Freshwater Fungi

Abstract The shift from terrestrial to freshwater ecosystems is accompanied by a dramatic shift in the decomposer fungal assemblages. The life cycle of freshwater fungi complicates our understanding of the phylogenetic relationships in this group. 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. The main ecological functions of freshwater fungi involve the breakdown of leaves, conifer needles, wood and other sources of coarse‐particulate organic matter. Training students in this field of fungi is imperative given the increase in global transportation of microbes, the rapid loss of healthy freshwater habitats due to human perturbation and global warming, the serious problem of antibiotic resistance and the rarity of trained taxonomists and molecular systematists in the field of mycology. Key Concepts Freshwater fungi are an ecological group of fungi and comprise both the meiosporic and mitosporic ascomycetes. They are important decomposers in freshwater habitats as they produce different types of enzymes to breakdown organic matter. The meiosporic ascomycetes are adapted to fresh water as their ascospores are equipped with gelatinous appendages or sheaths that are thought to help them attach to substrates in flowing or moving water. The mitosporic ascomycetes are adapted to freshwater habitats as they produce conidia, which are branched or long and filamentous, or have conidia which trap air for buoyancy and/or are hydrophobic. Freshwater fungi are understudied source of novel secondary metabolites. Phylogenetic studies on freshwater ascomycetes have shown that some species cluster with terrestrial ascomycete lineages while others belong in exclusive aquatic lineages. Distributional patterns of freshwater ascomycetes show distinct latitudinal and elevational structuring, but additional comparative studies are required to better predict distribution patterns of freshwater fungi. Species composition varies between lentic and lotic habitats, but more studies are warranted. Some species are distributed worldwide in similar habitats – A pantropical distribution of freshwater ascomycetes exists. As threats to freshwater biodiversity escalate, there is an urgent need to survey, collect and isolate freshwater fungal species, especially the poorly known but important taxa from the remaining high‐quality habitats. The knowledge of this group is essential to our understanding of ascomycete systematics in general as well as the systematics of freshwater ascomycete fungi.