Fuligo septica (“dog vomit”). Photo by Siga. Wikimedia Commons
(Click on this link to listen to an audio version of this blog … Slime molds
Back in September I wrote about the mushrooms growing in the over-watered yard of our next door neighbor. Almost immediately after sending out my observations, I got an email from Charlene (my cousin Randy’s wife) in which she talked about all of the fungi she had been seeing on her walks around their Webster, New York home (just north and east of Rochester). She told me about a stinkhorn fungus and something that looked like a pink shrimp growing on their mulch, and that, previously, she had seen a yellow fungus that someone had said was called “dog vomit.” She also reported that she thought she had seen the pink shrimpy-thing actually “walking” around!
I wrote back and told her that being surrounded by all those life forms must feel like Paradise (to which she did not reply)!
I started thinking about two things, though. One: I had heard of something called “dog vomit” before, and although it was blobby and yellow, it was not a fungus. Two: the thing I remembered called “dog vomit” could actually “walk” around or, at least, move! This “dog vomit” (a species much less colorfully referred to as Fuligo septica) was a slime mold, and slime molds are organisms that are quite different from fungi!
Slime mold (F. septica?) on birch log. Photo by brewbooks, Flickr
The first slime mold that I remember seeing was also in New York. Deborah and I were teaching at a summer research station near Cranberry Lake up in the Adirondacks. We were on a walk through the wet forest that ringed the lake and saw a neon yellow mass draped over a fallen birch log. Like Charlene, I swear that the mass was moving (although so slowly that it was really hard to detect)!
The term “slime mold” does not have any true, scientific standing. It describes a diverse group of eukaryotic (cells that have nuclei), free-living organisms that have the ability to live as single cells or as multicellular aggregations. Slime molds were once thought to be fungi, but two of the key features of fungi (chitinous cell walls and an absolutely sedentary life style) are not seen in slime molds (they have cellulose cell walls and can energetically crawl about!). Further, fungi secrete digestive enzymes into their environments and then absorb the digestion products from the organic materials around them. Slime molds, on the other hand, actively phagocytize their food (bacteria, protozoa, fungi and fungal spores) and digest it intracellularly inside their phagocytotic vacuoles.
Slime molds are now considered to be part of the Kingdom Protista. They are included in the super-group Amoebozoa along with several thousand other species of amoebae-like species. The “super-power” that separates the three types of slime molds from the rest of the Amoebozoa is, of course, their ability to form large, multicellular aggregations. These aggregations are typically less than a centimeter or two in size but can, in some species, cover several square meters and weigh up to20 kilograms!
Hemitrichia serpula. Photo by R. Prouidukhin. Biology Libre Texts
In some of the slime mold aggregations the amoeboid cells retain their individual structure and identity. These are the “cellular” slime molds (the “Dictyosteliidae”). In other aggregations the amoeboid cells lose their individual structural identity and fuse together to form a multinucleated syncytium. These are the “plasmoidial” slime molds (the “Myxogastria”). There is also a third group of slime molds that roughly resemble plasmoidial slime molds except that these slime molds make their spores on tiny, stalked fruiting bodies that extend from and cover their external surfaces (these are the “Protsteloids”).
Slime molds feed on the bacteria, fungi and protists that grow on decomposing plant materials. They can be found on wood, leaves, grass, mulch and even in rain gutters or in air conditioners. Moisture is the key for their abundance, persistence and activity.
Slime molds reproduce both asexually via simple mitotic division and also sexually via spore production. Spores are haploid cells (they have only one copy of the organism’s genetic information) that are encased in a tough, resistant layer of protein. These proteins enable spores to survive for very long periods of time (years, decades and, maybe, even centuries!) even under extremely stressful environmental conditions. When these spores “hatch” they form the haploid, single-celled amoeboid cell that is characteristic of all slime molds.
Cellular slime mold life cycle. Figure from Open Stax Biology
These amoeboid cells may live as independent entities or they may gather together to form their multicellular aggregations. Cellular slime molds pull their single-cellular forms together via chemical attractors. Within these aggregations cells that have appropriate genetic makeups specifically locate each other and fuse into large, diploid cells (they have two copies of the organism’s genetic information) that then grows into a structure called the “macrocyst” which then generates new, haploid spores via meiosis.
Plasmoidial slime mold life cycle. Open Stax Biology (from J. Gott)
In plasmoidial slime molds the amoeboid cells pair up with cells that have the appropriate genetic makeup and then fuse to form a diploid cell called the “zygote.” This zygote then rapidly grows and multiplies its genetic material (and nuclei) without dividing its cellular mass. This results in a very large, multinucleated mass of cytoplasm called the “plasmoidium.” This plasmodium is able to then form structures called sporangia that can synthesize new, haploid spores.
The key to the ecology of these slime molds, as I have previously indicated, is moisture. The plasmodia can only persist when there is abundant environment moisture. The spores can “hatch” only when moisture becomes available. Upstate New York (where both Charlene and I have seen our slime molds) is a very wet, humid place. I was sure, though, that there would not be any slime molds here in arid Colorado. But, I was wrong!
Physarum polycephalum.
Photo by D. Folds. Biology Libre Texts.
Colorado State University’s online “Plant Talk” series has a webpage entitled “Slime Molds.” They described the white, grey, yellow, purple, orange or brown “foam” that is regularly seen in lawns or areas covered with wood mulch all across the state of Colorado. These foams especially show up in warm weather after a rain or after a prolonged period of heavy, artificial watering. These foamy patches are slime molds!
It is very important to note that neither a lawn nor an expanse of wood mulch is a “natural” ecological system, and the Colorado State website does not mention any “natural” ecosystems in the state where slime molds might be found! The lawns and the mulches are all rich with materials that potentially have been transported great distances, often from places that are much more humid than Colorado and much more conducive to the survival and growth of slime molds. The highly resistant spores of these alien slime molds could easily survive the short-term transport of these grasses, soils and wood chips and shreds and rapidly “hatch” in their new habitats whenever moisture became available.
Slime molds here in the dry, arid plains of Colorado! Amazing!