**2.2 The life cycles of aquatic fungi**

Life cycles of aquatic fungi cover a broad spectrum from very simple cell divisions to very complex cycles, crossing the terrestrial-water boundary. Starting with basal fungal lineages, Microsporidia are intracellular parasites with an extremely reduced genome (down to 2.3 Mbp, which is half the genome size of the common enterobacterium *Escherichia coli*). They are transmitted passively with non-motile spores, which have a size range of 1 - 50 µm. Endospores are chitinous and mature inside host-cells, where they are eventually released by an extrusion apparatus (summarised by Keeling & Fast, 2002).

Aquatic Fungi 233

Mosquitoes, like other arthropods, are potential hosts for symbiotic trichomycetes (*Harpellales)* in many lentic and lotic freshwater habitats (Lichtwardt, 2004; Koontz, 2006; Strongman & White, 2008). These gut fungi disperse via trichospores through the water

Parasitic members of *Entomophthorales* also use arthropods as hosts. In insects with aquatic and terrestrial life stages, these parasites are well adapted to both habitats by developing asexual conidia for dispersal in air and typical tetraradiate conidia for dispersal in water. A detailed description has been given by Hywel-Jones & Webster (1986) and is depicted in

figure 2. The idea of a second host is especially inspiring, since it is known that

*Entomophthorales* are also parasites of planktonic desmids (green algae; Sparrow, 1960). Leaf decomposition is associated with high discharges of aquatic conidia of diverse shapes and sizes (e.g. Ingold, 1975), although the conidia of aquatic hyphomycetes are typically tetraradiate. Aquatic hyphomycetes reproduce asexually (figure 3), although in ca. 10 percent of all described species, teleomorphs have been found, e.g. on twigs at river margins

Fig. 2. Life cycle of *Erynia conica* on *Simulium sp.* (1) After oviposition (E), only infected females of *Simulium* stay at the river bank and become less active. (2) After 2-12 hrs, rhizoids (R) and pseudocystidia emerge from small swellings at the abdomen. The rhizoids anchor the animal to the ground and inhibit any further locomotion. After 15 hrs, conidiophores and primary aerial conidia emerge (C) and release. (3) After 24 hrs, when the ventral part of the fly is wetted or submerged, primary aquatic conidia are produced. Both types of conidia

can be produced simultaneously in a single fly for up to 96 hrs after arrival at the

descriptions of Hywel-Jones & Webster (1986).

oviposition site. Globose zygospores, however, stay embedded in the cadaver (teleomorphic form). (4) Aerial conidia can transform into secondary stellate aquatic conidia with typical tetraradiate symmetry upon submersion. Yet, it is not clear whether secondary hosts

(zooplankton or desmids) are required for *Erynia* development because aquatic conidia were never observed to cause infection of *Simulium* (Webster 1992). Illustration drafted after

column.

(Webster, 1992).

Members of "Rozellida" have a similar life cycle as *Chytridiomycetes*, although diversity of *Rozella* has been so far only marginally described and is mainly based on the description of *Rozella allomyces*, a parasite living on *Allomyces sp.* The environmental clade LKM11 (van Hannen et al., 1999), the other member of Rozellida, is so far completely undescribed with scarce information about its habitat and ecology. It is known that these organisms probably have zoospores in the size range of 0.2 – 5 µm, which are most abundant above lake sediments (Mangot et al., 2009). They are also found under reduced oxygen and anoxic conditions, (Slapeta et al., 2005; Luo et al., 2005). Under anoxic conditions potential relatives of the *Neocallimastigomycota*, an obligate anaerobic symbiotic group of ruminants can be found, too (Lockhart et al., 2006; Mohamed & Martiny, 2011). However, their life cycle is similar to that of the *Chytridiomycetes*. Briefly, a zoospore is chemically attracted to its host or substrate and attaches to its surface. Then a cyst forms and tiny rhizoids (or a penetration tube) grow into the substrate to gather nutrients for (endobiotic or epibiotic) sporangium formation. Thereafter, masses of zoospores can be discharged (up to 70 000 for *Rhizophlyctis petersenii*). Sexual recombination can occur when two zoospores fuse together either in the free-swimming stage or on the host/substrate surface. Alternatively, resting spores might be formed in a prosporangium or in a zygote (Sparrow, 1960).

In principle, the life cycle of *Blastocladiomycota* is quite similar to that of the *Chytridiomycetes*, although they have hyphal growth in addition to zoospores. An important group within the *Blastocladiomycota* is comprised of members of the *Coelomomycetes*, which are often speciesspecific for their mosquito host. Their complete life cycle, originally described by Whisler et al. (1975), is given in figure 1.

Fig. 1. Life cycle of *Coelomomyces psorophorae*. Zygote (A) infects larva of *Culiseta inornata* (B) leading to development of hyphal bodies, mycelium and, ultimately, thick-walled resistant sporangia. Under appropriate conditions these sporangia (C) release zoospores of opposite mating type (D) which infect the alternate host, *Cyclops vernalis* (E). Each zoospore develops into a thallus and, eventually, gametangia. Gametes of opposite mating type (F) fuse either in or outside of the copepod to form the mosquito-infecting zygote (Whisler et al., 1975, with permission).

Members of "Rozellida" have a similar life cycle as *Chytridiomycetes*, although diversity of *Rozella* has been so far only marginally described and is mainly based on the description of *Rozella allomyces*, a parasite living on *Allomyces sp.* The environmental clade LKM11 (van Hannen et al., 1999), the other member of Rozellida, is so far completely undescribed with scarce information about its habitat and ecology. It is known that these organisms probably have zoospores in the size range of 0.2 – 5 µm, which are most abundant above lake sediments (Mangot et al., 2009). They are also found under reduced oxygen and anoxic conditions, (Slapeta et al., 2005; Luo et al., 2005). Under anoxic conditions potential relatives of the *Neocallimastigomycota*, an obligate anaerobic symbiotic group of ruminants can be found, too (Lockhart et al., 2006; Mohamed & Martiny, 2011). However, their life cycle is similar to that of the *Chytridiomycetes*. Briefly, a zoospore is chemically attracted to its host or substrate and attaches to its surface. Then a cyst forms and tiny rhizoids (or a penetration tube) grow into the substrate to gather nutrients for (endobiotic or epibiotic) sporangium formation. Thereafter, masses of zoospores can be discharged (up to 70 000 for *Rhizophlyctis petersenii*). Sexual recombination can occur when two zoospores fuse together either in the free-swimming stage or on the host/substrate surface. Alternatively, resting spores might be

In principle, the life cycle of *Blastocladiomycota* is quite similar to that of the *Chytridiomycetes*, although they have hyphal growth in addition to zoospores. An important group within the *Blastocladiomycota* is comprised of members of the *Coelomomycetes*, which are often speciesspecific for their mosquito host. Their complete life cycle, originally described by Whisler et

Fig. 1. Life cycle of *Coelomomyces psorophorae*. Zygote (A) infects larva of *Culiseta inornata* (B) leading to development of hyphal bodies, mycelium and, ultimately, thick-walled resistant sporangia. Under appropriate conditions these sporangia (C) release zoospores of opposite mating type (D) which infect the alternate host, *Cyclops vernalis* (E). Each zoospore develops into a thallus and, eventually, gametangia. Gametes of opposite mating type (F) fuse either in or outside of the copepod to form the mosquito-infecting zygote (Whisler et al., 1975, with

formed in a prosporangium or in a zygote (Sparrow, 1960).

al. (1975), is given in figure 1.

permission).

Mosquitoes, like other arthropods, are potential hosts for symbiotic trichomycetes (*Harpellales)* in many lentic and lotic freshwater habitats (Lichtwardt, 2004; Koontz, 2006; Strongman & White, 2008). These gut fungi disperse via trichospores through the water column.

Parasitic members of *Entomophthorales* also use arthropods as hosts. In insects with aquatic and terrestrial life stages, these parasites are well adapted to both habitats by developing asexual conidia for dispersal in air and typical tetraradiate conidia for dispersal in water. A detailed description has been given by Hywel-Jones & Webster (1986) and is depicted in figure 2. The idea of a second host is especially inspiring, since it is known that

*Entomophthorales* are also parasites of planktonic desmids (green algae; Sparrow, 1960).

Leaf decomposition is associated with high discharges of aquatic conidia of diverse shapes and sizes (e.g. Ingold, 1975), although the conidia of aquatic hyphomycetes are typically tetraradiate. Aquatic hyphomycetes reproduce asexually (figure 3), although in ca. 10 percent of all described species, teleomorphs have been found, e.g. on twigs at river margins (Webster, 1992).

Fig. 2. Life cycle of *Erynia conica* on *Simulium sp.* (1) After oviposition (E), only infected females of *Simulium* stay at the river bank and become less active. (2) After 2-12 hrs, rhizoids (R) and pseudocystidia emerge from small swellings at the abdomen. The rhizoids anchor the animal to the ground and inhibit any further locomotion. After 15 hrs, conidiophores and primary aerial conidia emerge (C) and release. (3) After 24 hrs, when the ventral part of the fly is wetted or submerged, primary aquatic conidia are produced. Both types of conidia can be produced simultaneously in a single fly for up to 96 hrs after arrival at the oviposition site. Globose zygospores, however, stay embedded in the cadaver (teleomorphic form). (4) Aerial conidia can transform into secondary stellate aquatic conidia with typical tetraradiate symmetry upon submersion. Yet, it is not clear whether secondary hosts (zooplankton or desmids) are required for *Erynia* development because aquatic conidia were never observed to cause infection of *Simulium* (Webster 1992). Illustration drafted after descriptions of Hywel-Jones & Webster (1986).

Aquatic Fungi 235

diameter, only harbour a single fungal species with an evanescent low biomass. However, taking the size of a large water body and the high annual abundance of diatoms into account, the fungal biomass associated with these algae could exceed those growing within the whale carcass. Thus, substrate size is not the sole factor determining the

Aside from their dependence on substrate quality and quantity, fungi themselves harbour different morphologies, life stages and strategies. This is mainly due to the fact that aquatic fungi are derived from many fungal phyla comprising different cellular "blueprints" and life stages (see above). The diameter of a single fungal cell can roughly vary within an order of magnitude and there are numerous different spore morphologies extending from 1 µm small flagellated zoospores to several 100 µm large air-trapping conidia. An overview of

Fig. 4. Dimensions of vegetative growth forms and spores of aquatic fungi (republished

Many aquatic fungi are saprophytes, which consume dead organic matter (Dodds, 2002), but aquatic fungi may also be parasites or symbionts. In aquatic systems, the fungal community structure greatly differs between substrates (Shearer and Webster, 1985; Findlay et al., 1990; Bärlocher & Graça, 2002; Graça et al., 2002; Mille-Lindblom et al., 2006) and with the physico-chemical properties of the respective habitats, such as flow (Pattee & Chergui, 1995; Baldy et al., 2002), dissolved oxygen concentration (Field & Webster, 1983; Medeiros et al., 2009), nutrient concentrations (Gulis & Suberkropp, 2004; Rankovic, 2005), salinity (Hyde & Lee, 1995; Roache et al., 2006), temperature (Bärlocher et al., 2008) and depth (Wurzbacher et al., 2010). Therefore, fungal communities potentially differ between streams, shallow lakes and wetlands, deep lakes, and other habitats such as salt lakes and estuaries.

Upland stream habitats are characterised by a pool and riffle structure with relatively swift flow and high levels of dissolved oxygen. These streams are narrow and tend to be lined by overhanging riparian vegetation. These characteristics create an ideal habitat for aquatic hyphomycetes. Nikolcheva & Bärlocher (2004) have investigated the structure of fungal

importance of aquatic fungi in their natural habitat.

fungal dimensions in aquatic systems is shown in figure 4.

from Jobard et al., 2010, with permission).

**2.4.1 Fungal diversity in streams** 

**2.4 Diversity in large-scale aquatic habitats** 

Other filamentous fungi, such as endophytes or VAM fungi have a still more or less unknown life cycle. Though, it is similar to *Mucor* species in sediments, an interesting phenomenon occurs in this genus, which may be relevant to other fungi with yeast-like life stages. While *Mucor* usually grows in hyphal networks when oxygen is available, under certain circumstances (especially when growing anaerobically, at elevated pCO2), growth changes to a yeast-like morphology (Orlowski, 1991). This dimorphism is known of several yeast-like species such as *Aureobasidium pullulans* or *Candida* sp. and triggers a fast adaptation to changing environmental conditions. Yeasts and yeast-like organisms have often been isolated from freshwaters, a habitat varying greatly in time and space. For example, waves, chemical gradients and currents may be highly variable over time and hence, the capability to adapt rapidly to such changes is of great advantage.

Fig. 3. Asexual life cycle of aquatic hyphomycetes. Figure reproduced from Gulis et al. (2009) with permission.

### **2.3 Differences in fungal morphology and ecology**

Fungi can grow into the largest known organism on earth if the substrate is suitable and the environmental conditions favourable. In most cases, however, fungi remain invisible to the naked eye. Therefore, their global importance is seldom recognised even by scientists. Fungi literally tend to grow to the limit of their natural potential; the size of their cellular network is not genetically encoded, but defined by substrate and other environmental parameters. If, in the very unlikely event that a scientist attempted to prove that a whale could survive in freshwater, the whale's inevitable death would be rapidly followed by colonization of the gigantic carcass by coprophilus fungal species (as observed for various fish carcasses; Fenoglio et al., 2009). These fungi would flourish throughout the decomposition of the carcass and a single species could potentially establish an extensive network, exploiting a substantial portion of the whale's biomass. Most likely, the whale's carcass would harbour a very diverse fungal flora of several phyla and hundreds of species, supporting a whole benthic food web with nutrients and energy for years. In contrast, tiny substrates such as single celled diatoms of a few µm in

Other filamentous fungi, such as endophytes or VAM fungi have a still more or less unknown life cycle. Though, it is similar to *Mucor* species in sediments, an interesting phenomenon occurs in this genus, which may be relevant to other fungi with yeast-like life stages. While *Mucor* usually grows in hyphal networks when oxygen is available, under certain circumstances (especially when growing anaerobically, at elevated pCO2), growth changes to a yeast-like morphology (Orlowski, 1991). This dimorphism is known of several yeast-like species such as *Aureobasidium pullulans* or *Candida* sp. and triggers a fast adaptation to changing environmental conditions. Yeasts and yeast-like organisms have often been isolated from freshwaters, a habitat varying greatly in time and space. For example, waves, chemical gradients and currents may be highly variable over time and

hence, the capability to adapt rapidly to such changes is of great advantage.

Fig. 3. Asexual life cycle of aquatic hyphomycetes. Figure reproduced from Gulis et al.

Fungi can grow into the largest known organism on earth if the substrate is suitable and the environmental conditions favourable. In most cases, however, fungi remain invisible to the naked eye. Therefore, their global importance is seldom recognised even by scientists. Fungi literally tend to grow to the limit of their natural potential; the size of their cellular network is not genetically encoded, but defined by substrate and other environmental parameters. If, in the very unlikely event that a scientist attempted to prove that a whale could survive in freshwater, the whale's inevitable death would be rapidly followed by colonization of the gigantic carcass by coprophilus fungal species (as observed for various fish carcasses; Fenoglio et al., 2009). These fungi would flourish throughout the decomposition of the carcass and a single species could potentially establish an extensive network, exploiting a substantial portion of the whale's biomass. Most likely, the whale's carcass would harbour a very diverse fungal flora of several phyla and hundreds of species, supporting a whole benthic food web with nutrients and energy for years. In contrast, tiny substrates such as single celled diatoms of a few µm in

(2009) with permission.

**2.3 Differences in fungal morphology and ecology** 

diameter, only harbour a single fungal species with an evanescent low biomass. However, taking the size of a large water body and the high annual abundance of diatoms into account, the fungal biomass associated with these algae could exceed those growing within the whale carcass. Thus, substrate size is not the sole factor determining the importance of aquatic fungi in their natural habitat.

Aside from their dependence on substrate quality and quantity, fungi themselves harbour different morphologies, life stages and strategies. This is mainly due to the fact that aquatic fungi are derived from many fungal phyla comprising different cellular "blueprints" and life stages (see above). The diameter of a single fungal cell can roughly vary within an order of magnitude and there are numerous different spore morphologies extending from 1 µm small flagellated zoospores to several 100 µm large air-trapping conidia. An overview of fungal dimensions in aquatic systems is shown in figure 4.

Fig. 4. Dimensions of vegetative growth forms and spores of aquatic fungi (republished from Jobard et al., 2010, with permission).
