Fungal zoospores in aquatic ecosystems

Article By:

Sime-Ngando, Télesphore Laboratoire Microorganismes: Génome et Environnement, Université Blaise Pascal, Aubière Cedex, France.

Last reviewed:2013

DOI:https://doi.org/10.1036/1097-8542.YB130024

Fungi represent one of the last frontiers of the undiscovered biodiversity and the related functions that challenge aquatic microbial ecology today. The number of fungi present on Earth has been estimated to be approximately 1.5 million species, of which about 97,000 have so far been identified, corresponding mostly to species thriving in moist soils, lotic (actively moving water) systems, mangroves, and wetlands, or to economically interesting pathogens of humans, animals, and plants. Recent environmental ribosomal deoxyribonucleic acid (rDNA) surveys of microbial eukaryotes have unveiled a large reservoir of unexpected fungal diversity in pelagic (open water) systems, emphasizing their ecological potentials for ecosystem functioning, and have opened new perspectives in the context of food-web dynamics. Typical pelagic fungi are members of the phylum Chytridiomycota, which occupies the basal branch of the kingdom Fungi. These true fungi usually produce small zoospores that typically have a single, posteriorly directed flagellum. The so-called zoosporic fungi, or chytrids, are universally present in the world's pelagic ecosystems, where they play major roles, primarily as parasites and saprotrophs (utilizers of dead organic matter). These roles, however, remain mostly cryptic in classical microscopy studies because chytrids are small in size and lack conspicuous morphological features, a situation that makes them hardly distinguishable from many flagellated protists (for example, sessile choanoflagellates or bicosoecids). Previously, the modes of nutrition for all heterotrophic flagellates (which ingest organic compounds for their nutrition) in the plankton were thought to be restricted to bacterivory, but zoosporic fungi are not bacterivores (that is, bacterial feeders). It is now clearly evident that (1) not all heterotrophic flagellates thriving in pelagic systems are either protists or bacterivores, and (2) parasitism and saprophytism from fungal flagellates might represent important potential functions in these ecosystems.

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