Preliminary insights into the evolutionary relationships of aquatic hyphomycetes and endophytic fungi

Aquatic hyphomycetes play a key role in leaf litter decomposition and are mediators of organic matter turnover in streams. Molecular studies have shown that some aquatic fungi are also plant endophytes, however, more evidence is needed to evaluate their multiple ecological abilities. To date, little information is available on fungal lineages that might have undergone convergent evolution to adapt to multiple ecological modes. We examined the phylogenetic relationships and evolutionary divergences of aquatic hyphomycetes, endophytic aquatic hyphomycetes and other fungal endophytes of riparian/terrestrial plants by analyzing ITS1-5.8S-ITS2 sequences retrieved from the National Center for Biotechnology Information (NCBI). Sequences with close phylogenetic affinity to aquatic fungi can occur as endophytes of terrestrial plants or in soil far from streams. To fully understand the ecological impact of aquatic hyphomycetes, we need to document and interpret their niches more broadly.     

Does the growth rate hypothesis apply to aquatic hyphomycetes?

The growth rate hypothesis states that in many organisms or tissues, the specific growth rate μ correlates with RNA concentrations. Since RNA often accounts for much of the phosphorus content of cells, μ may also correlate positively with P concentrations and negatively with C:P and N:P ratios. We tested this hypothesis with broth cultures of five aquatic hyphomycete species. Samples were harvested on eight occasions after 3–56 d of incubation. Accumulation of biomass was fitted to a rectangular hyperbola, whose parameters were used to estimate μ. There were no consistent trends related to culture age or μ for C, N, P or ergosterol concentrations. RNA and DNA concentrations and RNA:DNA ratios were significantly and negatively correlated with culture age. Only RNA concentrations were positively and linearly correlated with μ. While RNA or DNA concentrations are unsuitable as indicators for total biomass, levels of fungal RNA combined with markers for fungal biomass may allow estimates of the extent to which the mycelia are metabolically active.     

Diversity patterns of leaf-associated aquatic hyphomycetes along a broad latitudinal gradient

Information about the global distribution of aquatic hyphomycetes is scarce, despite the primary importance of these fungi in stream ecosystem functioning. In particular, the relationship between their diversity and latitude remains unclear, due to a lack of coordinated surveys across broad latitudinal ranges. This study is a first report on latitudinal patterns of aquatic hyphomycete diversity associated with native leaf-litter species in five streams located along a gradient extending from the subarctic to the tropics. Exposure of leaf litter in mesh bags of three different mesh sizes facilitated assessing the effects of including or excluding different size-classes of litter-consuming invertebrates. Aquatic hyphomycete evenness was notably constant across all sites, whereas species richness and diversity, expressed as the Hill number, reached a maximum at mid-latitudes (Mediterranean and temperate streams). These latitudinal patterns were consistent across litter species, despite a notable influence of litter identity on fungal communities at the local scale. As a result, the bell-shaped distribution of species richness and Hill diversity deviated markedly from the latitudinal patterns of most other groups of organisms. Differences in the body-size distribution of invertebrate communities colonizing the leaves had no effect on aquatic hyphomycete species richness, Hill diversity or evenness, but invertebrates could still influence fungal communities by depleting litter, an effect that was not captured by the design of our experiment.     

Aquatic hyphomycetes in a changing environment

In 1942, Ingold documented an ecologically defined group of fungi, aquatic hyphomycetes, on autumn-shed leaves decaying in streams. They were shown to be vital intermediaries between the nutritionally poor leaf substratum and leaf-eating invertebrates. Research has subsequently emphasized functional aspects such as leaf decomposition and nutritional conditioning by fungi. Structural aspects (community composition) have attracted less attention, partly because of the difficulties of identifying fungal mycelia in situ. Extraction, amplification (PCR, qPCR) and characterization of DNA and RNA, and, more recently, of proteins, allow much greater insights into the presence of fungal taxa, their metabolic status (dead, dormant or active), and their potential and actual participation in decomposition processes. This approach can yield huge amounts of data, and major challenges today are the development and application of suitable bioinformatics techniques. The complexity of data collection and evaluation favour interdisciplinary teams of researchers. Fungi are major players in most ecosystems and are increasingly affected by human impacts. Changing land use, eutrophication/pollution and climate change are among the major factors that affect diversity and ecological functions of aquatic hyphomycetes.     

Colonization pattern of aquatic hyphomycetes on leaf packs in subtropical stream

Steralized leaf pieces of eight plants (Eucalyptus rostrata, Phoenix dactylifera, Phragmites australis, Musa nana, Salix subserrata, Cyperus alopecuroides, Ricinus communis and Eichhornia crassipes) were submerged in the Nile stream. Thirty-nine species of aquatic hyphomycetes were colonized on the plant leaves. Eucalyptus was the best substratum (30 species) for colonization by aquatic hyphomycetes. Phoenix (14 species), Phragmites (11 species), Salix (9 species), Musa (8 species), Ricinus and Cyperus (6 species) were also good substrata for aquatic hyphomycetes colonization. Echhornia (aquatic plant) was not suitable for aquatic hyphomycetes colonization. Alatospora acuminata, Triscelophorus monosporus and Tetracladium marchalianum were found to be the major colonizers on all leaf materials. Temperature and dissolved oxygen were the highest physico-chemical parameters affecting the aquatic hyphomycetes colonization. These results indicate that aquatic hyphomycetes have been shown to be successful colonizers on plants leaf in river Nile in Egypt as subtropical region. This revised version was published online in June 2006 with corrections to the Cover Date.     

Allelopathic inhibition of primary producer growth and photosynthesis by aquatic fungi

Autochthonous primary production is generally much reduced in forested headwater streams. Several hypotheses have been proposed for explaining this observation, among them, the low light intensity, or the strong constraints exerted by stream current. Allelopathic inhibition of competitors is a common ecological process in aquatic environments. Aquatic hyphomycetes are known to chemically inhibit bacteria and other fungi (including other aquatic hyphomycetes) but a possible allelopathic effect of aquatic hyphomycetes on primary producers has never been tested. The inhibitory effect of twelve aquatic hyphomycete species was tested on three diatom species. Nine aquatic hyphomycete species exhibited anti-diatom activity. Up to 100% diatom growth inhibition was observed. Our study reveals that such allelopathic interactions might be common in streams and probably involve an array of fungal compounds. We propose that the generally reduced primary production observed in forested headwater streams is, among other factors, due to the inhibition of primary producers by allelopathic compounds released by aquatic hyphomycetes.     

Effects of temperature and light on growth and sporulation of aquatic hyphomycetes

Nine isolates of the aquatic hyphomycetes Dactylella aquatica, Flagellospora penicillioides, Flagellospora saccata, Helicomyces sp., Lunulospora curvula, Phalangispora constricta, Tetracladium setigerum, Vermispora cauveriana and Wiesneriomyces laurinus were incubated at different temperatures (5–35 °C) to study their growth on MEA medium. Maximum growth was observed between 20 and 30 °C. Growth rate was highest in Vermispora cauveriana and lowest in Tetracladium setigerum. Colonies on submerged leaves showed maximum spore production at 25 °C. Light was confirmed as a stimulus to sporulation.     

Raised water temperature lowers diversity of hyporheic aquatic hyphomycetes

1. The hyporheic zone of a permanent first‐order stream was divided into a treatment and a control section using a 1 m deep sheet‐metal barrier. During a 4‐month pre‐treatment period, water temperatures in two transects of the two sections were not different. Upon heating, the water temperature in the treatment transect increased by an average of 4.3 °C over values in the control transect. 2. Eleven bimonthly core samples were taken from a treatment and a control transect, and recovered CPOM was classified as twigs, wood, grass, roots, cedar and deciduous leaves. 3. In both transects, twigs were the most common and deciduous leaves the least common substrates. The number of leaf fragments declined significantly in the heat‐treated transect. 4. Diversity and frequencies of occurrence of aquatic hyphomycetes were highest on leaves and lowest on grass and wood. On leaves, their frequency of occurrence was higher in control than in treatment samples. 5. Preliminary results with amplified and cloned 18S DNA sequences revealed many fungal taxa with high affinities to Basidiomycota, particularly to Limnoperdon incarnatum. 6. By itself, higher water temperature due to global warming is likely to lower the availability of substrates for, and therefore the occurrence of, aquatic hyphomycetes.     

Diversity of aquatic hyphomycetes in the aquatic ecosystems of the Western Ghats of India

Relationships between physico-chemical factors of waters, riparian vegetation, altitude and species richness of the hyphomycete communities in six rivers and a sulfur spring in the Western Ghats of Karnataka were analysed statistically. Linear regression and correlation between log fungal species and riparian vegetation was highly significant. The similarity in the fungal diversity between different rivers and the sulphur spring was evaluated by Sorensen's index. Generally, the indices of similarity in the mycoflora between the streams were high. However, in the streams of Lakya, Bhadra, Ranganathittu bird sanctuary and the sulfur spring the indices of similarity were low compared with the rest of the water systems. The low pH, low oxygen concentration, hardness, high iron, high temperature and sulfide contents of the waters of these streams are thought to be the reasons for the differences in these streams. Anguillospora longissima, Helicomyces roseus, Lunulospora curvula, Triscelophorus monosporus and Wiesneriomyces laurinus were found to be the most abundant species.     

Laccase activity of lignicolous aquatic hyphomycetes isolated from the River Nile in Egypt

Eleven species of aquatic hyphomycetes were isolated from 92 samples of different lignin sources (unidentified wood segments, skeleton and neck of leaves, bark). The most common species were Pyramidospora casuarina (on 3.7% of samples), Triscelophorus monosporus (3.2%) and Flagellospora curvula (3%). Varying levels of laccase activity were present in most of the fungi included in this study. The laccase plate assay was found to be much less reliable than the spectrophotometric assay. Several factors including type of growth medium, the media pH and assay pH had marked effects on laccase activity. A few species produced high levels of laccase in both malt extract (ME) medium and low N medium; however, a majority of the species produced laccase in low nitrogen (N) medium (pH 4.5) but not in the ME medium. When the tested species were grown in low N medium at pH 4.5, six species showed acidic laccase (pH 4.5) activity; of these, four also showed alkaline laccase (pH 8.2) activity. Alatospora acuminata and Tetracladium marchalianum exhibited laccase activity only when grown in the low N medium at pH 8.2. These results indicate that aquatic hyphomycetes may play a role in the decomposition of lignin materials in freshwater environments. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of cadmium,copper, and zinc on growth and thiol content of aquatic hyphomycetes

The effects of cadmium, copper, and zinc on the growth of ten strains ofaquatic hyphomycetes were investigated. On a solid medium, Cd and Cu reducedradial growth of most strains by 50% at concentrations between150–400 µM; in a liquid medium, the strains were more sensitive.The inhibitory effects of zinc were much less severe. Two isolates(Articulospora tetracladia and Tetracladium marchalianum) from a copper-minestream were more resistant against copper than conspecific strains from anon-polluted stream. Heliscus lugdunensis and Varicosporium elodeaeresponded to Cd exposure, but not to Cu or Zn exposure, by increasedsynthesis of SH-containing compounds. Glutathione levels showed a unimodalresponse to increasing Cd and Zn exposure. With copper, glutathionedecreased at intermediate levels of contamination. In the presence of Cd, H. lugdunensis synthesized several unknown sulfur-rich substances that wereabsent or produced at reduced rates in control cultures.     

Enzymatic studies of the extracellular mucilage of two aquatic hyphomycetes,Lemonniera aquatica andMycocentrospora filiformis

The effect of three carbohydrate-digesting enzymes, β-glucuronidase, lyticase and α-mannosidase and three proteolytic enzymes, α-chymotrypsin, papain and pronase E, on the strength of conidial attachment ofLemonniera aquatica andMycoentrospora filiformis was determined using the LH_Fowler cell Adhesion Measurement Module. Carbohydrate-digesting enzyme treatments showed significant differences in number of attached and detached, conidia versus control samples; little or no effect was observed for the proteolytic enzymes. Scanning and transmission electron microscopy showed different degrees of mucilage digestion by the carbohydrate-digesting enzymes on the germ hyphae, hyphae subtending appressoria, and appressoria of the two species. The loss of mucilage integrity and decrease in mucilage thickness were more pronounced on the hyphal sheaths than on the appressorial sheaths. Lyticase caused the most severe damage to the mucilage and cytoplasm of both fungi, particularlyL. aquatica. β-Glucuronidase and α-mannosidase exhibited more effective mucilage digestion onM. filiformis than onL. aquatica. Results indicate that the mucilage of the two species is mainly polysaccharide, containing more β-1,3-glucans than β-glucuronide and α-mannosyl residues. Variability of mucilage composition exists between these species and also between different structures of the same fungus.     

Intraspecific diversity affects stress response and the ecological performance of a cosmopolitan aquatic fungus

Fungi play an important role in organic matter turnover and ensure key ecosystem services in freshwaters. The relationships between intraspecific fungal diversity and key ecological processes remain largely unknown. We examined the effects of intraspecific diversity of Articulospora tetracladia, a cosmopolitan fungal decomposer thriving on plant detritus in streams. Alder leaves were inoculated with 1 or mixtures of 2–8 fungal strains for 35 d, and leaf litter decomposition and fungal reproduction were quantified in the presence and absence of 2 mg L⁻¹ of cadmium (Cd), a common stressor in polluted streams. Intraspecific diversity and identity affected fungal reproduction, but not leaf decomposition. Under metal stress, leaf decomposition slightly increased with intraspecific diversity. Fungal reproduction increased with intraspecific diversity and was greater in mixed assemblages, either in the absence or presence of Cd. Effect size of intraspecific diversity was higher under Cd stress for fungal reproduction, but no differences were found for leaf mass loss, with or without metal. The impacts of intraspecific diversity loss may jeopardize fungal survival and fungal functions, namely microbial leaf decomposition and leaf litter condition for invertebrate shredders in streams, particularly under metal stress.     

中国梧桐科上的尾孢类丝孢菌

报道生在中国梧桐科Sterculiaceae上的5种尾孢类丝孢菌（cercosporoid hyphomycetes）,其中在梧桐 Firmiana simplex 上新发现的2个新种,梧桐假尾孢 Pseudocercospora firmianae,梧桐生假尾孢 P. firmianicola和一个中国新记录种,野路葵假尾孢 P. corchorifoliae。提供了拉丁文特征简介、形态描述、绘图及讨论。研究的标本保存在中国科学院菌物标本馆（HMAS）。     

Biotic interactions between aquatic,aero-aquatic and terrestrial hyphomycetes in microcosm

The duplex coefficient of specific correlation was applied for the first time for the estimation of possible interaction between water and terrestrial hyphomycetes (Alternaria sp. and Fusarium sp.) and oomycete Achlya sp. under microcosm conditions. Values of the either direct or indirect coefficient for correlation between water and terrestrial hyphomycetes and oomycete Achlya sp. were predominantly negative. Values of the coefficient for correlation between more frequent water hyphomycete species were more often positive or near 0. Rare water hyphomycete species had usually more or less distinct negative correlation with other water hyphomycete species.     

Selective feeding by shredders on leaf-colonizing stream fungi: comparison of macroinvertebrate taxa

Summary Laboratory experiments were conducted to examine the effects of fungal species composition of leaf detritus on the feeding of distantly related macroinvertebrate shredders. Preferences of shredders representing three orders of insects (Diptera: Tipulidae; Plecoptera: Pteronarcidae; Trichoptera: Limnephilidae and Calamatoceridae) and one each of gastropods (Basommatophora: Planorbidae) and crustaceans (Amphipoda: Gammaridae) were compared. Shredder preferences were based on consumption of leaves separately colonized by one of eight species of aquatic hyphomycetes. The feeding patterns of the invertebrates ranged from lack of feeding to heavy consumption of fungal-colonized leaves. Where consumption occurred, rank orders of preference and degree of selectivity differed among invertebrate shredders. Differences in preferences together with relationships between degree of selectivity and the relative mobility and digestive specializations exhibited by shredders suggest that the exploitation of fungal-colonized leaf detritus by different taxa is affected by phylogenetic constraints. Our results suggest that fungal species composition affects the feeding of a variety of shredders and that fungal species composition may be as important as degree of conditioning in determining food selection by shredders.     

Effect of nutrients on the sporulation and diversity of aquatic hyphomycetes on submerged substrata in a Mediterranean stream

Eutrophication in streams often results in enhanced decomposition rates and chemical conditions that greatly affect the suitability of the habitat for many species. We performed an enrichment experiment in a 50-m reach of a Mediterranean stream by the continuous addition of N and P (enriched reach) while another consecutive reach upstream served as the reference (unenriched reach). After 1.5 years of enrichment, conidia production and species composition of aquatic hyphomycetes were measured in communities colonizing four streambed substrata (Platanus acerifolia and Populus nigra leaves, epipsammic and epilithic biofilms). Samples collected from the two reaches were subjected to two nutrient treatments in the laboratory [low nutrient (−NP) and high nutrient (+NP) concentrations] to test for short-term (48 h) nutrient effects. Long-term nutrient additions did not affect potential sporulation rates while short-term nutrient enrichment produced insignificant increase in these rates on most substrata collected in the unenriched reach. Major differences in potential sporulation rates were observed between substrata, the highest rate being for leaves (especially Platanus acerifolia). However, enrichment produced significant changes in fungal community composition. Clavariopsis aquatica, Alatospora acuminata and Lemonniera sp. were dominant in the enriched reach while Heliscella stellata, Trisceloporus acuminatus and Clavatospora longibrachiata were characteristic of the unenriched reach. Our results suggest that fungal N demands can be fulfilled at relatively low levels of dissolved nitrate and further increases in the nutrients availability may not result in enhanced fungal activity.     

Interspecies and intraspecies interactions in social amoebae

The stable co‐existence of individuals of different genotypes and reproductive division of labour within heterogeneous groups are issues of fundamental interest from the viewpoint of evolution. Cellular slime moulds are convenient organisms in which to address both issues. Strains of a species co‐occur, as do different species; social groups are often genetically heterogeneous. Intra‐ and interspecies 1 : 1 mixes of wild isolates of Dictyostelium giganteum and D. purpureum form chimaeric aggregates, following which they segregate to varying extents. Intraspecies aggregates develop in concert and give rise to chimaeric fruiting bodies that usually contain more spores (reproductives) of one component than the other. Reproductive skew and variance in the proportion of reproductives are positively correlated. Interspecies aggregates exhibit almost complete sorting; most spores in a fruiting body come from a single species. Between strains, somatic compatibility correlates weakly with sexual compatibility. It is highest within clones, lower between strains of a species and lowest between strains of different species. Trade‐offs among fitness‐related traits (between compatible strains), sorting out (between incompatible strains) and avoidance (between species) appear to lie behind coexistence.     

Species diversity of freshwater hyphomycetes in some streams of Pakistan. III. Autumnal colonization of freshwater hyphomycetes on bait leaves

Significantly more species of freshwater hyphomycetes colonized bait leaves of alder (16 species) and willow (14 species) during early submersion period (1 wk) than those of oak (8 species). Production of conidia was also higher on alder and willow leaves than on oak leaves. Higher amounts of dry mass were lost from the alder (28.6%) and willow leaves (29.4%) than from the oak leaves (18.7%). Rapid loss of dry mass was accompanied by rapid development of a fungal community. Fungal species took a longer time (6 wk) to reach their peak of occurrence on oak than on willow and alder leaves (3–4 wk).Flagellospora curvula dominated the assemblages of freshwater hyphomycetes on alder and willow leaves, andLunulospora curvula dominated the assemblage on oak leaves. The freshwater hyphomycete community showed higher values of species diversity (H value) on oak leaves than on alder and willow leaves.