Bioluminescent Fungi

© Brian Perry Ph.D.
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Original publication: Mycena News, March 2007

Bioluminescence, the emission of visible light by living organisms, is known to occur in many distantly related phyla, including the fungi. Although written observations of this phenomenon in fungal species are reported to date as far back as Aristotle (384-322 BC) and Pliny the Elder (23-79 AD), and detailed studies have been conducted with many other organisms (i.e., insects, fish, dinoflagellates, bacteria), we know surprisingly little about the nature of the chemical reactions responsible for fungal bioluminescence, or even why this phenomenon occurs in fungal species.

Like all other organisms in which it occurs, bioluminescence in fungi is an oxygen-dependent reaction involving substrates generically termed luciferans, which is catalyzed by one or more of an assortment of unrelated enzymes referred to as luciferases. In fungi, both the luciferans and luciferases involved remain largely unidentified. During the luciferan-luciferase reaction, unstable chemical intermediates are produced. As these intermediates decompose excess energy is released as light emission, causing the tissues in which this reaction occurs to glow or luminesce. Although the older literature reports some fungal species as producing white or blue light, all recent studies and observations indicate that bioluminescent fungi emit a greenish light with a maximum around 520-530 nm. For those of you who have observed such fungi, you know the light they emit is often quite faint, and typically requires very dark conditions to see. I recently locked myself in a darkened room with several cultures of a bioluminescent species of Mycena, and even in the presence of actively growing mycelium, it required several minutes for my vision to adapt to the dark before I could perceive the faint light these cultures produced. This, however, is certainly not always the case. The basidiomes of Poromycena manipularis are reported to be visible from a distance of up to 40 meters, and other species have been reported to emit light that is bright enough to read by.

Bioluminescence has thus far been reported in approximately 50 species of fungi, all of which are white-spored basidiomycetes traditionally placed in the family Tricholomataceae. Impressively, more than two thirds of these bioluminescent species are members of the diverse and widespread genus Mycena. Additional genera containing luminescent species include Armillaria, Omphalotus (including Lampteromyces and luminescent Pleurotus species), Gerronema, Panellus and Dictyopanus (these latter two genera actually represent additional species of Mycena, but they have not yet been transferred to this genus). Luminescence, the intensity level and location of which varies by species, typically occurs in the mycelium and/or basidiomes, or some portion thereof. In Mycena lamprospora, however, it is the mature spores rather than these tissues that have been observed to luminesce. In many instances it is the hyphae present in decaying plant tissues that luminesces, resulting in the appearance of luminescent wood or leaves.

Here in California, Armillaria mellea, Omphalotus olivascens and Panellus stipticus occur regularly and are likely your best bet for observing luminescence locally. While many species of Mycena are present in California, it is primarily the tropical members of the genus that are luminescent. Studies by Bermudes and colleagues have demonstrated that M. haematopus (a locally abundant temperate species) does emit low levels of light, but this is not detectable by the human eye and was determined with the use of a photometer. The mycelium of several additional temperate Mycena species (e.g., M. epipterygia, M. galopus, M. maculata , M. pura, M. sanquinolenta) have also been reported to luminesce in European collections, but this condition has not yet been confirmed for North American material. The basidiomes of both O. olivascens and P. stipticus are luminescent, while for A. mellea it is only the mycelium (and rhizomorphs) that emits light. Due to the high levels of light pollution present in most regions, not to mention the moon, it is often necessary to relocate specimens of these species to darker locales in order to observe their luminescence. David Arora recommends locking yourself in a dark closet with fresh specimens, as well as a grilled cheese sandwich, to help combat the boredom while allowing your eyes to adjust to the dark!

Our current understanding of the evolutionary relationships of the fungi, based on the phylogenetic analyses of the Assembling the Fungal Tree of Life collaboration, suggest that bioluminescence within the basidiomycetes has evolved independently in a least three distinct lineages. Within the Mycena lineage (i.e., Mycenaceae) which contains the majority of these luminescent species (and likely well over 500 species in total), it remains to be seen whether luminescence evolved a single time early in the evolutionary history of this group and was then subsequently lost by numerous species, or whether it evolved numerous times independently as it appears to have done in the basidiomycetes as a whole.

Of course, all of this glowing business begs the question of why some species of fungi are luminescent while most others (as least as far as we have thus far observed) are not? Does producing light infer some form of selective advantage to the species in which this phenomenon occurs, and if so why is it not more widespread throughout the fungi? These, I must confess, are the sorts of as yet unanswered questions that evolutionary biologists lay awake at night puzzling over. Fungal luminescence has been hypothesized to attract invertebrates that aid in spore dispersal, which may be a suitable explanation for those species with luminescent basidiomes, but not for those in which only the mycelium emits light. Additional hypotheses include the attraction of predators of mycetophagous invertebrates, and even the function of emitted light as a warning to nocturnal heterotrophs that might consume the fungus or its substrate, similar to warning colorations observed in other organisms. In a study by Sivinski, leaves and twigs covered with a luminescent mycelia, as well as glowing basidiomes, were placed in sealed glass vials coated with a sticky substance, and used to trap arthropods at night. When compared with control tubes containing basidiomes that had been rendered non-luminescent by soaking in alcohol, significantly more Collembola and Diptera species were attracted to the tubes with bioluminescent material, lending support for such hypotheses.

It is also possible that bioluminescence in these fungi is nothing more than the by-product of some other metabolic process. Because the reaction is oxygen-dependent, it has been hypothesized that bioluminescence may have evolved as a method to consume excess oxygen produced in the cells of organisms during other metabolic processes (i.e., an antioxidant). Studies have suggested a link between lignin degradation and luminescence in fungi, with the oxygen-consuming reaction acting as a means of dealing with peroxides generated during the process. In the words of P. J. Herring, however, "such a prosaic explanation seems somehow unworthy of so splendid a phenomenon." Fortunately, these hypotheses are not all mutually exclusive. It is possible that bioluminescence may have arisen as a by-product of a beneficial metabolic process, and subsequently been co-opted in a relatively small number of taxa for secondary use in attracting spore dispersal agents or deterring fungivores.

Whatever role bioluminescence may play in the lifecycle of the fungal species in which it occurs, it is by all means an intriguingly beautiful phenomenon to observe and study. Desjardin and colleagues recently reported six new observations of luminescence in Mycena species from a single site in Brazil, and they are currently investigating the origin of luminescence in mycenoid fungi and the mechanisms responsible for light emission. There are undoubtedly many additional luminescent species of fungi out there that we have yet to document. After all, most mycologists do not spend a great deal of time wandering the woods in complete darkness, at least not by choice! Next time you are out in the woods at night, take the time to turn off your flashlight, let your eyes adjust, and have a look around you. You just never know what you might encounter.

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