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4RESONANCE April 2005
GENERAL ARTICLE
A fungus hidden beneath the soil surface in forest, esti-
mated to be 1500-2400 years old, is the largest and oldest
living organism.
If asked which is the largest and the oldest living organism, the
California redwood tree, scientifically named Sequoia sempervirens,
comes to mind. This tree grows to heights of 300 feet or even
more (as a comparison the Qutab Minar in Delhi is 238 feet
high). The oldest redwood tree is 2,200 years of age. Some might
say that it is the blue whale, Balaenoptera musculus, which has an
average length and life span are 23 feet and 110 years, respec-
tively. In 1992, a fungus, Armillaria bulbosa (synonym Armillaria
gallica) was discovered in the forests of Michigan in North
America which was stated to be the largest and oldest living
organism. Subsequently, another species, Armillaria ostoyae, was
discovered in Oregon, USA, with a spread of approximately 890
hectares, and 2400 years old! These findings were startling
because fungi generally are microscopic organisms, the excep-
tions being the fleshy mushrooms often seen in lawns and
pastures, or the leathery bracket fungi growing on fallen trees
and decaying timber.
The Fungi
Fungi lack chlorophyll and are therefore non-photosynthetic.
The discovery of a giant fungus is therefore a testimony of the
success of fungi, whose vegetative body is composed of thread-
like filaments called hyphae, which secrete digestive enzymes
and breakdown the complex polymeric constituents of living or
dead plant remains. A part of these compounds is absorbed by
the fungus for its own growth, whereas the major portion is
made available for growth of plants and other microorganisms.
The fungi play a vital role in sustaining the carbon cycle in the
The Largest and Oldest Living Organism
Ramesh Maheshwari
Keywords
Largest and oldest organism.
Ramesh Maheshwari is at
the Department of
Biochemistry, IISc,
Bangalore. His research
interests are in biochemis-
try and genetics of fungi.
GENERAL ARTICLE
5
RESONANCE April 2005
biosphere. An American mycologist, B O Dodge (1872-1960),
who discovered Neurospora crassa (a fungus used by George W
Beadle and Edward L Tatum for their experiments which led to
the formulation of the one gene-one enzyme concept for which
they were awarded the Nobel Prize) summed up his view of fungi
as: “The fungi…are progressive, ever changing and evolving
rapidly in their own way, so that they are capable of becoming
readily adapted to every condition of life. We may be rest assured
that as green plants and animals disappear one by one from the
face of the globe, some of the fungi will always be present to
dispose of the last remains.” The fungi are the ultimate winners.
The hyphae generally have a diameter of 2-10 µm (Figure 1), and
are concealed in the substratum, such as soil or a dead tree. How
then was this fungus discovered, its age and mass estimated?
Discovery
In the 1990s aerial survey at the boundary of Michigan (USA)
and Canada showed yellowing, wilting of leaves and killing of
mature forest trees [1]. The culprit was a fungus that infected the
roots and extended into the trunk of mature, killed trees. The
fungus, scientifically named as Armillaria bulbosa, is commonly
called the ‘honey fungus’ because it forms honey-coloured,
edible, mushroom-like fruit bodies (basidiocarps) at the base of
the killed tree (Figure 2). It is classified in the Phylum Basidi-
omycetes (or Basidiomycotina) of Kingdom Fungi (Eumycota).
Figure 1. Hypha of a fun-
gus is divided by transverse
walls into multinucleate
compartments (cells) . The
septa have a pore through
which protoplasmic move-
ment occurs.
Figure 2. A postage stamp
showing the mushroom-
like, spore-bearing fruiting
bodies (basidiocarps) of
Armillaria mellea.
6RESONANCE April 2005
GENERAL ARTICLE
A characteristic feature of Armillaria is that hundreds of hyphae
aggregate into strands or cord-like structures, called the
rhizomorphs (Figure 3). These structures tap a living tree for
nutrients; extend through soil until they find a new victim.
Although an individual fungal hypha is barely visible to the
naked eye, the rhizomorphs are visible as black cords or shoe-
laces, growing beneath the bark of the infected tree (Figure 3) or
in the soil.
A Fungal Individual
Was the large area of the forest killed by a single fungal indi-
vidual that had begun its growth from a single spore (Figure 4)?
Mapping of the mycelial spread by microscopic examination was
precluded because soil is an opaque medium, and the area very
large. Therefore, microbiological and molecular methods were
used to answer this question. Rhizomorphs were sampled from
the infected area and portions thereof cultured on nutrient agar
medium to induce the component hyphae to fan out on medium,
and the pure mycelium (mass of hyphae) to be obtained for
genetic and molecular analyses. Were all isolates (mycelial
growth) those of a single individual?
Testing Individualism
One method to test this was to place the inoculum (a tiny amount
of mycelium from which the culture develops) adjacent to each
other on nutrient medium. If they are genetically the same, then
Figure 3. Rhizomorph of
Armillaria mellea growing
beneath the bark of a pine
tree.
Source: http:journeytoforever.
org
Figure 4. Diagram of a fun-
gal individual. The myce-
lium may become frag-
mented but the colony is
regarded as a single indi-
vidual.
7
RESONANCE April 2005
GENERAL ARTICLE
the growing mycelium from the inocula will come together and
anastomose (a compatible reaction). If not, there will be a zone of
confrontation with ‘killed’ cells in between (a ‘barrage reac-
tion’). The results showed that a single individual had parasit-
ized trees in approximately 15 hectare area.
The nucleotide sequence of DNA is a constant feature of the
genome of a particular organism. To confirm that all isolates
were genetically the same, molecular analyses called restriction
fragment length polymorphism (RFLP) and rapid amplified
polymorphic DNA (RAPD) were used. In the RFLP method,
the DNA of the isolates is cut with a bacterial enzyme, called
restriction endonuclease which has a defined target sequence
and the resultant DNA fragments are separated according to
their size by electrophoresis. If the isolates are different even by
one nucleotide difference in their DNA, then the distance be-
tween specific target cut sites on the DNA by restriction endo-
nuclease will differ. On the other hand, if the DNA of isolates
came from those of a single genetic individual, then every one of
them will have band(s) at the same position (Figure 5). The
products were separated on an agarose gel by electrophoresis and
the RFLP patterns visualized by molecular hybridization with a
labeled complimentary DNA strand as a probe. The pattern of
restriction fragments served as a fingerprint of a DNA molecule.
In the RAPD method, an arbitrarily designed 10-base-pair
5
21 345 6 12 3 4 6
55
21 345 6 12 3 4 6
21 345 6 12 3 4 6
Figure 5. (left) Diagram
of RFLP of genetically
different isolates. (right)
Diagram of RFLP of ge-
netically same isolates.
8RESONANCE April 2005
GENERAL ARTICLE
sequence was annealed to DNA isolated from the strains and a
polymerase chain reaction is carried out. Let us suppose that
DNA extracted from six isolates showed different bands, the
inference would be that the DNA came from different Armillaria
individuals. However, the surprising finding was that DNA
fingerprints of all isolates from an extensive area were identical!
It was inferred that rhizomorphs sampled from the area, were
parts of an individual fungus, regardless whether its mycelium
had remained intact, or become fragmented because of preda-
tion by microfauna or due to physicochemical regions in soil.
A feature of fungal growth is that different parts of a genetically
identical mycelium may reunite by hyphal fusion and advance,
exploiting resources in the direction of available organic food.
Another feature is that the mycelium is an interconnected sys-
tem of hyphae which have a pore in their transverse walls (septa)
which can be sealed by a proteinaceous material, called Woronin
bodies. Thus the pores can be selectively closed, as for example
the valves in a city water supply system, allowing cytoplasmic
flow and redistribution of nutrients in any direction. The suc-
cess of fungi is based on cooperation, not competition, among
hyphae – an obvious message for the human beings!
Age and Mass of Giant Fungal Colony
What might be the age of this fungal colony? From the rate of
growth of rhizomorphs on wooden posts buried in the ground,
as well as from the rate of expansion of mycelium on nutrient
media in Petri dishes, the age of the colony was estimated to be
approximately 1500 years! And what might be the mass of this
colony? From the numbers of rhizomorphs in a representative
area of the forest, the mass of this colony was conservatively
estimated to be 10,000 kg. Remarkably, RFLP and RAPD analy-
ses also indicated that these two fungal colonies have remained
genetically stable!
Presently, approximately 70,000 species of fungi are known.
Therefore, we may ask: What is unique about the genus Armillaria
9
RESONANCE April 2005
GENERAL ARTICLE
that has contributed to the giant size of its colony. Its rhizo-
morphs grow beneath the bark of the tree, and tap xylem vascu-
lar tissue for water, and the phloem vascular tissue for organic
compounds synthesized by leaves by photosynthesis and trans-
located downwards. With a water and nutrient supply base in a
tree, the rhizomorphs can extend several meters through soil,
even if the terrain is inhospitable, until it encounters another
victim. Additionally, the rhizomorph is resistant to heat and can
survive forest fires. Furthermore, this fungus is opportunistic -
it is a facultative parasite which means that it lives as a saprophyte,
breaking down organic polymeric compounds present in dead
leaf litter or dead wood in soil and absorbing them for its growth
requirements; but it also has the capability of parasitizing a
living host. These fungal colonies may still be growing, and may
continue to do so indefinitely, unless there is some holocaust.
Suggested Reading
[1] M Smith, J N Bruhn and J B Anderson, The fungus Armillaria bulbosa
is among the largest and oldest living organisms, Nature, Vol. 356,
pp.428-431, 1992.
[2] http://www.anbg.gov.au/fungi/mycelium.html
[3] http://journeytoforever.org/
Address for Correspondence
Ramesh Maheshwari
Department of Biochemistry
Indian Institute of Science
Bangalore 560 012, India.
Email:
fungi@biochem.iisc.ernet.in
We are glorious accidents of an
unpredictable process with no
drive to complexity, not the
expected results of evolutionary
principles that yearn to produce
a creature capable of under-
standing the mode of its own
necessary construction.
Stephen Jay Gould
