Beneath the quiet waters of a small lake, bog, swamp or within the waters of a running brook or river, unseen but active, a distinctive group of fungi lives and reproduces. The filamentous bodies of these fungi invade submerged substrates (dead stems and leaves of herbaceous and woody plants). These fungi are aquatic ascomycetes. Penetrating their substrates, aquatic ascomycetes release enzymes that break down the ligno-cellulose of plant cell walls, the pectins that hold cells together, and starch stored in plant tissues. The fungi then use the resulting simple sugars and amino acids to grow. In carrying out their enzymatic activities, these fungi contribute to the decomposition of dead plant material and serve as food for invertebrate grazers.

The ascomycetes grow vegetatively as long, septate filaments (hyphae) about 3-5 microns wide, but eventually undergo sexual reproduction to produce a fruiting body in the shape of a disc, cup or flask with a narrowed neck. Most of the freshwater ascomycetes are very small (fruiting bodies are usually less than 0.5 mm in diameter) and cannot be seen in detail without the aid of the microscope. Within the fruiting body they reproduce sexually to form ascospores in a sac (ascus). The ascospores are liberated from the ascus in a variety of ways and are then dispersed via water currents to new substrates. The ascospores may be variously shaped: round, oval or long and filamentous. Many of the ascospores are equipped with gelatinous appendages or sheaths that are thought to help them attach to substrates in flowing or moving water. To observe some of the morphological features of the freshwater ascomycetes, click on the Species Monograph. In the browsing mode, click on a species name to bring up the pictures of the organism.

The freshwater ascomycetes (teleomorphs) often have asexual states (referred to as anamorphs or mitosporic states) in their life cycles. The asexual states of freshwater ascomycetes are often adapted to aquatic life by producing spores (conidia) that are long and filamentous or branched; this facilitates their attachment to substrates in moving water. Some species have conidia that are tightly coiled to trap air or are hydrophobic and hollow; these adaptations allow them to float on the surface of water until they attach to a new substrate. For a list of the asexual states known for freshwater ascomycetes, click on the Teleomorph/Anamorph Database.

For more detailed information about these fungi, read the following Background section.

Definition. About 530 species of ascomycetes have been reported from freshwater habitats (Shearer, 2001; Shearer et al. 2007). This number has increased by about 300 species since an initial comprehensive review of freshwater ascomycetes (Shearer 1993). Freshwater ascomycetes occur in both lentic and lotic habitats, primarily as parasites and endophytes of aquatic macrophytes and algae, and as saprophytes on dead plant material. As with aquatic and wetland plants (Correll and Correll 1975), a precise definition of what constitutes a freshwater ascomycete is problematic. Presence in water, alone, may not be an appropriate definition. Some ascomycetes reported from freshwater habitats also occur in terrestrial habitats. Whether their occurrence in freshwater is fortuitous or whether they are able to grow, reproduce and maintain their populations without continuous immigration from land is not known for species.

In most aquatic habitats there is a submersion gradient from land to water. The positions of such gradients may fluctuate seasonally and/or episodically with weather events, thus it is difficult to establish precise aquatic boundaries. Likewise, the degree of submergence of the substrates of freshwater ascomycetes will vary with fluctuations in water level. Thus if the definition of presence in water is used, the same species may be considered terrestrial or freshwater depending on the water level at the time that they are observed. As more collections from freshwater habitats are made, our ability to define this mycota more precisely will improve. The freshwater ascomycetes, although monophyletic at the class level, comprise a diverse assemblage of species with representatives in a variety of disparate orders (e.g., Leotiales, Pezizales, Rhytismatales, Eurotiales, Amphisphaeriales, Diaporthales, Hypocreales, Halosphaeriales, Sordariales, Xylariales, Pleosporales and Melanommatales). Thus definition by taxonomy is not useful. As a working definition, all ascomycetes that occur on submerged or submerged parts of partially submerged substrates are considered freshwater ascomycetes. Results from past studies indicate that many freshwater ascomycetes are known only from water and occur over broad geographic areas. Recently a new order Jahnulales was established within the Dothideomycetes (loculoascomycetes) for a monophyletic group of freshwater lignicolous ascomycetes (Pang et al. 2002; Campbell et al. in review). Other species are known only from water and occur at only a few sites but occur in consecutive years at their respective sites. This suggests that a distinctive freshwater ascomycota does exist. It is only through detailed, extensive worldwide surveys and sequencing the fungi found in these habitats, along with phylogenetic analyses that a freshwater ascomycota can be confirmed and characterized.

Geographical Distribution. There are so few comprehensive studies of freshwater ascomycetes of different geographical areas that little can be said about their worldwide geographical distribution patterns, except that they reflect the geographical distribution of their collectors (Shearer 1993, Hyde et al. 1997, Shearer 2001). In recent surveys conducted by the Shearer Lab, the ranges of all the previously known species collected were extended. For example, Pseudoproboscispora caudae-suis was known from only its type locality in the Lake District of England (Ingold 1951). We have collected this fungus 37 times in North America. It occurs at latitudes ranging from 30°-55° N and longitudes 3°-124° W; however, it has not yet been reported from the eastern or southern hemisphere. The southern most distribution of Pseudoproboscispora caudae-suis in North America is the Apalachicola National Forest (29-30° N) in Florida. Distinctive species distribution patterns for a number of frequently occurring species were revealed as a result of our collections: (1) Species which occur over a broad latitudinal range, e.g., Aniptodera chesapeakensis, Pseudoproboscispora caudae-suis, Halosarpheia retorquens, Kirschsteiniothelia elaterascus, Lutrellia estuarina, Massarina ingoldiana, Nais inornata, Pseudohalonectria lignicola, Submersisphaeria aquatica and Savoryella lignicola. Not surprisingly, many of these species were among the first to be described from freshwater habitats. (2) Species restricted to latitudes of about 45° and above, e.g., most Discomycetes, Macrospora scirpicola; (3) Species restricted to latitudes of about 45° and below, e.g., Cleistothecial sp. A-99, Boerlagiomyces websteri, and Zopfiella lundqvistii, (4) Species that appear to occur only in the tropics, e.g., Halosarpheia heteroguttulata, Fluviatispora reticulata, Phaeonectriella lignicola, Aniptodera lignatilis, Aniptodera palmicola, Aniptodera inflatiascigera; and (5) Species that appear to be limited to their host distribution patterns, e.g., Massariosphaeria scirpina, Ascovaginospora stellipala, Phaeosphaeria typhae (Shearer 2001), Discomycete species F-47 (Raja and Shearer unpublished). For the vast majority of species, however, occurrences are too few in number to elucidate distribution patterns at this time. Clearly much more work is needed in this area. With respect to species richness along latitudinal gradients, if all species reported worldwide are considered, a pattern of greatest richness in mid-latitudes in both the northern and southern hemispheres, with lower richness at low and high latitudes is revealed (Shearer et al. 2003).

Preliminary Beta diversity (variation in species composition among sites in a geographic area) values between freshwater ascomycetes collected in Wisconsin (46 °N latitude), and Florida (29-25 °N latitude) were quite high (about 0.90) suggesting a complete difference in species composition between the temperate and subtropical fresh water sites within North America. These preliminary beta diversity results provide important implications for conservation of freshwater habitats, since high species turnover reflects deterministic processes such as adaptations to climate, substrate or habitat type and influences diversity. Knowledge of the geographical distribution patterns of freshwater ascomycetes is important since freshwater habitats are declining at an alarming rate due to human activities and climate change (Dudgeon et al. 2006). Perturbation of aquatic habitats can lead to loss of species, which may further lead to alteration of ecological processes and ecosystem function (Firth and Fisher 1991, Kareiva et al. 1993).

Substrate Distribution. A variety of aquatic macrophytes, including species of Carex, Juncus, Phragmites, Scirpus and Typha serve as substrates for freshwater ascomycetes. Although species occurrences on aquatic macrophytes have been reported sporadically in taxonomic treatises (Mueller 1950, Holm 1957, Munk 1957, Leuchtmann 1984), the first systematic efforts to collect freshwater ascomycetes on aquatic macrophytes were by Ingold (1951, 1954, 1955) and Ingold and Chapman (1952). These pioneering studies were followed by those of Webster and Lucas (1961), Dudka (1963), Pugh and Mulden (1971), Apinis et al. (1972a, b), Taligoola et al. (1972), Magnes and Hafellner (1991) and Fallah and Shearer (2000). Although Ingold (1955) predicted a rich pyrenomycetous mycota on aquatic macrophytes, this substrate type has been largely unstudied. Data from recent studies support Ingold's prediction and amplifies it in that the ascomycota on aquatic macrophytes is not only rich in pyrenomycetes but also in other groups such as the loculoascomycetes and discomycetes (Magnes and Hafellner 1991, Fallah and Shearer 1998a-c, Fallah et al. 1998, Fallah and Shearer 2000). Allochthonous woody debris as a substrate for freshwater fungi has been studied more frequently (e.g., Dudka 1963, Shearer 1972, Willoughby and Archer 1973, Lamore and Goos 1978, Minoura and Muroi 1978, Shearer and von Bodman 1983, Shearer and Crane 1986, Revay and Gonczol 990, Shearer and Webster 1991, Hyde et al. 1998, Ho et al. 2002, Raja and Shearer 2003; 2006a,b; Raja et al. 2005), and numerous ascomycetes have been reported from these studies (Shearer 2001; Shearer et al. 2007). Ascomycetes have also been reported from water-saturated wood in cooling towers, a sort of artifical tropical habitat (Eaton and Jones 1971a, b, Eaton 1972, Udaiyan 1989, Udaiyan and Hosagoudar 1991). In addition, many new taxa of ascomycetes have been described from wood in freshwater (e.g., Webster 1959, Webster, 1965, Webster and Lucas 1961, Tubaki 1966, Jones and Eaton 1969, Iqbal 1972, Minoura and Muroi 1978, Shearer 1978, Shearer and Crane 1978, Shearer and Crane 1980a, b, Abdullah and Webster 1981, Fisher and Petrini 1983, Fisher and Webster 1983, Monod and Fisher 1983, Shearer 1984, Shearer 1989 a, b, Webster et al. 1991, Crane et al. 1992, Hyde 1992a-c, 1993, 1995a,b, 1996, Crane and Shearer 1995, Shearer and Crane 1995, Hyde et al. 1996, Shearer and Hyde 1997, Hyde 1998, Shearer and Crane 1998, Fallah et al. 1998, Shearer et al. 1999; Tsui et al. 2002, 2003; Ho et al. 200; Dhanasekaran et al. 2005; see the World Database for a complete summary). Although the fungi colonizing allochthonous deciduous leaves have been studied more intensively than those on wood, only a few meiosporic ascomycetes have been reported from this substrate type (Shearer 1992). Allochthonous deciduous leaves are colonized primarily by Ingoldian mitosporic ascomycetes and a few basidiomycetes (Bäerlocher 1992).

Habitat Distribution. A number of studies on freshwater ascomycetes have been conducted at spatially limited sites in either lotic (Lamore and Goos 1978, Shearer and Von Bodman 1983, Hyde and Goh, 1997; Hyde et al. 1998, Hyde and Goh 1998b, Hyde and Goh 1999, Sivichai et al. 2000, 2002, Tsui et al. 2001, 2003, 2003, Tsui and Hyde 2004, Kane et al. 2002, Ho et al. 2001, 2002, Cai et al. 2003, Fryar et al. 2004) or lentic habitats (Shearer and Crane 1986, Hyde and Goh 1998a, Goh and Hyde 1999, Fallah et al. 1999, Cai et al. 2002, Luo et al. 2004). However, there have been no studies that target both habitat types within a small relatively homogenous geographical area. To better understand the distribution of organisms, it is important to understand their associations with different habitat types. More studies have been carried out in lotic waters than in lentic waters, therefore, basic knowledge of freshwater ascomycetes exists but at present, it is not yet clear whether there is a unique group of ascomycetes in lentic habitats. Fallah (1999) conducted a study of freshwater ascomycetes on submerged wood and herbaceous material in lentic habitats in Northern Wisconsin and reported 10 new species and two new genera of ascomycetes with little species overlap in species composition among the lakes and bogs studied. Goh and Hyde (1999) conducted a study of freshwater fungi from a single reservoir in Hong Kong and found eight new species of ascomycetes. More recently, several species of ascomycetes were described from peat swamps in Thailand (Pinruan et al. 2002, Pinruan et al. 2004a, b, Pinnoi et al. 2003, Pinnoi et al. 2004). A number of these fungi appear to be unique to lentic habitats and have not been previously reported from lotic waters. Additional studies comparing lentic and lotic habitats are needed to determine whether lentic habitats contain species of freshwater ascomycetes that are restricted to these habitat types. Studies from our lab thus far suggest that the composition of the freshwater ascomycota differs between lentic and lotic habitats, but that some geographically broadly distributed species occur in both habitats but numerous species occur in either lotic or lentic habitats. Lentic habitats might support more unique communities of freshwater ascomycetes, but we need additional comparative collections in different geographical locations to support this initial observation.

Systematics. The systematics of ascomycetes in general is at present controversial and taxonomic schemes abound (Hawksworth, et al. 1995). The classification system of Barr (1987, 1990) as modified by new molecular based information (Myconet, http://www.fieldmuseum.org/myconet/ ) is used in World Monograph. Based on Barr's classification scheme, ~ 500 species of ascomycetes in 20 orders have been reported from freshwater habitats. Of the known species, 111 are Discomycetes, 257 are Pyrenomycetes and 162 are Loculoascomycetes. Orders with 10 or more representatives from fresh water are Sordariales (82 spp.), Leotiales (101 spp.), Pleosporales (106 spp.), Melanommatales (30 spp), Eurotiales (25 spp.), Halosphaeriales (25 spp.), Hypocreales (14 spp.), Jahnulales (18 spp). These orders represent distinctly different evolutionary lines, indicating that adaptation to the freshwater habitat has occurred numerous times. At the same time, it appears that certain evolutionary lines (Leotiales, Pleosporales, Sordariales) have been more successful than others in the freshwater habitat. Numerous newly described freshwater ascomycetes are incertae sedis above the generic level. Hopefully molecular phylogenetic studies of freshwater ascomycetes will help resolve the phylogenetic relationships and taxonomic placement of these taxa.

The mitosporic ascomycetes found in fresh water can be divided into several groups: the Ingoldian hyphomycetes, the aeroaquatic hyphomycetes, the dematiaceous hyphomycetes, and the coelomycetous hyphomycetes or the (miscellaneous mitosporic fungi) Shearer et al. (2007). These taxa are somewhat easy to identify because of their morphologically distinctive spores. Their phylogenetic relationships, for the most part, are unknown except for a few studies (see Belliveau and Bärlocher 2005; Baschein et al. 2006; Campbell et al. 2006). Isolations of freshwater ascomycetes and phylogenetic analyses of molecular sequences should help to identify the phylogenetic relationships of freshwater mitosporic ascomycetes.

The taxonomic diversity of freshwater ascomycetes and the absence of taxonomic keys for their identification make them difficult to identify. There are several general works on ascomycetes that have proved to be useful (Dennis 1978, Kohlmeyer and Kohlmeyer 1979, Kohlmeyer and Volkmann-Kohlmeyer 1991, Korf 1973, Luttrell 1973, Mueller and v. Arx 1973, Sivanesan 1984) and monographs of specific groups (Mueller 1950; Holm 1957; Munk 1957; Eriksson 1967; Hedjaroude 1969; Lundqvist 1972; Barr 1978, 1987, 1990; Leuchtmann 1984, Shoemaker and Babcock 1989). More recently two books on Freshwater ascomycetes have been published by Tsui and Hyde (2003), and Cai et al. (2006), which can be used for the identification of some tropical freshwater ascomycetes. Some descriptions and illustrations are now available on this site in the Species Monograph Database.

Evolution. It is likely that most freshwater ascomycetes have evolved from terrestrial ancestors via a variety of evolutionary pathways. A recent study by Dhanasekaran et al. (2006) on origins of freshwater ascomycetes supports the above hypothesis. One such pathway is that of pathogens, endophytes and saprophytes of wetland and aquatic plants. As these plants invaded freshwater habitats, they no doubt brought their associated microorganisms with them. Fungal species able to survive and adapt to aquatic habitats may have been ancestral to present-day species that occur on freshwater macrophytes. Included in this group are primarily Discomycetes and Loculoascomycetes. A second route to freshwater habitats may have been on riparian vegetation in the form of tree and shrub litter. This debris carries with it a large variety of fungi capable of adapting to freshwater. Included in this assemblage are Discomycetes, Hypocreales, Sordariales, and Loculoascomycetes. A third route to freshwater habitats may have been through the run-off of rainwater and sediments that carry propagules of species in the Eurotiales, Sordariales and Hypocreales. One way to test these hypotheses is by collecting and isolating freshwater ascomycetes for use in molecular phylogenetic analyses of ascomycetes from terrestrial and freshwater habitats. Kohlmeyer and Kohlmeyer (1979) suggested that extant marine fungi could be divided into two groups based on their origins. Primary ascomycetes (e.g., Halosphaeriales) evolved directly from a marine ancestor, probably one common to both marine fungi and red algae. Secondary marine ascomycetes originated from terrestrial ancestors and are mostly saprophytic.

Kohlmeyer and Kohlmeyer (1979) agreed with Savile (1968) that parasites are more ancient than saprophytes and they suggested that the most ancient types of marine ascomycetes are Spathulosporales and some parasitic Xylariales (sub Sphaeriales) on marine Rhodophyta. Molecular data now available do not support the red algal ancestry of marine ascomycetes (Spatafora et al. 1998). Based on 18s rDNA, the scolecosporus freshwater ascomycetes do not appear to be closely related to species in the marine scolecosporous ascomycete genus, Lulworthia (Spatafora et al. 1998; Chen et al. 1999).

A large number of freshwater ascomycete taxa, ~70, are considered inc. sed. and cannot be accommodated in any existing orders or families. There are a number of single taxa (belonging to about 17 genera) that differ in morphology from known taxa of Ascomycetes and likely represent new evolutionary lineages. New, rare or unusual taxa found during the surveys need to be sequenced for18S small-subunit, SSU and 28S large-subunit LSU ribosomal genes to estimate their appropriate taxonomic placement at the generic and ordinal levels. These genes are appriopriate based upon the availability of pre-existing data in GenBank, which have been used successfully in previous phylogenetic studies of freshwater ascomycetes and in accordance with the AFTOL project, which is a broad-scale phylogenetic study of fungi.

Adaptations. Freshwater ascomycetes appear to have adapted morphologically to aquatic habitats in a variety of ways. One type of modification involves the presence on ascospores of viscous, sticky appendages that may enable the ascospores to stick onto substrates in moving water. Species of freshwater ascomycetes in Aniptodera, Aquadiscula, Pseudoproboscispora, Ceriosporopsis, Halosarpheia and Nais have ascospores equipped with viscous appendages. The appendages of Aniptodera and Halosarpheia spp. unwind to form extremely long, sticky threads which may help to entangle the ascospores with substrates (Shearer and Crane 1980b). The ascospores of several loculoascomycetes are equipped with sticky gelatinous sheaths (Aliquandostipite, Jahnula, Leptosphaeria, Luttrellia, Massarina, Phaeosphaeria, Pleospora, Trematosphaeria) or apical gelatinous appendages (Flammispora, Jahnula, Lophiostoma, Rebentischia, Wettsteininia). Since most of the foregoing loculoascomycete genera also contain terrestrial species with ascospore sheaths or apical appendages, aquatic representatives may have been pre-adapted. When introduced into water, terrestrial ancestors with ascospore sheaths or appendages may have been better able than species lacking these features to attach to substrates in water and thus were more likely to succeed. The cells at the base of the ascomata of all species of Jahnulales produce broad, brown, thick-walled hyphae that spread across the substrate and often connect adjacent ascomata. Initially these broad hyphae were considered to be algal associates (Hyde and Wong 1999), but more recently Pinruan et al (2002) considered this unlikely. These thick filaments are connected to and/or develop from peridial cells, and hence, are fungal (Raja and Shearer 2006). These large, brown hyphae are produced in cultures derived from single ascospores of Jahnulales species. It is hypothesized that these repent, connecting hyphae may play an important role in colonizing and holding fungi onto softened wood in wet or aquatic habitats, which may represent another adaptative feature for an aquatic existence (Raja and Shearer 2006; Dhanashekaran et al. 2006). However, unlike other adaptive characters, which are homoplastic due to convergent evolution, the thick walled hyphae developing from the ascomata of members of the Jahnulales is an evolutionarily informative character (Pang et al. 2002; Campbell et al. in review).

Many marine ascomycetes have evolved a fascinating array of ascospore appendages formed by the fragmentation and/or extension of outer ascospore walls (Jones and Moss 1978). None of the appendages reported thus far for freshwater ascomycetes appear to result from these mechanisms. The sigmoid spore form as an adaptation to freshwater and marine habitats has been discussed frequently (Webster and Davey 1984, Webster, 1987; Jones, 2006). The sigmoid shape increases the area of orthogonal projection and the long filamentous structure enhances entanglement with substrates. Sigmoid ascospores are found in numerous freshwater Discomycetes (Apostemidium, Loramyces, Niptera, Obtectodiscus, Vibrissea) and Pyrenomycetes (Ophioceras, Plagiosphaeria, Pseudohalonectria).

Ascomal and ascus structure and function do not seem to be modified for aquatic habitats. There is little difference in these structures between aquatic and terrestrial counterparts. Deliquescent asci are present in some genera (Aniptodera, Fluviatispora, Halosarpheia, Nais). It is thought that this is an adaptation to water, as ascospores can be washed from ascomata and forcible discharge is not necessary. An interesting situation exists in Pseudohalonectria (Shearer 1989b) and Ophioceras (Conway and Kimbrough 1978) in which asci separate from ascogenous hyphae, are discharged, and assume a sigmoidal shape in water. Ascospores and asci are released simultaneously in submerged culture. The larger size of the ascus, compared to a single ascospore may improve the chances of impaction on substrates.

Numerous freshwater ascomycetes have adapted to freshwater via their anamorphs (Shearer 1993, Sivichai and Jones 2003; this site – Anamorph/Teleomorph Database). Anamorphs with branched, tetraradiate, sigmoid, helicosporous and multicellular air-trapping conidia have been reported for freshwater ascomycetes. The functional significance of helical spore forms in aquatic habitats has been discussed by Ingold (1975) and Webster (1987, 1992). The sigmoidal conidial form appears to have evolved in several different evolutionary lines. It occurs in discomycetes, pyrenomycetes, and loculoascomycetes. Comparable adaptation through modification of anamorphs has not occurred as frequently in marine ascomycetes. Only three similar teleomorph-anamorph connections have been made for marine ascomycetes (Shearer and Crane 1971, Nakagiri and Tubaki 1983; Kohlmeyer et al. 1998). This is not surprising given that seawater apparently selects against conidial forms (Shearer 1972). Connections between freshwater ascomycete teleomorphic states and aquatic hyphomycete anamorphic states are relatively rare. Continued efforts to connect anamorphs and teleomorphs of freshwater ascomycetes are needed to shed light on evolutionary relationships in both the freshwater ascomycetes and Ingoldian, aeroaquatic and the miscellaneous mitosporic fungi.

© Carol Shearer & The University of Illinois at Urbana-Champaign