FUNGI AND THE CARBON CYCLE
Barron, G. L. 2003. Predatory fungi, wood decay, and the carbon cycle. Biodiversity, Volume 4: 3-9.
**** See also War of the Microworlds - How do I kill thee? Let me count the ways. Apologies to EBB.
Abstract: Predatory fungi attack nematodes and other microorganisms using a remarkable array of trapping devices to attract, capture, kill, and digest nematodes for food. The novelty of these relationships, however, has deflected attention from a more fundamental role played by these fungi in the ecosystem. The primary function of predatory fungi appears to be that of wood decay and hence they are cellulolytic or ligno-cellulolytic fungi that attack other organisms as sources of nitrogen to supplement a primarily carbohydrate (woody) diet. The Carbon to Nitrogen ratio (C:N) of wood is extremely high and nitrogen is the limiting factor for growth. For decay fungi, predation of nematodes or other organisms adds extra protein (nitrogen) to the system and reduces the C:N to manageable proportions. More importantly, nematode predation, although dramatic, is perhaps of less importance than the ability of wood decay fungi to attack bacteria and perhaps other life forms as nutrient supplements. By definition, therefore, many (most?) wood decay fungi, are not saprobes (i.e. live on dead organic material) but are facultative parasites (saprobes that can also parasitize living organisms for part of their life in which the predatory parasitic phase runs parallel to the saprophytic wood decay phase and both are essential to success. Based on their roles in building up woody material through mycorrhizal associations, and destroying it through biological decay, it is not surprising that the biomass of fungi in forest soils reaches 90% of the total and exceeds all other micro- and meso-organisms combined. Based on the respiration of their massive amounts of hyphal material, fungi are the driving force in the biological component of the terrestrial Carbon Cycle.
Predatory fungi capture, kill, and digest other microorganisms such as nematodes (Phylum Nematoda) for food. Nematodes are tiny, non-segmented animals that are protected by a transparent cuticle. They are elongate, taper at both ends, and mostly measure from 100-1000 microns. They are also known as roundworms or eelworms but are not related to the true segmented worms (Phylum Annelida). Nematodes can be parasitic on plants, animals, or fungi. The majority, however, are free living and feed on bacteria, fungus spores, etc. for sustenance. In active soils the number of nematodes can reach twenty million per square metre.
Predatory fungi have developed an astonishing array of trapping devices to capture nematodes as a food source. Species of the asexual fungus Arthrobotrys (Hyphomycetes, Deuteromycota) are probably the most common and certainly the best known predators of nematodes and have been the topic of many scientific and popular articles. The strategy for capture is fairly straightforward. Branching hyphae of the fungus ramify through soil, compost, dung, rotting wood, or wherever nematodes abound. Traps are initiated in response to a trap-inducing compound released by the nematodes themselves and are formed at intervals along the length of the hyphae. The trapping system is analogous to a fishing line with hooks at intervals.
In species of Arthrobotrys, traps take many forms but the best known are adhesive (sticky) knobs, adhesive nets (click here and click here), or rings (click here). The most sophisticated trap is a constricting ring composed of three cells. When the nematode sticks its head or tail in a ring, the physical contact triggers the ring and the cells expand rapidly inwards crushing the victim (click here and click here), which is then penetrated and digested within hours. Fungal enzymes break down the body contents of the nematode and the nutrients are translocated elsewhere within the hyphal system for growth or to produce conidiophores and conidia (asexual spores, click here and click here). Spores are carried off by wind, water, mites, insects or whatever and start the predatory cycle again at a new location. It has been shown that trapping devices produce a chemical lure that is attractive to nematodes and this bait attracts the victims to the site of their destruction. Much of the earlier work on this group of fungi has been reviewed in detail (Barron 1977).
For many years these dramatic and fascinating methods for capture of nematodes by predatory fungi distracted observers from the true significance of this unique relationship. We had occasion however, to do a detailed study on another genus of the asexual, predatory fungi called Nematoctonus and this work gave exciting clues to a much more fundamental relationship.
Nematoctonus is unique amongst predators in possessing clamp connections (click here) scattered along the hyphae (Drechsler 1941) and uses distinctive, hour-glass-shaped, adhesive cells to capture the nematodes (click here). Clamp connections are specialized features of certain hyphae that indicate unequivocally that the fungus is a species of the Basidiomycota (mushroom group). Thus, Nematoctonus, classified in the Hyphomycetes of the Deuteromycota, is closely related to mushrooms. It is also well established that species of the Basidiomycota are the most capable degraders of woody stuff (Hudson, 1972). Most significantly, the fungal nematode predator belonging to the genus Nematoctonus, produces both cellulases and lignases, the principal enzymes used by wood decay fungi! Interestingly, and equally significant, in our earlier studies we tested eighteen species of Arthrobotrys and found all of them produced cellulases and some were potent producers of this enzyme (click here). Cellulose and lignin are the stuff of plants, never animals, and at first thought production of cellulases and lignases seems odd for animal predators such as Arthrobotrys and Nematoctonus.
It was our practice at Guelph to isolate predatory fungi from parasitized nematodes and grow them in pure cultures on nutrient agar in Petri dishes. Amongst these, we recovered isolates of several Nematoctonus species. To our surprise and delight, one of our Nematoctonus isolates produced a mushroom (Basidiomycota) in the Petri dish (click here). This mushroom, although small, had fully developed gills, the basidia were fully formed and the basidiospores were discharged actively from the gills. The mushroom was identified by David Malloch (University of Toronto) as a species of Hohenbuehelia, a close relative of Oyster Mushroom (Pleurotus ostreatus).
To establish the proof of parasitism, we discharged the basidiospores onto nematode cultures in Petri dishes. In the presence of nematodes the basidiospores germinated to produce hour-glass-shaped, adhesive knobs typical of Nematoctonus (click here). The spores adhered to the nematodes cuticle by means of these adhesive knobs then the fungus penetrated and killed the host. We had confirmed the connection between Nematoctonus (asexual state) and Hohenbuehelia (sexual state) and discovered the first carnivorous mushroom (Barron and Dierkes 1977).
In our paper on these observations, we suggested that nematodes trapped by the fungus hyphae are merely dietary supplements and that lignin and cellulose may be the main energy sources. The implication is that the mushroom fungus Hohenbuehelia (with a conidial state called Nematoctonus) is primarily a wood-rotting fungus that has learned the trick of capturing nematodes as a nutritional supplement, and the predatory habit that has attracted so much attention over the years is a secondary capability.
Wood Decay and Nitrogen Availability
In natural habitats, population densities are high with severe competition for available nutrients. In many habitats nitrogen is in such short supply that it is the limiting factor to growth. Most organisms cannot grow and multiply successfully unless they can obtain a continuing source of useable nitrogen for the production of all the nuclei acids, proteins, enzymes and other compounds essential to growth. Nitrogen compounds are not just lying around in the habitat or at least not for long. Most of the organic nitrogen in the natural world is already tied up in other living things and, if the need is desperate, then living things are the obvious available source. Freshly dead stuff is OK too, but getting access is a bit of a dogfight with stiff competition from a myriad of opportunistic microorganisms. Exploitation of recently-dead, organic remains also favours organisms with mobility and this excludes the predatory fungi. Thus, in response to their nitrogen requirements, wood decay fungi, such as Hohenbuehelia, have evolved unique methods to satisfy their nutritional demands and capture animals as nutritional sources. Why wood decay fungi? Why nematodes?
The Earth is covered with a mantle of organic debris from generations past. The vast bulk of this is from dead, herbaceous or woody plants. The persistent fractions of woody debris are mostly carbohydrates, composed of cellulose and lignified cellulose. The latter is a hard, recalcitrant compound that is very difficult to degrade. But as noted earlier fungi, particularly Basidiomycota, are adept at rotting wood (click here).
We are well aware that in our own nutrition for a healthy diet, we need the proper balance of protein (N) and carbohydrate (C). This is no less true for other organisms, and a proper balance of C to N is essential for successful growth of all living things. This is often expressed as the ratio of C to N or C:N. Although the ratio varies a lot amongst organisms, most require a C:N ranging around 30:1 for good growth.
There is very little nitrogen available in wood and, therefore, the C:N in wood is very high. It can be 300:1 to 1000:1 or even higher. In theory, because of these extremely low nitrogen levels, it does not seem possible for saprobic fungi to decay wood! There are many theories as to how fungi manage to rot wood and I do not have space to evaluate these at this time. Suffice to repeat that:
This leads to the general hypothesis that to satisfy their nitrogen requirement it is necessary for wood decay fungi to obtain nitrogen compounds directly from other life forms and that the predatory capability of wood decay fungi has evolved in response to this biological imperative. The predation of nematodes by Hohenbuehelia/Nematoctonus is considered a specific example of this phenomenon.
As a corollary, it might be mentioned that once fungi have found a way to capture and kill nematodes some might take the easy road, give up the cut and thrust of competition for woody debris and restrict their interest strictly to nematodes with associated loss of cellulolytic and lignolytic enzymes. We have found that some predatory fungi produce much less cellulase than others (unpublished results). Such fungi would eventually become specialized predators with nematodes as the only nutritional source and eventually lose cellulase capability.
Other Carnivorous Mushrooms
The discovery of a carnivorous mushroom and the hypothesis proposed above prompted two directions for further study. First, if a supplementary source of nitrogen is essential for the biodegradation of wood by fungi, then what other wood decay fungi use living nitrogen sources to satisfy this requirement? Second, what alternative living organisms can be utilized as nutrient sources?
There are about 25 species of Hohenbuehelia. We isolated or obtained cultures of a number of these (Thorn and Barron 1984) and found all of them captured nematodes in the same way as the original Hohenbuehelia isolate (N90). It is probable that all species of Hohenbuehelia capture nematodes using adhesive knobs and that any species thought to be a Hohenbuehelia that doesnt capture nematodes is not a Hohenbuehelia! Speculative statements like this can now be tested using molecular methods. Some Hohenbuehelia species were originally described in the closely related genus Pleurotus, a common wood decay mushroom with about 50 known species. We tested five species of Pleurotus, and found that they also exploited nematodes as a nutrient source but in a remarkably different fashion (Barron and Thorn 1987).
Pleurotus ostreatus, better known as the Oyster Mushroom (click here), produces tiny appendages on the vegetative hyphae and these secrete droplets of a potent toxin (click here). This toxin paralyses nematodes in seconds but does not kill them. The paralyzed victims are located by specialized directional hyphae, the cuticle is penetrated, and the contents digested (click here). All five species of Pleurotus tested captured and killed nematodes in this way. The toxin, to which we gave the trivial name ostreatin, was potent enough that white worms (Phylum Annelida) several mm long were eventually inactivated and died when given free rein to wander around on a culture of Pleurotus. Also, and remarkably, culture mites wandering around colonies of Pleurotus are either repelled or killed (unreported observation). It is noteworthy that the toxin is apparently not produced in the fruitbodies of Pleurotus, so mycophiles can relax!
The sticky knobs of Hohenbuehelia are nematode specific and these fungi are, apparently, restricted to this source of biological nitrogen. Pleurotus, on the other hand, with its non-specific toxin, has a much wider range of potential victims to attack. The host limits for Pleurotus have never been fully established. Toxin droplets can be instrumental, not only in supplying a nitrogen source to Pleurotus, but can also function as antifeedents that discourage grazing and help protect Pleurotus hyphae from fungus-feeding microfauna such as mites, springtails, water bears, as well as fungal-feeding nematodes. It is tempting to suggest that the predatory habit might have originated as a defence response by fungi against grazing microfauna. It was found that secretory appendages similar to those of Pleurotus are produced on hyphae of the lawn mushroom Conocybe lactea (click here). These apparently function to repel attacks by fungal feeding nematodes and although the nematodes can be killed by the toxin, Conocybe shows no interest in utilizing their bodies as a food source (Hutchison et al. 1995).
Bacteria as a Source of Nitrogen for Wood Decay Fungi
If nematodes are a nutritional supplement for wood decay fungi such as Hohenbuehelia and Pleurotus, we might ask what alternative organisms can serve this purpose?
To test for nematode predation using Pleurotus, we place a square cm from a pure culture of the fungus onto a water agar plate (very low nutrient) and the hyphae then grow thinly over and through the clear agar. Fifty or so washed nematodes are added to this culture and allowed to wander around until eventually they are paralysed, killed and digested by the fungus as described above. In this experiment the nematodes are washed, but not sterile, and they carry along with them bacteria that are present in our stock nematode cultures. As the nematodes move across the agar, some bacteria are dislodged, or are defecated, and eventually form small microcolonies (100-300 microns diam) scattered over the surface of the water agar (click here). Remarkably, we observed that the Pleurotus attacked, destroyed, and consumed the microcolonies of bacteria in the same way as if they were nematodes . From hyphae in the vicinity of a bacterial colony, very fine directional hyphae originated de novo and grew towards the bacterial colonies (click here) and (click here).
These hyphae then penetrate the microcolony (click here), form specialized feeding cells inside, secrete lytic and other enzymes that kill and degrade the bacteria (click here), and the fungus then absorbs the nutrients. Most importantly (as with nematodes) the fungus does not proliferate in the vicinity of the kill but translocates the nutrients back out through the directional hyphae to the hyphal grid for further growth and/or reproduction elsewhere. In the end, the bacterial colony has completely disappeared and the only evidence of its existence is a clump of empty, absorption cells that define the previous size and shape of the colony (click here). The predatory capability against bacteria has since been demonstrated for a number of wood decay fungi (Thorn and Tsuneda 1993).
Nematophagous fungi were originally thought to be strictly asexual with no sexual state known. In our work, we discovered the sexual state of Nematoctonus is the mushroom Hohenbuehelia (Basidiomycota). An exciting breakthrough by Pfister (1994) and more recently by others has discovered that several Arthrobotrys species have a sexual state in the cup fungus Orbilia (Ascomycota). Orbilia belongs to an entirely different group of fungi than Hohenbuehelia and is also commonly associated with wood decay. This is an interesting example of convergent evolution. It is presumed that the same biological forces that have resulted in wood-decay mushrooms developing predatory capabilities are also operative in wood decay sac fungi developing similar predatory capabilities but, of course, with different trapping devices for capturing nematodes. The comments made about the biology of Hohenbuehelia/Nematoctonus, therefore, hold equally for Orbilia/Arthrobotrys.
Stephanocysts are two-celled, hemispherical bumps produced on the hyphae of some species of Hyphoderma (click here). Hyphoderma is related to the bracket fungi and belongs to a group of common, wood decay fungi in which the fruitbodies are flat (=resupinate) and often look like paint blotches on twigs, branches or logs. Tzean and Liou (1993) showed that stephanocysts were in fact trapping devices to capture nematodes (click here). There are about 85 species described in Hyphoderma and not all produce stephanocysts. As with Hohenbuehelia, I suspect that those that dont produce stephanocysts are not in the same genus as those that do! We will have to see if the type species of Hyphoderma has stephanocysts or not before we resolve this problem. However, the importance of this contribution is the extension of nematode capture to another major group of wood decay fungi.
The results from our studies and those of others with wood decay fungi (mushrooms, bracket fungi etc) and litter fungi (including the commercial mushroom, Agaricus brunnescens, showed that a large percentage could attack and digest bacterial colonies in the same way as Pleurotus (Barron 1988). In the case of Pleurotus this involves the following sequence of events:
All of our studies were carried out in vitro on water agar (almost no nutrient), and I suggest that the results reflect the reality in the natural habitat. In my opinion, the complex sequence of events required to utilize nematodes or bacterial colonies is too sophisticated to be explained as cultural artifacts.
The conclusion, therefore, is that many of the fungi involved in decay that we have referred to as saprobes (or saprophytes) have a Jekyll and Hyde existence and, as well as being apparently innocuous saprobes, they also have the faculty of becoming aggressive predators of a variety of other microscopic life forms including nematodes, bacteria, pollen grains, yeasts etc. A fungus that is basically a saprophyte but has a parasitic phase in its life cycle is referred to as a facultative parasite. In the case of plant parasites this process is usually sequential, where a saprophytic phase is followed by a parasitic phase or vice versa. In the case of predatory wood decay fungi, however, the saprophytic phase and parasitic phase run concurrently and the two phases are parallel rather than sequential. There is also evidence to show that wood decay fungi can attack other wood decay fungi (= mycoparasitism) and this may eventually prove to be an important alternative for some species. Much of the background literature in this critical area, by a number of individuals and research groups, has been reviewed previously (Barron 1992).
Fungi possess a battery of enzymes that make them well suited to breaking down plant debris of all types. As well, the fungus hypha has the power of intrusive growth. When a hypha reaches a lignified wall it produces a narrow peg and, by using osmotic forces of growth, this peg penetrates the toughest walls with cell after cell succumbing to the fungus attack. Softening up the surface of the wall with enzyme activity often facilitates penetration. By virtue of their enzymatic versatility and intrusive penetration, fungi are wood destroyers par excellence.
The Role of Nitrogen-Fixing Bacteria
Nitrogen-fixing bacteria deserve special mention as they are the primary catalysts for much of what we are discussing. Slowly but inexorably the nitrogen levels in forests are built up through eons of time to fuel the system, maintain the nitrogen balance, and spur biological activity. Nematodes eat bacteria and the predatory fungi capture the nematodes. It makes sense to cut out the middle man and go directly to the bacteria. Taking this one step further it would be a useful attribute to go to the mother lode and attack the nitrogen-fixing bacteria directly and we found this to be true for some fungi (Hutchison and Barron 1996). Whether all of these are plausible hypotheses or flights of speculative fancy remains to be determined.
Fungi and the Carbon Cycle
So! What about the Carbon Cycle? Ecological studies have found that, in forest soils, the biomass of fungi is 90% of the total and outweighs the biomass (= living stuff) of all other microorganisms and mesoorganisms combined. ( Microorganisms include fungi, bacteria, nematodes, protozoa, rotifers, algae and all organisms that need a microscope to observe and identify. Mesoorganisms (= mesofauna) are small to very small but are still big enough to see with the unaided eye and include springtails, mites, worms, small insects and insect larvae). This massive fungal component is based largely on the role of fungi in two major biological systems: 1. As decay organisms of plant debris. 2. As mycorrhizal partners with trees and other plants.
The biodegradation of cellulose and lignified cellulose reaches staggering levels and is responsible for the return of hundreds of billions of tons of C02 annually to the atmosphere and is a major biological component of the terrestrial Carbon Cycle (Hudson 1962). Fungi are the major players in this process through biodegradation of cellulose by decay fungi and synthesis of cellulose by virtue of their role in mycorrhizal associations. As far as mycorrhizal fungi are concerned, each forest tree is associated with hundreds of thousands of kilometres of fungal hyphae (estimates of 2 km per cc, see Read, 1992). Therefore, the photosynthetic process in the leaves of the trees, that forms the basis for all the organic carbon compounds that constitute the tree, is mediated through fungi as suppliers of almost all the essential minerals and water. When we consider the amount of active fungal hyphae utilized in the biodegradation of all the woody debris and add the enormous amount of mycorrhizal hyphae associated with each tree, it is no longer surprising that the biomass of fungi is 90% of the total living organic material in forest soils.
The forest tree obtains almost all the water and minerals required through extensive networks of hyphae of mycorrhizal fungi. These fungi are particularly adept at obtaining low mobility phosphorus ions. They either grow to the source ions or alternatively they utilize difficult sources of phosphorus such as rock phosphate. As with decay fungi, nitrogen is also in short supply. Attacking living animals could kill two birds with one stone!
Ectomycorrhizal fungi (Hymenomycetes, Basidiomycota) are those fungi that produce a hyphal sheath around the outside (= ecto) of roots of forest trees and supply their essential nutrients. Most ectomycorrhizal fungi are mushrooms and, therefore, closely related to many of the wood decay fungi that are also mushrooms (Hymenomycetes, Basidiomycota). It is not unreasonable to suggest, therefore, that mycorrhizal fungi may be as versatile as wood decay fungi and employ similar or novel methods to access limiting nutrients. It was shown by Zhao and Guo (1989) that a number of mycorrhizal fungi were capable of attacking the plant parasitic fungus Rhizoctonia solani (a facultative parasite that attacks the roots of many plants). In their discussion, they suggested that this ability (mycoparasitism) allowed mycorrhizal fungi to protect the tree from attack by opportunistic parasitic fungi such as Rhizoctonia. This is a very plausible hypothesis but, in the light of the present discussion, there is an alternative explanation i.e. that mycorrhizal fungi can attack other fungi as nutrient sources including perhaps even other mycorrhizal fungi. Both these explanations could be valid and a specific mycorrhizal fungus could not only eliminate the competition of other mycorrhizal fungi but also utilize the nutrients that become available from this conquest. If 90% of the living organic material (=biomass) in forest soils is fungi, this would surely be the largest pool of nitrogen and phosphorus available and afford the best solution to any nutritional limitation. It was shown recently that mycorrhizal fungi can kill and consume a species of springtail (Collembola). Along with the earlier work of Zhao and Guo (1989), this is another indicator of predatory potential by mycorrhizal fungi.
It has been demonstrated that many mycorrhizal fungi secrete external proteases. This recalls the unexpected production of cellulases by nematophagous fungi. Why would mycorrhizal fungi produce proteases? The argument that these mycorrhizal proteases break down free proteins in soil for utilization by the tree symbiont is plausible. Simple proteins that become available through death, however, are rapidly depleted by a host of microorganisms and competition here is very tough. The ability of mycorrhizal fungi, however, to break down complex, difficult proteins, unavailable to most other microorganisms, is still a reasonable consideration. On the other hand, proteases are also essential components in the enzymatic arsenal of any potential predator or parasite of other organisms, and this might well be a role in e mycorrhizal fungi. Any mycorrhizal fungus that is a facultative parasite (as are their wood decay relatives) certainly has the edge in obtaining useable forms of nitrogen and phosphorus in a nutrient limiting environment. Whatever the truth, there remains much to do in untangling the many complex ecological relationships amongst fungi and the other life forms that abound in forest soils.
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