Another terrestrial role for fungi that's minimized in aquatic habitats is that of the mycorrhizae. Just about every terrestrial flowering plant, including each species of tree in the rain forest, has its own species of co-evolved mycorrhizal fungi, which coat its microscopic root hairs and penetrate the very cells of the root. The mycorrhizae bring water to the roots and share with the plant those essential nutrients that only fungal enzymes can "digest" out of the soil. The "higher" or vascular plants originally evolved on land. When some of them colonized freshwater, they largely left behind their symbiotic fungi. The mycorrhizae weren't necessary in the aquatic environment, and most of our familiar water plants don't have them.

Fungal roles in water. Fungi rot the wood and leaf litter in tropical water. The fungi that have been most thoroughly studied are terrestrial, but their ecological role in freshwater is important: fungi are the main decomposers of softer plant and animal tissues . And they are the only organisms that can oxidize lignin directly.

Water-saturated wood is a suitable substrate for fungal growth, as long as there is enough oxygen in the water: indeed the powerful carbon-to-carbon bonds of lignin that form the skeletal structure of wood can only be broken down by fungal enzymes. (Bacteria can break down the cellulose in plant walls, but not lignin.) Those enzymes need plentiful oxygen to create highly oxidative free radicals in order to crack the powerful chemical bonds of lignin and cellulose. By contrast, without oxygen--— for instance at the bottom of an anaerobic bog--— wood can lie for thousands of years without rotting. Closer to home, the enzyme cellulase, derived from fungi, produces your "stone-washed" jeans by dissolving away the outermost layers of cellulose-rich cotton fibers, thus releasing some dye.

Two of the main divisions in the kingdom of fungi are the basidiomycetes and the ascomycetes.

Comparatively few basidiomycetes are found on submerged wood, according to a recent Thailand study. Though the particular assemblage of fungi was different at each site, most of the fungi in leaf litter that has been washed into tropical streams turn out to be ascomycetes, normal terrestrial mitosporic fungi, the same Thailand study found. These are the fungi that break down leaf litter on the forest floor. "Mitosporic" simply means that these fungi normally reproduce vegetatively, dividing and elongating and forming asexual spores, rather than sexually, by exchanging genetic material within the nucleus.

In contrast to the basidiomycetes, a wide assemblage of the other major group of fungi, the ascomycetes, are more or less adapted to life in the water. All they need is some vascular plant material, alive or dead, either growing in the water, or emerse like the stems of reeds and rushes, or else to be washed in from the surrounding terrain on woody debris or even in leaf litter. The planted aquarium is a haven for them.

The filaments of the ascomycetes invade submerged plant stems, woody substrates and senescent leaves. They secrete enzymes to break down the cellulose of cell walls, break down the pectins that hold cells together and assimilate stored sugars and starches. These decomposers serve as food for the primary grazers in the biofilm.

Without passing through their sexual state, the freshwater ascomycetes constantly produce, at the ends of some hyphae, filamentlike or hollow spores ("conidia") that float or entangle or stick to new substrates. Eventually, if conditions are good, the ascomycetes may also form sexual fruiting bodies, in the form of a disc or a sac or cup that is more or less closed, according to the species, usually less than half a millimeter across. Then they can release their spores, which are dispersed in the water currents to fetch up on new substrates. Many of the microscopic spores of these aquatic ascomycetes are provided with filaments and gelatinous coverings to help them get entangled and stick fast in suitable places. The structures of these details are extremely various. So both the sexually produced spores and the conidia are distributed all through the freshwater planted aquarium. They provide mysterious subjects in floc to puzzle amateur microscopists.

At the website "Freshwater Ascomycetes and their anamorphs" (anamorphs are the non-sexual fungal life stages) you can see micropix of ascomycetes species and get some background information about the group of ascomycetes to be found in freshwater.


Yeasts. Also at home in water are yeasts, the unconnected, rounded, budding forms of fungi. Instead of forming a filamentous mycelium from hyphae like most of the fungi, the yeasts are constantly budding and pinching off, fragmenting and multiplying in small clusters of cells. The yeasts have a page to themselves in this folder.


Water molds. As the biofilm develops, the spores of funguslike water molds (Oomycetes) will also settle on any suitably "biodegradable" surface that they are able to penetrate with their extending rootlike process and digest.

The water molds or Oomycetes, Saprolegnia and its clan (the "Saprolegniales"), are partners with bacteria and true fungi in decomposing cellulose and lignin. In the aquarium, at the early stage in the biofilm's development, the fungal mycelia have few competitors for space. Saprophytic water molds can form large whitish colonies, especially on wood, in the newly set-up tank. But fungi have such a messy manner of feeding that they attract tablemates. Fungi must exude enzymes to decompose their nourishing substrate, to break down complex organic structures into soluble sugar units and other simple molecules that can diffuse through their cell walls. Then they absorb the molecular soup they have created in a microscopic layer surrounding the fungal mycelium. The immediate neighborhood of thriving water molds and fungi offers rich rewards for bacteria. Before long, as benthic populations mature, open space will be at a premium, and saprophytic fungi will assume their usual less important role in underwater decomposition, as symbiotic partners of bacteria. Ordinarily you won't see mats of fungal hyphae forming unless there's been a temporary windfall of degradable tissues, like a dead fish. Then the saprophytic fungi will experience a population boom. But fungi and fungal spores are a major food source for many protists, and for nematodes and other minute organisms. So the resource that sparks a boom is followed by a crash, like the population of a gold-rush camp.

As a rule fungi are aerobic, but under anoxic conditions the Oomycetes can switch to fermentation like yeasts. There are more than 500 species of water molds, but that figure includes those oomycetes that inhabit the water films of damp terrestrial soils, such as the one that caused the potato blight in Ireland of 1845-1848, or the closely related ones that are now causing "sudden oak death" among native Californian oaks and Coast Redwoods and the East Coast canker of beeches.

Most water molds prefer clean cool waters, but a few thrive in polluted streams. Most water molds don't tolerate much salinity. Thus you raise the temperature and add salt to counter "fungal fin rot." (But since the kingdom of fungi always offers exceptions, a few of the Saprolegniales are found in slightly brackish waters.)

Oomycetes do operate like typical fungi in many ways. When a fungal spore germinates, it begins a budding process, building a branching and self-grafting network of hyphae. Throughout their structure hyphae remain only a single cell thick. Fungal growth is largely confined to the tips of the hyphae, which elongate in the water and will also penetrate cells. You can easily see that, in comparison to its volume, the network structure offers huge surfaces for nutrient absorption. That's important for oomycetes and fungi, for absorption is the only way they can "feed."

The oomycetes are saprobic, that is, subsisting on dead organic matter and helping decay it. But since they absorb their pre-digested food rather than ingest it whole or envelope it, say, as an amoeba does, a parasitic life-style is a natural opportunity for them. When they invade living organic matter, we consider them parasites. Opportunistic Saprolegnia can attack a weakened living fish, in the guise of "mouth fungus" or "body fungus." Saprolegnia and its kin are the only group of water molds that can attack fish eggs or tissues of living fish.

But are Oomycetes fungi after all? Freshwater oomycetes are among the "primitive" group of fungi that produce motile zoospores in sac-like spore-cases called sporangia --that is, if they are in fact fungi at all! Some aspects of oomycetes set them apart from all others in the kingdom of fungi. Their zoospores have flagella, which enable them to swim in water. That's not very fungal. And there are other very fundamental differences from the typical members of the fungal kingdom. The Oomycetes contain a unique mix in their cell walls of cellulose compounds and glycan, whereas the other four phyla of fungi construct cell walls containing chitin, (which is also the material of insect exoskeletons). That's a pretty basic metabolic difference. And since oomycetes spend most of their life in the diploid state (like plants and animals), rather than in the haploid state (like other fungi), some biologists are questioning now whether the oomycetes have any true connection with the fungi at all! DNA analysis seems to confirm that they're only very distantly related, if they do have any common ancestry. So stay tuned! There's more about the possible "kingdom of Chromists," where some biologists would associate oomycetes with diatoms and even with kelp, at the Berkeley website, www.ucmp.berkeley.edu

But in the aquarium we still think of the Saprolegniae as fungi.

Fungi link. A really broad portal to web information concerning every aspect of fungi is at http://mycology.cornell.edu/fteach.html

Slime molds (myxomycetes). Slime molds only make rare appearances in aquaria. Probably just as well, since a blob of protoplasm that very slowly shifts from one place to another may raise alarms. Slime molds commonly occur in microhabitats where bacterial populations are dense. Decaying submerged wood is the usual substrate for the aquatic myxomycetes, but their common lifestyle is ordinarily in the form of microscopic amoeboflagellate cells that live independent lives, feeding mostly on bacteria but also ingesting fungal spores and algal cells. But then, at a chemical signal, the cells congregate, moving together to join into a plasmodium, the single, amorphous, slowly-shifting mass you might see. It's a whitish blob of protoplasm like a big amoeba but with many nuclei, which very slowly shifts about. Beverly Erlebacher's slime-mold post "What was the white monster crawling in my tank?" in response to a newsgroup post is archived at theKrib.com.

Myxomycetes are more common in damp forest ecosystems than underwater. The Myxomycetes homepage gives you some general information but makes the merest mention of aquatic slime molds: "Didymium aquatile has been found on submerged plant material and Didymium difforme is capable of completing its entire life cycle under water. Plasmodia have been observed submerged in glass flower vases."

In Veracruz, Mexico, the plasmodia are collected and fried and eaten as the untranslatable caca de luna.