ryckegem (2003) describe fungal hyphae as self-extending digestive tracts that have been turned inside

The aquatic fungi which typically decompose leaf litter and wood with a hyphal network are the polyphyletic group known as "aquatic hyphomycetes". Aquatic hyphomycetes are most common in clean, well oxygenated, flowing waters (Ingold, 1975; Bärlocher, 1992), and are characterised as anamorphic fungi with tetraradiate or sigmoid conidia (asexual reproductive structures). Taxonomically, they are mainly associated with the *Ascomycota*, and only a small percentage is affiliated with the *Basidiomycota*. In contrast, aero-aquatic hyphomycetes colonise submerged plant detritus in stagnant and slow flowing waters, such as shallow ponds and water-filled depressions. Taxonomically, most aero-aquatic fungi are classified as *Ascomycota*, although four aero-aquatic species have been classified as *Basidiomycota*, and one as O*omycete* (Shearer et al., 2007). These fungi are adapted to habitats with fluctuating water levels subjected to periodic drying, low levels of dissolved oxygen, and elevated levels of sulfide. Therefore, they have buoyant conidia that are released at the water surface as water levels recede. Along with aquatic fungi, terrestrial fungi enter the aquatic realm as pioneer decomposers and endophytes of allochthonous plant debris. In the water, however, they are partially replaced by true aquatic hyphomycetes. After colonising the substrate and forming internal hyphal networks, the POM is macerated at least partly by the fungi themselves. This process is often accelerated by the feeding activity of macroinvertebrates, which find colonised leaves to be more palatable (compiled in Bärlocher, 1992; Gessner & Van Ryckegem, 2003). With the aid of an array of extracellular enzymes, aquatic fungi are able to degrade most of the polymeric substances in leaves (hemicelluloses, cellulose, starch, pectin and to some extent lignin; Krauss et al., 2011). Depending on leaf litter type and water chemistry, fungal leaf decomposition can extend over 1 to 6 months. The situation is slightly different for fungal decomposition of emergent macrophytes, because decomposition starts in standing shoots. Over 600 species of fungi have been recorded from the litter of *Phragmites australis* alone (Gessner & Van Ryckegem, 2003). Ninety four percent of these 600 species were members of *Ascomycota* and only 6%

**plagues & pathogens** 

out growing hidden inside the substrate.

decomposers.

(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 investigation (Lindley et al., 2007).

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 various aquatic systems.
