**3. Hidden biodiversity of aquatic fungi**

Actual fungal biodiversity suggests that the most species-rich regions of the globe are situated in temperate rather than in tropical regions. Given that many fungal species are host or substrate specific, and that biodiversity of plants and animals is highest in tropical regions, this notion is counter-intuitive. It is very likely that sampling efforts for fungal biodiversity have been largely restricted to temperate regions, where most fungal taxonomists are situated (Shearer et al., 2007). Alternatively, seasons, cooler temperatures and moist conditions may be more amenable to fungal evolution and niche differentiation. From the above mentioned discrepancies and gaps of knowledge in diversity of aquatic fungi, it appears timely to commence co-ordinated world-wide

Aquatic Fungi 239

Known Substrates Remarks

obligate

fungi obligate

phytoplankton, zooplankton, animals, plant debris, seeds, pollen, fruits, chitin, keratin, cellulose, twigs

fruits, twigs, animal

vegetable debris, excrements of amphibians

coprophilous

*Piromyces* ruminant cellulose obligate anaerobe

debris

endoparasites esp. of fishes and arthropods

mycoparasites, common at anoxic

obligate and opportunistic endoparasites & ectoparasites; saprophytes

symbionts, potentially in sediments

endoparasites of malaria mosquito Anopheles

obligate VAM building symbionts

metabolism possible

endoparasites & saprophytes

endoparasites & ectoparasites or predatory fungi

species and trichomycetes (symbionts of aquatic arthropods)

sites

Hosts

animals (incl. protists and zooplankton)

mycoplankton, phytoplankton, zooplankton, animals, macrophytes

insect larvae, eggs of liver fluke, nematodes, aquatic

fungi

macrophytes

**Mucoromycotina** *Mucor* debris fermentative

insects, desmids, rotifers, nematodes

nematodes, fungi (e.g. Mucor)

Chironomidae)

Table 1. Lower fungal phyla of *Eumycota* in accordance to Hibbett et al. (2007) and Lara et al. (2010). Detailed information was obtained mainly from Sparrow (1960), Hywel-Jones & Webster (1986), Ebert (1995), Keeling & Fast (2002), Lichtwardt (2004) and Benny (2009).

systems is an ideal habitat for fungi and hence should be the focus of future fungal biodiversity research. Littoral food webs are very complex and a wealth of invertebrates, vertebrates and progeny suggest close interaction with a diverse community of fungi including parasitic, symbiotic and endophytic fungi. Littoral zones are highly structured by large emerged macrophytes, floating macrophytes and submerged macrophytes, which can

Phyla Representatives Known

*Telohania Pleistophora*

*LKM11* 

*Endochytrium Batrachochytrium*

*Catenomyces* 

**Glomeroycota** *Glomus* roots of aquatic

*Ancylistes Macrobiotophthora*

**Zoopagomycotina** *Zoophagus* amoebae, rotifers,

**Kickxellomycotina** *Harpellales* arthropods (e.g.

*Erynia* 

Asterisks mark not yet confirmed phyla.

**Microsporidia\*** *Glugea* 

**Rozellida\*** *Rozella* 

**Neocallimastigo-**

**mycota** 

**Subphyla of Glomeromycota** 

**Entomophthoromycotina** 

**Chytridiomycota** *Rhizophydium* 

**Blastocladiomycota** *Coelomomyces* 

sampling programs using consistent methodology to evaluate fungal biodiversity in various aquatic systems around the globe.

Gessner & Van Ryckegem (2003) estimated the total number of aquatic fungal species to a maximum of 20 000 different species based on the assumption that only 5% have been described so far. Whereas only a few newly described fungal species have been added in recent years, an increasing number of genetically distant environmental DNA sequences have been found (Hibbett et al., 2011). For example, biodiversity of basal fungal lineages, which bear numerous aquatic species, seems to be much higher than expected. In addition, biodiversity of these basal phyla is elevated in aquatic sediments when compared to terrestrial soil (Mohamed & Martiny, 2011). The highest estimates of global fungal diversity reach up to 5 million species (Blackwell, 2011). The above mentioned "lower fungi" belonging to *Eumycota*, excluding congruously *Oomycetes* and *Thaustrochytrids*, are listed in table 1.

Currently, the species ratio of terrestrial fungi to land plants is approximately 10.6:1. Most likely, this ratio will increase in the future since mycologists have largely increased their efforts to find new fungal species. Freshwater ecosystems can be considered as rather unexplored fungal habitats whereby the few, presently available molecular studies point to a high species diversity. Blackwell (2011) gives helpful suggestions on where to search for these hidden species and highlights insects and other animals as potential fungal habitats. For example, in a single pilot-study in 2005, Suh et al. have isolated 196 new yeast species from guts of mushroom eating beetles and thereby increased the total number of worldwide described yeast species by more than 30%. Next to fungi residing in arthropod guts, endophytes in freshwater ecosystems are another budding source of high fungal biodiversity. For example, when applying molecular tools Neubert et al. (2006) found >600 fungal operational taxonomic units (a measurement of environmental DNA sequence diversity) in single plants (*Phragmites australis*) of a single lake (Lake Constance). This remarkably high diversity of endo- and ectophytic fungi points to a so far largely hidden fungal diversity associated with higher aquatic organisms.

As already mentioned, fungal parasites in pelagic systems can greatly add to global fungal diversity, which should by far exceed even that of saprophytic fungi. This is due to the following features of parasitic fungi: (1) the presence of a specialised attack-defence coevolution based on the red queen hypothesis and (2) a high specificity to host species of various eukaryotes. A precise estimation of their diversity is difficult since parasites can be either host strain specific (De Bruin et al., 2008) or cover a wider spectrum of hosts such as *B. dendrobatidis.* In addition to parasitic fungi, many opportunistic saprophytic fungi are hostspecific (Sparrow, 1960). Nevertheless, variability in host and substrate specificity is high among aquatic fungi and it is difficult to generalise.
