Slime moulds – the most remarkable organisms

“Slime-moulds may occur on objects of any and every sort. Their minuteness retires them from ordinary ken; but such is the extreme beauty of their microscopic structure, such the exceeding interest of their life history, that for many years enthusiastic students have found the group one of peculiar fascination, in some respects, at least, the most interesting and remarkable that falls beneath our lenses.’

– Thomas Macbride 1899 The North American Slime moulds.

At least once a year slime moulds make the news. On December 21 2016 the headline in Science Daily was ‘Giant cell blob can learn and teach’. The article explained how the plasmodial stage of a slime mould—essentially a moving feeding giant brainless cell—can transmit what it has learned to a fellow slime mould when the two combine.

Acellular or plasmodial slime moulds, also known as myxomycetes or myxogastrids, are ubiquitous opportunistic organisms. They are most abundant in temperate forests but they also occur in tropical forests, alpine areas, heathlands, grasslands, deserts, and arctic and sub‑Antarctic regions. In short, wherever there is organic material.

Myxomycetes were once included with plants when all living things were placed in just two kingdoms – plants and animals. When more kingdoms were created to encompass a vastly more complex world than was originally imagined, slime moulds were moved to the kingdom fungi. Indeed, they are often described as fungus‑like organisms and, like most fungi, they make only a relatively brief appearance at their fruiting body stage. However, they do not have any structures analogous to fungal hyphae, so when their pulsating feeding plasmodial stage was discovered they were moved to the animal kingdom. Then their single‑celled amoeboid feeding stage was observed and they were moved again, this time to the kingdom protozoa. However, this kingdom does not encompass organisms that have an amoeboid, plasmodial AND spore-bearing stage. They are now considered to be Amoebozoans but whether Amoebozoa is a supergroup or a kingdom is matter of some debate.

Before molecular research undertaken in 2013 established the 1000 million years of evolutionary history of the supergroup Amoebozoa to which myxomycetes belong, two fossils from 35–40 million-year-old Baltic Amber, the extant Stemonitis splendens and the extinct Arcyria sulcata, were the earliest evidence of myxomycetes. It is now believed that the common ancestor of myxomycetes was marine and existed before plants colonised the land, and that myxomycetes possibly co-evolved with plants in terrestrial habitats.

The terrestrial amoebozoan with the largest number of described species include the cellular and acellular slime moulds—the Dictyostelids and Myxomycetes respectively. Their predatory amoebae feed on bacteria and single-celled fungi such as yeasts. They live in soils rich in organic matter and play important roles including enhancing soil fertility though nutrient recycling. Research suggests that amoebae possibly participated in the initial enrichment of soils prior to the colonisation of land by plants although more work is needed to confirm this.

My research is about another stage in the lives of these intriguing organisms. The reproductive or spore-bearing stage comprises fruiting bodies that range in size from large amorphous blobs to tiny delicate structures little more than 2 mm high.

Study site:

Black Sugarloaf (41° 23′ 42″ S 146° 48′ 23″ E) is located in central north Tasmania. It has well-drained, rocky, dolerite-derived soils and forests dominated by Eucalyptus species. Over the past one hundred or so years these have been cut for sawlogs, railway sleepers, telegraph poles and, more recently, for woodchips. 130 acres of forest at approximately 450 m above sea level is privately owned land protected by a conservation covenant. Current land management practices do not include removal or burning of coarse woody debris or the understorey vegetation. The areas most often surveyed for slime moulds (Big tree track, Thismia gully and Swamp) are all within about 100-200 metres from Home. Big Tree Track is visited at least once each day; the other areas are usually visited once a week.

Home
‘Home’ is open eucalypt forest dominated by gumtopped stringybark (Eucalyptus delegatensis), black peppermint (E. amygdalina), stringybark (E. obliqua), white gum (E. viminalis) and swamp gum (E. ovata). The diverse understorey has silver banksia (Banksia marginata), forest daisybush (Olearia lirata), native cherry (Exocarpos cupressiformis), guitarplant (Lomatia tinctoria), prickly moses (Acacia verticillata), dollybush (Cassinia aculeata) and mountain correa (Correa lawrenciana). Ground layer vegetation includes sagg (Lomandra longifolia), forest flaxlily (Dianella tasmanica), cutting grass (Gahnia grandis), swordsedge (Lepidosperma sp.), bracken fern (Peridium esculentum) and other ferns. Some clearing of understorey vegetation for fire protection occurs around the house but old logs are left in situ.

Swamp
‘The Swamp’ is approximately 100 metres north of ‘Home’. It is a damp shady area with a permanent small creek that overflows onto a level flood plain after extended rainy periods. The closed canopy forest is dominated by blackwood (Acacia melanoxylon) and swamp gum (E. ovata) with a mid layer of tall swamp paperbark (Melaleuca ericifolia) and scented paperbark (M. squarrosa). The ground is mostly devoid of vegetation except for cutting grass, hard waterfern (Blechnum watsii) soft waterfern (B. nudum), treefern (Dicksonia antarctica) and mother shieldfern (Polystichum proliferum). There is a small patch of rainforest species including sassafras (Atherosperma moschatum) and the endemic heartberry (Aristotelia peduncularis). There are numerous logs and lots of leaf litter.

Big Tree Track
The Big Tree Track is so called because when we first moved to Black Sugarloaf a large old-growth tree, the only remaining such tree on Black Sugarloaf, was still standing. The tree fell in about 1990 and the stump and large log now provide substrate for numerous bryophytes, fungi and slime moulds. The 1 km track meanders through closed wet forest dominated by stringybark (Eucalyptus obliqua) and blackwood (Acacia melanoxylon). The midstorey species include silver banksia (Banksia marginata), dogwood (Pomaderris apetala), blanketleaf (Bedfordia salicina), treefern (Dicksonia antarctica), forest daisybush (Olearia lirata), and musk (O. argophylla). The ground layer is mostly devoid of vegetation except for patches of hard waterfern (Blechnum nudum). There is a deep layer of leaf litter and numerous rotting logs covered in mosses, leafy liverworts and lichens.

Thismia Gully
Thismia Gully (TG) is a southeast facing slope approximately 100 metres downslope from ‘home’. It has a watercourse with an ephemeral creek that flows only after extended rainy periods. It has a closed wet forest dominated by blackwood and swamp gum. The mid layer vegetation is mostly dogwood (Pomaderris apetala), musk (Olearia argophylla) and native currant (Coprosma quadrifida). The ground is devoid of vegetation except for occasional ferns including hard waterfern, treefern and tender brake (Pteris tremula). There is a litter layer of leaves, many fallen small trees, particularly dogwood, and numerous very large old bryophyte-covered eucalypt logs.