Slime mold (or
slime mould, see
spelling
differences) is a broad term describing
fungus-like organisms that use spores to reproduce.
They were formerly classified as fungi, but are no longer
considered part of this group.
Their common name refers to part of some of these organism's
lifecycles where they can appear gelatinous (hence the name slime).
However, this feature is mostly seen with the myxomycetes, which
are the only macroscopic slime molds.
Slime molds have been found all over the world and feed on
microorganisms that live in any type of dead plant material. For
this reason, these organisms are usually found in soil, lawns, and
on the
forest floor, commonly on
deciduous logs. However, in tropical areas they are also common on
inflorescences,
fruits and in aerial situations (e.g., in the canopy
of trees). In urban areas, they are found on
mulch or even in the
leaf
mold in gutters. One of the most commonly encountered slime
molds, both in nature in forests in the temperate zones of the
earth as well as in classrooms and laboratories is the yellow
Physarum
polycephalum.
Most slime mold are smaller than a few centimetres, but the largest
recorded reached an area of up to thirty square metres, making them
the largest undivided cells known (although one could argue that
slime molds are made up from individual cells). Many have striking
colours such as yellow, brown and white.
Taxonomy
Slime molds can generally be divided into two main groups.
- A plasmodial slime mold
involves numerous individual cells attached to each other, forming
one large membrane. This "supercell" is essentially a bag of
cytoplasm containing thousands of
individual nuclei.
- By contrast, cellular slime molds spend most of their lives as
individual unicellular protists, but when a
chemical signal is secreted, they assemble into a cluster that acts
as one organism.
Slime molds as a group, are
polyphyletic. They were originally represented
by the subkingdom
Gymnomycota in the
Fungi kingdom and included the defunct phyla
Myxomycota,
Acrasiomycota and
Labyrinthulomycota. Today, slime molds
have been divided between several supergroups and not one of them
is included in the kingdom Fungi.
In more strict terms, slime molds comprise the group of the
mycetozoans (
myxomycetes,
dictyostelids and
protostelids). However, even at this level there
are conflicts to be resolved. Recent molecular evidence shows that
the first two groups are likely to be monophyletic; however the
protostelids seem to be polyphyletic, too. For this reason,
scientists are trying to elucidate the relations between these
three groups.
Bikont
The
Acrasidae have a life style similar
to Dictyostelids, but their amoebae behave differently and are of
uncertain taxonomic position.
The
Plasmodiophorids also form
coenocytes but are internal parasites of
plants (e.g.,
club root disease of
cabbages).
Finally, the
Labyrinthulomycetes
are marine and form labyrinthine networks of tubes in which amoebae
without
pseudopods can travel.
Amoebozoa
Slime mold (
Physarum
polycephalum)
- Mycetozoa, which
includes the defunct phylum Myxomycota, belong to the supergroup
Amoebozoa and include:
A common slime mold which forms tiny brown tufts on rotting logs is
Stemonitis. Another form which
lives in rotting logs and is often used in research is
Physarum polycephalum. In logs it
has the appearance of a slimy webwork of yellow threads, up to a
few feet in size.
Fuligo forms
yellow crusts in mulch.
The Protostelids life cycle is very similar to the above
descriptions, but they are much smaller, the fruiting bodies only
forming one to a few spores.
The
Dictyosteliida, cellular slime
molds, are distantly related to the plasmodial slime molds and have
a very different life style. Their amoebae do not form huge
coenocytes, and remain individual. They live in similar habitats
and feed on microorganisms. When food runs out and they are ready
to form sporangia, they do something radically different. They
release signal molecules into their environment, by which they find
each other and create swarms. These amoeba then join up into a tiny
multicellular slug-like coordinated creature, which crawls to an
open lit place and grows into a fruiting body. Some of the amoebae
become spores to begin the next generation, but some of the amoebae
sacrifice themselves to become a dead stalk, lifting the spores up
into the air.
Opisthokont
Fonticula is a cellular slime
mold which forms a fruiting body in a volcano shape. As long ago as
1979 it has been known to not have a close relationship with either
the Dictyosteliida or the Acrasidae. A 2009 paper finds it to be
related to
Nuclearia, which in
turn is related to
fungi.
Life cycle
Slime mold growing out of bin of wet
paper
They begin life as
amoeba-like
cells. These unicellular amoebae are commonly
haploid and multiply if they encounter their
favorite food,
bacteria. These amoebae can
mate if they encounter the correct mating type and form
zygotes which then grow into plasmodia. These contain
many
nuclei without
cell membranes between them, which can grow
to be meters in size. One variety is often seen as a slimy yellow
network in and on rotting logs. The amoebae and the plasmodia
engulf microorganisms. The plasmodium grows into an interconnected
network of protoplasmic strands.
Within each protoplasmic strand the cytoplasmic contents rapidly
stream. If one strand is carefully watched for about 50 seconds the
cytoplasm can be seen to slow, stop, and then reverse direction.
The streaming protoplasm within a plasmodial strand can reach
speeds of up to 1.35 mm per second which is the fastest rate
recorded for any micro-organism. Migration of the plasmodium is
accomplished when more protoplasm streams to advancing areas and
protoplasm is withdrawn from rear areas. When the food supply
wanes, the plasmodium will migrate to the surface of its substrate
and transform into rigid fruiting bodies. The fruiting bodies or
sporangia are what we commonly see, they superficially look like
fungi or molds but are not related to the true fungi. These
sporangia will then release spores which hatch into amoebae to
begin the life cycle again.
Plasmodia
In
Myxomycetes, the plasmoidal
portion of the life cycle only occurs after
syngamy, which is the fusion of cytoplasm and
nuclei of myxoamoebae or swarm cells.
Therefore, all of the nuclei are
diploid at
this stage and mitosis occurs simultaneously throughout the
organism. Myxomycete plasmodia are multinucleate masses of
protoplasm that move by cytoplasmic streaming. In
order for the plasmodium to move, cytoplasm must be diverted
towards the leading edge from the lagging end. This process results
in the plasmodium advancing in fan-like fronts. As it moves,
plasmodium also gains nutrients through the
phagocytosis of bacteria and small pieces of
organic matter.
The Myxomycete plasmodium also has the ability to subdivide and
establish separate plasmodia. Conversely, separate plasmodia that
are genetically similar and compatible can fuse together to create
a larger plasmodium. In the event that conditions become dry, the
plasmodium will form a sclerotium, essentially a dry and dormant
state. In the event that conditions become moist again the
sclerotium absorbs water and an active plasmodium is restored. When
the food supply wanes, the Myxomycete plasmodium will enter the
next stage of its life cycle forming haploid
spores, often in a well-defined
sporangium or other spore-bearing
structure.
In popular culture
- Traditional Finnish
lore
describes how malicious witches used yellow
Fuligo (there called "paranvoi," or butter of the familiar) to spoil milk.
- Mycologist Tom Volk reports that the
plasmodium of Fuligo is eaten in
Mexico
.
- The popular webcomic GPF has as one of its
main characters two sentient, talking
slime molds by the names of Frederick Physarum
and Persephone.
- The computer game NetHack has Slime Mold
as an edible, re-nameable fruit.
- The giant amoeba-like alien that
terrorizes the small community of Downingtown, Pennsylvania, in the
1958 American horror/science-fiction film The Blob might be based on slime molds.
See also
References
External links
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