March 10, 2021

Our friends, the fungi

We're likely the only fish geeks you'll encounter who constantly sing the praises of biofilms and fungi huh?

Fungi.

Yeah, those guys.

They're all over our botanical-style aquariums, regardless of how we feel about them....

Not the ones that we vilify for attacking our fish or their eggs, of course. It's easy to just heap them in with the "bad guys" and the nasty implications they have.

Nope. There's more to it than that.

Fungi are "omnipresent" in aquatic ecosystems- even our allegedly "pristine" aquariums.

Fungi reproduce by releasing tiny spores that then germinate on new and hospitable surfaces (ie, pretty much anywhere they damn well please!). These aquatic fungi are involved in the decay of wood and leafy material. And of course, when you submerge terrestrial materials in water, growths of fungi tend to arise.

Anyone who's ever "cured" a piece of aquatic wood for your aquarium can attest to this!

Fungi tend to colonize wood because it offers them a lot of surface area to thrive and live out their life cycle. And cellulose, hemicellulose, and lignin- the major components of wood and botanical materials- are degraded by fungi which posses enzymes that can digest these materials! Fungi are regarded by biologists to be the dominant organisms associated with decaying leaves in streams, so this gives you some idea as to why we see them in our aquariums, right?

And of course, fishes and invertebrates which live amongst and feed directly upon the fungi and decomposing leaves and botanicals contribute to the breakdown of these materials as well! Aquatic fungi can break down the leaf matrix and make the energy available to feeding animals in these habitats.

And look at this little gem I found in my research:

"There is evidence that detritivores selectively feed on conditioned leaves, i.e. those previously colonized by fungi (Suberkropp, 1992; Graca, 1993). Fungi can alter the food quality and palatability of leaf detritus, aecting shredder growth rates. Animals that feed on a diet rich in fungi have higher growth rates and fecundity than those fed on poorly colonized leaves. Some shredders prefer to feed on leaves that are colonized by fungi, whereas others consume fungal mycelium selectively..."

"Conditioned" leaves, in this context, are those which have been previously colonized by fungi! They make the energy within the leaves and botanicals more available to higher organisms like fishes and invertebrates!

Interestingly, many fungi are "specialists", with what ecologists classify as "Ingoldian anamorphic fungi" being most numerous on decaying leaves, while fungi known as ascomycetes (order Dothideomycetes, Sordariomycetes) are prevalent on submerged/exposed woody substrata, like twigs, tree trunks, and bark. Unique fungi are found in tropical waters and differ from those in temperate locations.

There are around 3000 species that are known to be associated with aquatic habitats! That's A LOT of species!

Aquatic “true fungi” are known as "osmoorganotrophs", a fancy way of saying that they absorb nutrients across their cell wall. Most of them have a "filamentous" morphology at some point during their life cycle. This morphology enables them to invade deep into substrates and to directly digest particulate organic matter (POM) to acquire nutrients for growth and reproduction.

The fungal community consumes microscopic algae, aquatic macrophytes and terrestrial plant litter (including wood). Aquatic fungi act as very significant decomposers of particulate organic matter (POM), specifically coarse particulate organic matter (CPOM), which includes both plant and animal material .

We see this in Nature- and absolutely in our aquariums!

The aquatic fungi which will typically decompose leaf litter and wood are the group known as “aquatic hyphomycetes”. Another group of specialists, "aero-aquatic hyphomycetes," colonize submerged plant detritus in stagnant and slow- flowing waters, like shallow ponds, puddles, and flooded forest areas. Fungal communities differ between various environments, such as streams, shallow lakes and wetlands, deep lakes, and other habitats such as salt lakes and estuaries.

And we see them in our own tanks all the time, don't we?

As we know by now, fungi play a huge role in the decomposition of leaves, both in the wild snd in the aquarium. By utilizing special enzymes, aquatic fungi can degrade most of the molecular components in leaves, such as cellulose,, hemicelluloses, starch, pectin and even lignin.

Depending on various factors, such as leaf litter type and the local water chemistry, fungal decomposition of leaves can take anywhere from 1 month to 6 months.

The fungi known as aquatic hyphomycetes produce enzymes which cause leaf maceration, and in as little as 2 to 3 weeks, as much as 15% of the decomposing leaf biomass in many aquatic habitats is processed by fungi, according to one study I found.

Fungi, although not the most attractive-looking organisms, are incredibly useful...and they "play well" with a surprisingly large number of aquatic life forms to create substantial food webs, both in the wild and in our aquariums!

Natural habitats are absolutely filled with this stuff...it's like the whole game here- an explosion of life-giving materials, free for the taking...

Another "mental shift", I suppose...one which many of you have already made, no doubt. I certainly look forward to seeing many examples of us utilizing "what we've got" to the advantage of our fishes! AGAIN: A truly "Natural" aquarium is not sterile. It encourages the accumulation of organic materials and other nutrients- not in excess, of course.

Biofilms, fungi, algae...detritus...all have their place in the aquarium. Not as an excuse for lousy or lazy husbandry- but as supplemental food sources to "power" the life in our tanks.

And of course, as we've discussed many times here, fungi are actually an important food item for other life forms in the aquatic environments tha we love so much! In one study I stumbled across, gut content of over 100 different aquatic insects collected from submerged wood and leaves showed that fungi comprised part of the diet of more than 60% of them, and, in turn, aquatic fungi were found in gut content analysis of many species of fishes!

Still need more convincing about the value of fungi to food webs? Check out this passage from a paper I found by Werzbacher Christian, et al.:

"Microbial mineralisation of plant litter supports a complex food-web including all kinds of microbes (Archaea, Bacteria, fungi, protozoans) and invertebrates (nematodes, trematodes, gammarids, insects, snails). As a consequence, plant litter even supplies top predators such as crayfishs, amphibians, birds, fishes and bats with organic matter and energy via the microbial food web. The main basis of the microbial food web consists of fungi and bacteria growing in and on the plant debris..."

Fungi perform key roles in transferring terrestrial materials, like leaves- otherwise unavailable for aquatic organisms - to higher trophic levels. Ya' know- like, our fishes!

These small, seemingly "annoying" life forms are actually the most beautiful, elegant, beneficial friends that we can have in the aquarium. When they arrive on the scene in our tanks, we should celebrate their appearance.

Stay the course.

Don't be afraid.

Don't reach for the scrub brush.

Open your mind.

Study what is happening. Draw parallels to the natural aquatic ecosystems of the world. Look at this with wonder, awe, and courage. And know that the pile of decomposing goo and fungal strands that you're looking at now all over your leaf litter is just a metaphorical "stepping stone" on the journey to an aquarium which embraces Nature in every conceivable way.

Stay inspired. Sty curious. Stay patient. Stay open-minded...

And Stay Wet.

Scott Fellman

Tannin Aquatics