The role of fungi in carbon and nitrogen cycles in freshwater ecosystems

Introduction Fungi are adapted to a diverse array of freshwater ecosystems. In streams and rivers, flowing water provides a mechanism for downstream dispersal of fungal propagules. The dominant group of fungi in these habitats, aquatic hyphomycetes, have conidia that are morphologically adapted (tetraradiate and sigmoid) for attachment to their substrates (leaf litter andwoody debris from riparian vegetation) in flowing water (Webster, 1959; Webster & Davey, 1984). In freshwater wetlands and lake littoral zones, production of emergent aquatic macrophytes is often extremely high, resulting in an abundance of plantmaterial that eventually enters the detrital pool. The dead shootmaterial of thesemacrophytes (leaf blades, leaf sheaths and culms) often remains standing for long periods of time before collapsing to the sediments or water. This plant matter is colonized by fungi that are adapted for surviving the harsh conditions that prevail in the standing-dead environment (Kuehn et al., 1998). There are a number of other freshwater ecosystems where fungi are present and exhibit interesting adaptations, e.g. aero-aquatic fungi in woodland ponds, zoosporic organisms (Chytridiomycota and Oomycota) in a variety of habitats including the pelagic zones of lakes, and Trichomycetes that inhabit the guts of a variety of aquatic insects. Despite the well-known occurrence of these fungal groups in aquatic habitats, virtually nothing is known concerning their roles in biogeochemical processes. Overall, the contributions of fungi to biogeochemical cycles have been understudied in most freshwater ecosystems. Most studies examining fungal participation in biogeochemical cycles in freshwater ecosystems focused on the role of fungi in the decomposition of plant litter.Historically, the lackof appropriatemethods toaccuratelyquantify